Class 8

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 12 Introduction to Acid and Base Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 12 Introduction to Acid and Base Question Answer Maharashtra Board

Class 8 Science Chapter 12 Introduction to Acid and Base Question Answer Maharashtra Board

1. Identify the following solutions, whether they are acid or base.

Question a.

Answer:

2. Write chemical names from given formulae.
H₂SO₄, Ca(OH)₂, HCl, NaOH, KOH, NH₄OH

Question a.
Write the chemical names from given formulae: H₂SO₄, Ca(OH)₂, HCl, NaOH, KOH, NH₄OH
Answer:

1. H₂SO₄ – Sulphuric acid
2. Ca(OH)₂ – Calcium hydroxide
3. HCl – Hydrochloric acid
4. NaOH – Sodium hydroxide
5. KOH – Potassium hydroxide
6. NH₄OH – Ammonium hydroxide

3. Sulphuric acid has highest importance in chemical Industry. Why?

Question a.
Sulphuric acid has highest importance in chemical Industry. Why?
Answer:
Answer:
Sulphuric acid has highest importance in the chemical industry because it is used in the manufacturing of fertilizers like ammonium sulphate and superphosphate of lime. It is used in the manufacturing of hydrochloric acid, nitric acid, phosphoric acid, ether, plastics and metal sulphates. It is also used in the manufacturing of dyes, drugs, perfumes, disinfectants and glue.

4. Give answers.

Question a.
Which acid is used for getting chloride salt?
Answer:
Hydrochloric acid is used for getting chloride salt.

Question b.
By squeezzing lemon on a piece of rock the gas liberated turned lime water milky. Which compound is present in the rock?
Answer:
Metal carbonate is present in the rock.

Question c.
The label on the bottle of chemical is spoiled. How will you find whether the chemical is acidic or not?
Answer:
The chemical is tested with blue litmus paper. If it turns red it is an acidic otherwise tested with red litmus paper, if it turns blue, it is an alkaline.

5. Answer the following questions. 

Question a.
Explain the difference between acid and base.
Answer:
Acids:

1. Acids have sour taste.
2. Acids turn blue litmus red.

Bases:

1. Bases have bitter taste.
2. Bases turn red litmus blue.

Question b.
Why indicator does not affect by salt?
Answer:
Mostly indicators are organic compounds which do not react with a salt.

Question c.
Which substances are produced i by neutralization process?
Answer:
Salt and water are produced in the f neutralization process.

Question d.
Which are the industrial uses of acids?
Answer:
Acids which are used in the industry are:

1. Sulphuric acid
2. Hydrochloric acid
3. Nitric acid.;

1. Sulphuric acid: Sulphuric acid is 5 used (a) in the manufacture of chemical? fertilizers like ammonium sulphate, (b) for cleaning gold and silver articles and (c) in car batteries.

2. Hydrochloric acid: Hydrochloric acid is used (a) to clean toilets and (b) to obtain glucose from starch and also for producing gelatine.

3. Nitric acid: Nitric acid is used (a) in the manufacture of perfumes (b) for engraving on copper, brass or silver and (c) in the manufacture of paints and explosives.

6. Select proper word given in bracket and fill in the blanks.

Question a.
Main constituent of acid is ……………. .
Answer:
Main constituent of acid is H⁺ ion.

Question b.
Main constituent of alkali is …………….. .
Answer:
Main constituent of alkali is OH^– ion.

Question c.
Tartaric acid is a …………… acid.
Answer:
Tartaric acid is a weak acid.

7. Match the pairs. 

Question a.

Group A	Group B
1. Tamarind	a. Acetic acid
2. Curd	b. Citric acid
3. Lemon	c. Tartaric acid
4. Vinegar	d. Lactic acid

Answer:

Group A	Group B
1. Tamarind	c. Tartaric acid
2. Curd	d. Lactic acid
3. Lemon	b. Citric acid
4. Vinegar	a. Acetic acid

8. State true or false.

Question a.
Oxides of metals are alkaline in nature.
Answer:
True.

Question b.
Salt is acidic.
Answer:
False. (Salt is neutral)

Question c.
Metal corrodes due to salts.
Answer:
False. (Acids and bases corrode metals)

Question d.
Salts are neutral.
Answer:
True. (Not all salts neutral)

9. Classify the following substances into acidic, basic and neutral group:
HCl, NaCl, MgO, KCl, CaO, H₂SO₄, HNO₃, H₂O and Na₂CO₃.

Question a.
Classify the following substances into acidic, basic and neutral group:
HCl, NaCl, MgO, KCl, CaO, H₂SO₄, HNO₃, H₂O and Na₂CO₃.
Answer:

Group	Substances
Acid	HCl, H2SO4, HNO3
Base	CaO, MgO, Na2CO3
Neutral	H2O, NaCl, KCl.

Project:

Question a.
Write in your own language the uses and importance of neutralization reaction in daily life.

Class 8 Science Chapter 12 Introduction to Acid and Base Important Questions and Answers

Fill in the blanks:

Question 1.
Acid reacts with metal to form …………. gas.
Answer:
Acid reacts with metal to form hydrogen gas.

Question 2.
DNA is an acid present in our body, it decides …………. properties.
Answer:
DNA is an acid present in our body, it decides heredity properties.

Question 3.
The chemical formula of milk of magnesia is ………….
Answer:
The chemical formula of milk of magnesia is Mg (OH)₂.

Question 4.
…………. are used to control hyperacidity.
Answer:
Antacids are used to control hyperacidity.

Question 5.
Alkali has …………. taste.
Answer:
Alkali has bitter taste.

Question 6.
Proteins are made up of ………….
Answer:
Proteins are made up of amino acids.

Question 7.
…………. is used in batteries.
Answer:
Dil. H₂SO₄ is used in batteries.

Rewrite the following statements by selecting the correct options:

Question 1.
The colour of phenolphthalein indicator in alkaline solution is ………….
(a) yellow
(b) green
(c) orange
(d) pink
Answer:
The colour of phenolphthalein indicator in alkaline solution is pink.

Question 2.
…………. is sour to taste.
(a) An acid
(b) An alkali
(c) A salt
(d) Alcohol
Answer:
An acid is sour to taste.

Question 3.
When phenolphthalein is added to NaOH, the colour of the solution will become ……………… .
(a) colourless
(b) red
(c) pink
(d) yellow
Answer:
When phenolphthalein is added to 5 NaOH, the colour of the solution will become pink.

Question 4.
When phenolphthalein is added to HCl, the colour of the solution will be ………….. .
(a) red
(b) pink
(c) green
(d) colourless
Answer:
When phenolphthalein is added to HCl, the colour of the solution will be colourless.

Question 5.
…………. is a natural indicator.
(a) Phenolphthalein
(b) Methyl orange
(c) Litmus
(d) Methyl red
Answer:
Litmus is a natural indicator.

Question 6.
The litmus paper or the litmus solution is obtained from …………. plants.
(a) moss
(b) rose
(c) hibiscus
(d) lichen
Answer:
The litmus paper or the litmus solution is obtained from lichen plants.

Question 7.
…………. is not an alkali.
(a) Sodium hydroxide
(b) Potassium hydroxide
(c) Copper hydroxide
(d) Calcium hydroxide
Answer:
Copper hydroxide is not an alkali.

Question 8.
…………. is a weak acid.
(a) Hydrochloric acid
(b) Nitric acid
(c) Carbonic acid
(d) Sulphuric acid
Answer:
Carbonic acid is a weak acid.

Question 9.
When methyl orange is added to HCl, the colour of the solution will be …………… .
(a) red
(b) pink
(c) yellow
(d) colourless
Answer:
When methyl orange is added to HCl, the colour of the solution will be pink.

Question 10.
When blue litmus paper is added to NaOH, the colour of the litmus paper will be ……………. .
(a) blue
(b) red
(c) pink
(d) yellow
Answer:
When blue litmus paper is added to NaOH, the colour of the litmus paper will be blue.

State whether the following statements are true or false. If a statement is false, correct it and rewrite:

Question 1.
Red cabbage is a natural indicator.
Answer:
True

Question 2.
Fats of our body are formed by fatty acids.
Answer:
True.

Question 3.
Ammonium hydroxide is used in the production of fertilizers.
Answer:
True.

Question 4.
Lime water is a weak acid.
Answer:
False. (Limewater is a weak base)

Question 5.
Orange colour of methyl orange turns yellow in acid.
Answer:
False. (Orange colour of methyl orange turns pink in acid)

Question 6.
Methyl red turns yellow in alkali.
Answer:
True.

Question 7.
Oxalic acid is used in aerated cold drinks.
Answer:
False. (Carbonic acid is used in aerated cold drinks).

Find the odd one out and justify:

Question 1.
Acetic acid, carbonic acid, hydrochloric acid, nitric acid.
Answer:
Acetic acid. (Others are mineral acids.)

Question 2.
Hydrogen chloride, sodium hydroxide, calcium oxide, ammonia.
Answer:
Hydrogen chloride. (Others are bases.)

Question 3.
HCl, CH₃COOH, H₂SO₄, HNO₃
Answer:
CH₃COOH. (Others are strong acids.)

Question 4.
NaOH, Ca(OH)₂, NH₄OH, Ba(OH)₂
Answer:
NaOH (Others are weak bases.)

Question 5.
H₂SO₄, H₂CO₃, HCl, HNO₃
Answer:
H₂CO₃ (Others are strong acids.)

Question 6.
Citric acid, formic acid, lactic acid, nitric acid.
Answer:
Nitric acid. (Others are organic acids.)

Question 7.
Lime, litmus, phenolphthalein, methyl orange.
Answer:
Lime. (Others are indicators.)

Match the following:

Question 1.

Column ‘A’	Column ‘B’
1. Strong acid	a. Magnesium hydroxide
2. Weak alkali	b. Carbonic acid
3. Weak acid	c. NaOH
4. Strong alkali	d. Nitric acid

Answer:

Column ‘A’	Column ‘B’
1. Strong acid	d. Nitric acid
2. Weak alkali	a. Magnesium hydroxide
3. Weak acid	b. Carbonic acid
4. Strong alkali	c. NaOH

Question 2.

Column ‘A’	Column ‘B’
1. Sodium hydroxide	a. Whitewashing
2. Magnesium hydroxide	b. Fertilizers
3. Calcium hydroxide	c. Washing soap
4. Ammonium hydroxide	d. Antacid

Answer:

Column ‘A’	Column ‘B’
1. Sodium hydroxide	c. Washing soap
2. Magnesium hydroxide	d. Antacid
3. Calcium hydroxide	a. Whitewashing
4. Ammonium hydroxide	b. Fertilizers

Define the following:

1. Acid: A substance which gives H⁺ ions in water is called an acid.

2. Alkali: A substance which gives OH^– ions in water is called an alkali.

3. Indicator: Substances which change their colours in presence of acid or base are called indicators.

4. Neutralization: The reaction between an acid and an alkali to form the r salt and water is termed as neutralization.

Answer the following questions in one sentence each:

Question 1.
What are natural acids?
Answer:
Acids present in foodstuffs are called natural acids.

Question 2.
State three vegetables from which natural indicators can be prepared.
Answer:
Natural indicators: Red cabbage, radish, tomato.

Question 3.
State names of any three strong acids.
Answer:

1. Hydrochloric acid
2. Sulphuric acid
3. Nitric acid.

Question 4.
State the names of the acids found in lemon juice, Amla, tamarind water and curd.
Answer:

1. Lemonuice – citric acid
2. Amla – ascorbic acid
3. tamarind water – tartaric acid
4. curd – lactic acid.

Question 5.
State names of any three alkalis.
Answer:

1. sodium hydroxide
2. potassium hydroxide
3. calcium hydroxide.

Question 6.
State the names of three indicators.
Answer:

1. Litmus paper
2. Methyl orange
3. Phenolphthalein.

Question 7.
State the names of three weak alkalis.
Answer:

1. Calcium hydroxide
2. ammonium hydroxide
3. magnesium hydroxide.

Question 8.
From which plant is litmus obtained?
Answer:
Litmus is obtained from plants called lichens.

Question 9.
Name the antacid which is used to control the hyper acidity.
Answer:
Milk of magnesia [Mg(OH)₂] is used to control the hyper acidity.

Question 10.
How will you neutralize the excess of acid present in the soil?
Answer:
Lime stone or lime water is mixed in the soil to neutralize the excess of acid.

Question 11.
When lemonuice falls on a marble kitchen counter, which is the gas that bubbled out?
Answer:
Carbon dioxide.

Answer the following questions:

Question 1.
What are indicators?
Answer:
Substances which change their colours in presence of acid or base are called indicators.

Question 2.
State two acid-base indicators and mention their colour change.
Answer:
Phenolphthalein and methyl orange are two acid-base indicators. Phenolphthalein is colourless in an acidic solution while it turns pink in a basic solution. Methyl orange gives orange colour with an acidic solution and yellow colour with a basic solution.

Question 3.
State the colour change for each of the following solutions with red litmus, blue litmus, phenolphthalein and methyl orange solutions.
Answer:

Question 4.
Name the acid present in (1) Orange (2) Vinegar (3) Tamarind (4) Curd (5) Tomato (6) Lemon.
Answer:

Substance	Acid
Orange	Citric acid
Vinegar	Acetic acid
Tamarind	Tartaric acid
Curd	Lactic acid
Tomato	Oxalic acid
Lemon	Citric acid

Question 5.
Classify the following into acidic substances, basic substances and salts.
(1) Lime water, (2) soda water, (3) soap water, (4) sea water, (5) sweet limeuice, (6) sugar caneuice, (7) buttermilk, (8) a mixture of ash in water, (9) tamarind water.
Answer:
a. Acidic substances: (1) Soda water, (2) sweet limeuice, (3) buttermilk, (4) tamarind water.
b. Basic substances: (1) Lime water, (2) Soap water, (3) a mixture of ash in water.
c. Salts: (1) Sea water, (2) sugar caneuice.

Question 6.
What happens when sour substances like limeuice, tamarind water falls on shahabad stones or kitchen platform? Why?
Answer:
When sour substances like limeuice, tamarind water falls on shahabad stones or kitchen platform, citric acid present in limeuice and tartaric acid in tamarind water reacts with metal carbonates present in shahabad stones or kitchen platform, carbon dioxide gas is evolved resulting in the formation of uneven surface.

Question 7.
Collect soil samples from your surroundings and find out whether it is acidic or alkaline or neutral?
Answer:
Soil samples collected from the surroundings are acidic in nature.

Question 8.
Which substances are used to clean greenish stains on copper vessels and to shine blackish silver utensils?
Answer:
Tamarind pulp is used to clean greenish stains on copper vessels. To shine blackish silver utensils, ammoniacal solution or limeuice or detergent is used.

Question 9.
Why toothpaste is used for brushing teeth?
Answer:

1. Toothpaste contains fluorides and alkali to neutralize the mouth acid.
2. Hence, the tooth paste, which is generally alkaline, is used for cleaning the teeth as it can neutralize the excess acid in i the mouth and prevent tooth decay.

Question 10.
State the properties of acids.
Answer:

1. Acids are sour in taste.
2. Acid molecules contain hydrogen ion (H⁺) as a main constituent.
3. Acid reacts with metal to form hydrogen gas.
4. Acid reacts with carbonates and liberates CO₂ gas.
5. Blue litmus turns red in acid.

Question 11.
State the uses of acids.
Answer:

1. Acids are used in the production s of chemical fertilizers.
2. Acids are used in the production of explosives, oil purification, medicines, dyes and paints.
3. Hydrochloric acid is used for the preparation of different types of chloride salts.
4. Dil. H₂SO₄ acid is used in the batteries (electric cell).
5. Dil. HCl is used for sterilization of water.
6. Acid is used for making of white paper from wood pulp.

Question 12.
The iron knife shines better after cutting the sour fruits like lemon, raw mangoes. Why?
Answer:
Acids present in sour fruits clean and dissolves the salts present on the surface of iron knife as a result iron knife shines better after cutting the sour fruits.

Question 13.
What is meant by neutralization? Give example.
Answer:
When an acid reacts with an alkali to form a salt and water, it is called neutralization.

Give scientific reasons:

Question 1.
When we have stomachache, doctors prescribe medicine containing magnesium hydroxide.
Answer:

1. There is hydrochloric acid in our stomach which helps in the digestion of food.
2. Sometimes in the stomach, there is more hydrochloric acid than we need.
3. This excess acid causes acidity and indigestion resulting in stomachache.
4. When magnesium hydroxide is administered it neutralizes the extra acid in the stomach.
5. Therefore, magnesium hydroxide is the best cure for stomachache.
6. So, when we have stomachache, doctors prescribe medicine containing magnesium hydroxide.

Question 2.
Quicklime or slaked lime is added to the soil.
Answer:

1. When chemical fertilizers are added to the soil, the soil becomes acidic.
2. To reduce its acidity quicklime or slaked lime is added to the soil.

Question 3.
Tarnished and stained copper vessels are brought to a shine by using tamarind pulp.
Answer:
In due course, unus6d copper vessels
are covered with a coating of black copper oxide and other compounds of copper which tarnish the vessel. Tamarind contains tartaric acid. When the vessel is rubbed with tamarind pulp, tartaric acid reacts with copper oxide and removes the layer. Thus, the copper vessel regains its shine.

Question 4.
Copper and brass utensils are tinned.
Answer:
Copper and brass reacts with i organic acids present in food materials and poisonous salts are formed. Tin does not react with organic acids present in food materials. Therefore, copper or brass vessels are tinned.

Distinguish between acid and alkali. OR Distinguish between the following:

Question 1.
Acid and Alkali:
Answer:

Acid	Alkali
1. Acid has sour taste.	1. Alkali has bitter taste.
2. Acid turns blue litmus red.	2. Alkali turns red litmus blue.
3. An acid in an aqueous solution gives H+ ions.	3. An alkali in an aqueous solution gives OH+ ions.
4. Oxides of non – metals form acids.	4. Oxides of metals form bases.

Activity-based questions:

Activity 1:
Apparatus: Hibiscus, rose, turmeric, red cabbage leaves, filter paper, etc.
Activity: Rub red petals of hibiscus flower on the white filter paper. This gives hibiscus indicator paper. Similarly rub, rose petals on the white filter paper. Cut strips of this paper, it is a rose indicator paper. Take turmeric powder, add a little water in it. Dip filter paper or ordinary paper in the turmeric water for some time.

After drying make strips of that paper. Prepare turmeric indicator paper in this way. Put leaves of red cabbage in small quantity of water and heat it. Once solution of cabbage leaves cool down, dip papers in it and dry it. Make strips of dried paper. In this way prepare red cabbage indicator paper. Put some drops of following substances on the indicator papers prepared by the above method and write the effect in the following table:

Substance	Effect on turmeric paper	Acidic/basic
1. Lime juice	Yellow	Acidic
2. Lime water (calcium hydroxide)	Red	Basic
3.

Activity 2:
Take baking powder. Add a little water to it. Add this solution on to limeuice, vinegar, orangeuice, appleuice, etc. and note the findings.
What do you observe on addition of baking soda solution in the fruituice? Whether bubbles formed or effervescence came out of fruituice?
From the above first activity we came to know that yellow turmeric indicator paper’s turns red in certain solutions. Similarly on addition of baking soda solution in the acidic solution bubbles come out or effervescence is produced.
By these simple and easy activity we can identify acidic or alkaline substance:
[Note for Activity 1 and Activity 2: Students should perform the experiments under the guidance of school teachers and record their observations.]

Activity 3:
Under the guidance of teacher take limeuice, ammonium hydroxide (NH₄OH), dil. hydrochloric acid (dil. HCl) and nitric acid (HNOs) in different test-tubes. Add drops of following indicators in them. Also dip litmus papers in the solutions. Observe and record in the following table.

8th Std Science Questions And Answers:

• Human Body and Organ System Class 8 Questions And Answers
• Introduction to Acid and Base Class 8 Questions And Answers
• Chemical Change and Chemical Bond Class 8 Questions And Answers
• Measurement and Effects of Heat Class 8 Questions And Answers
• Sound Class 8 Questions And Answers
• Reflection of Light Class 8 Questions And Answers
• Man-made Materials Class 8 Questions And Answers
• Ecosystems Class 8 Questions And Answers
• Life Cycle of Stars Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 7 Metals and Nonmetals Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 7 Metals and Nonmetals Question Answer Maharashtra Board

Class 8 Science Chapter 7 Metals and Nonmetals Question Answer Maharashtra Board

1. Complete the table:

Question a.

Answer:

Property of metal	Use in everyday life
i. Ductility	i. Gold, silver ornaments
ii. Malleability	ii. Aluminium sheets, galvanised sheets
iii. Conduction of heat	iii. Stainless steel vessels, copper vessels, boilers
iv. Conduction of electricity	iv. Copper wires
v. Sonority	v. Brass articles

2. Identify the odd term.

Question a.
Gold, Silver, Iron, Diamond.
Answer:
Diamond. (Others are metals.)

Question b.
Ductility, Brittleness, Sonority, Malleability.
Answer:
Brittleness. (Other properties are metallic properties.)

Question c.
Carbon, Bromine, Sulphur, Phosphorus.
Answer:
Bromine. (Others are solids.)

Question d.
Brass, Bronze, Iron, Steel.
Answer:
Iron. (Others are alloys.)

3. Give scientific reasons:

Question a.
The stainless steel vessels in kitchen have copper coating on the bottom.
Answer:

1. Stainless steel is an alloy of iron ; with carbon, chromium and nickel.
2. The conductivity of copper is higher than that of iron in steel. Copper heats uniformly and faster. The time for cooking is reduced, as a result it saves fuel. Hence, the stainless steel vessels in kitchen have copper coating on the bottom.

Question b.
Copper and brass vessels are cleaned with lemon.
Answer:

1. Copper undergoes oxidation in air to form black copper oxide. Copper oxide reacts slowly with carbon dioxide in air and gains a green coat. This green substance is copper carbonate.
2. Lemon contains acid. The acid dissolves the green coating of basic copper carbonate present on the surface of a tarnished copper and brass vessels and makes them shiny again.

Question c.
Sodium metal is kept in kerosene.
Answer:

1. Sodium reacts so vigorously with atmospheric oxygen and water that it catches fire if kept in the open.
2. It does not react with kerosene and sinks in it. Hence, to protect sodium and to prevent accidental fires it is always kept in kerosene.

4. Answer the following: 

Question a.
What is done to prevent corrosion of metals?
Answer:
By applying a layer of paint, oil, grease or varnish on the surface of a metal to prevent corrosion. Also plating with noncorroding metal is done. Iron is coated with thin layer of zinc. Due to these processes the contact of metal surface with air is lost and corrosion is prevented.

Question b.
What are the metals that make the alloys brass and bronze?
Answer:
The alloy brass is formed from copper and zinc and the alloy bronze is formed from copper and tin.

Question c.
What are the adverse effects of i corrosion?
Answer:

1. A reddish coloured deposit (rust) is formed on iron by reaction with oxygen gas.
2. A greenish coloured deposit (copper carbonate) is formed on copper by reaction with carbon dioxide.
3. A blackish coloured deposit is formed (silver sulphide) on silver.
4. Corrosion causes damages to car bodies, bridges, iron railings, ships specially those of iron, silver articles and copper vessels.

Question c.
What are the uses of noble metals?
Answer:
Uses of Noble Metals:

1. Gold, silver and platinum are used to prepare ornaments.
2. Silver is used in medicines. (It has antibacterial property).
3. Gold and silver are also used to make metals and few electronic devices.
4. Platinum, palladium metals are used as catalyst.

5. Three experiments to study the process of rusting are given below. Observe the three test tubes and answer the following questions.

Question a.
Why the nail in the test tube 2 is not rusted?
Answer:
In the test tube 2, oil cuts the supply of air to nail due to which oxidation of nail is prevented and boiled water is free from gases. Hence, the nail in the test tube 2 is not rusted.

Question b.
Why is the nail in the test tube 1 is rusted highly?
Answer:
The nail in the test tube 1 is highly rusted because nail is in contact with water and air. The oxidation process is fast.

Question c.
Would the nail in the test tube 3 get rusted?
Answer:
No change is observed in the test tube 3. Nail remains as it is because the calcium chloride absorbs moisture, making the air dry, thus preventing rusting of the nail.

Project:

Question a.
How is the Varkha or silver foil used in sweets made? Collect the information about which metals are used to make ‘Varkha’.

Class 8 Science Chapter 7 Metals and Nonmetals Additional Important Questions and Answers

Rewrite the sentences after filling the blanks:

Question 1.
Gold of 100% purity is …………… carat gold.
Answer:
Gold of 100% purity is 24 carat gold.

Question 2.
The stainless steel utensils used at home are made of an alloy of iron with carbon, …………… and ……….. .
Answer:
The stainless steel utensils used at home are made of an alloy of iron with carbon, nickel and chromium.

Question 3.
…………….. is used in medicines.
Answer:
Silver is used in medicines.

Question 4.
To prepare ornaments ………… carat gold is used.
Answer:
To prepare ornaments 22 carat gold is used.

Question 5.
Non-metals are ………… conductors of heat and electricity.
Answer:
Non-metals are bad conductors of heat and electricity.

Question 6.
Non-metals form ………….. oxides.
Answer:
Non-metals form acidic oxides.

Question 7.
Some element can be hammered into thin sheets. This property is called …………….. .
Answer:
Some element can be hammered into thin sheets. This property is called malleability.

Question 8.
Iodine and ……………. are the two non – metals having typical metallic lustre.
Answer:
Iodine and diamond are the two non¬metals having typical metallic lustre.

Question 9.
Non-metals form ………. ions by gain of electrons.
Answer:
Non-metals form negative ions by gain of electrons.

Question 10.
During formation of positively charged ion, atom of metal ………….. .
Answer:
During formation of positively charged ion, atom of metal lose electrons.

Rewrite the following statements by selecting the correct options:

Question 1.
Metalloids have properties of …………. .
(a) metals
(b) non-metals
(c) both metals and non-metals
(d) neither metals nor non-metals
Answer:
(c) both metals and non-metals

Question 2.
………….. is a metal.
(a) Hg
(b) S
(c) P
(d) Br
Answer:
(a) Hg

Question 3.
…………… is a non-metal.
(a) Aug
(b) Ag
(c) Br
(d) Cu
Answer:
(c) Br

Question 4.
……………. is a metalloid.
(a) Aluminium
(b) Antimony
(c) Zinc
(d) Mercury
Answer:
(b) Antimony

Question 5.
……………. is a metal which is in liquid form at ordinary temperature.
(a) Gallium
(b) Magnesium
(c) Sodium
(d) Scandium
Answer:
(a) Gallium

Question 6.
…………….. is an acidic oxide.
(a) Na₂O
(b) CO₂
(c) FeOs
(d) H₂O
Answer:
(b) CO₂

Question 7.
The valence electron in …………… atom is 1.
(a) magnesium
(b) sodium
(c) silicon
(d) aluminium
Answer:
(b) sodium

Question 8.
The electronic configuration of oxygen is …………….. .
(a) 2, 5
(b) 2, 6
(c) 2, 4
(d) 2, 8, 6
Answer:
(b) 2, 6

Question 9.
Pure ………… is soft.
(a) aluminium
(b) silver
(c) gold
(d) platinum
Answer:
(c) gold

Question 10.
…………… is a highly malleable metal.
(a) Iron
(b) Nickel
(c) Manganese
(d) Aluminium
Answer:
(d) Aluminium

Question 11.
22 carat gold is gold of …………… purity.
(a) 100%
(b) 91.66%
(c) 75%
(d) 44%
Answer:
(b) 91.66%

State whether the following statements are True or False:

Question 1.
Metals are sonorous.
Answer:
True.

Question 2.
Diamond is the softest natural substance.
Answer:
False. (Diamond is the hardest natural substance.)

Question 3.
The density of lithium is lower than water.
Answer:
True.

Question 4.
Sulphur is brown in colour.
Answer:
False. (Sulphur is yellow in colour.)

Question 5.
Germanium is a metalloid.
Answer:
True.

Question 6.
Ornaments are made from 24 carat gold.
Answer:
False. (Ornaments are made from 22 carat gold.)

Question 7.
A reddish coloured deposit is formed on iron by reaction with oxygen.
Answer:
True.

Question 8.
Paladium and rhodium are noble metals.
Answer:
True.

Question 9.
The metal oxides are acidic in nature.
Answer:
False. (The metal oxides are basic in nature.)

Question 10.
The metal reacts with dilute acids to form a metal salt.
Answer:
True.

Identify the odd term:

Question 1.
Magnesium, Mercury, Sodium, Iron.
Answer:
Mercury. (Others metals are in solid state at normal temperature.)

Question 2.
Magnesium, Aluminium, Sulphur, Copper.
Answer:
Sulphur. (Others are metals.)

Question 3.
Iron, Copper, Graphite, Phosphorus.
Answer:
Phosphorus. (Others are good conductors of electricity.)

Question 4.
Tin, Bronze, Steel, Stainless steel.
Answer:
Tin. (Others are alloys.)

Consider the relation between the items in the first pair and write the correlation for second pair:

Question 1.
Mercury : Metal : : Carbon : …………. .
Answer:
Non-metal

Question 2.
Wood : Bad conductor of electricity : : Copper : ………………… .
Answer:
Good conductor or electricity

Question 3.
Gold : Noble metal : : Copper: ……………. .
Answer:
Metal

Question 4.
Iron and carbon : Steel : : Copper and Tin : ……….. .
Answer:
Bronze

Question 5.
0 : 2, 6 : : Mg : …………….. .
Answer:
2, 8, 2.

Match the columns:

Question 1.

Column ‘A’	Column ‘B’
1. Copper	a. Low melting point
2. Sodium	b. Liquid
3. Magnesium	c. High melting point
4. Boron	d. Conduction of heat
	e. 2, 8, 2

Answer:

Column ‘A’	Column ‘B’
1. Copper	d. Conduction of heat
2. Sodium	a. Low melting point
3. Magnesium	e. 2, 8, 2
4. Boron	c. High melting point

Question 2.

Column ‘A’	Column ‘B’
1. Silver	a. Copper + tin
2. Mercury	b. Medicines
3. Platinum	c. Thermometer
4. Brass	d. Catalyst

Answer:

Column ‘A’	Column ‘B’
1. Silver	b. Medicines
2. Mercury	c. Thermometer
3.Platinum	d. Catalyst
4. Brass	a. Copper + tin

Define the following:

1. Ductility: The property due to which a substance can be drawn into a thin wire is called ductility.
2. Malleability: The property due to which a substance can be hammered into a thin sheet is called malleability.
3. Metalloids: The element which shows the properties of metals as well as those of non-metals is called a metalloid.
4. Corrosion: Gases in the air react with metals in presence of moisture to form metal compounds on its surface, this destroys the metal gradually and is called corrosion.
5. Alloy: A homogeneous mixture of two or more metals or a homogeneous mixture of metal with non-metals is called alloy.

Answer the following questions in one sentence:

Question 1.
Name an alloy of copper and tin.
Answer:
An alloy of copper and tin is termed as bronze.

Question 2.
Name a metal which is in liquid state at ordinary temperature.
Answer:
Mercury is in liquid state at ordinary temperature.

Question 3.
Name two metals which are malleable.
Answer:
Iron and aluminium are malleable metals.

Question 4.
Name two metals which are ductile.
Answer:
Gold and silver are ductile metals.

Question 5.
Name two metals which are good conductors of heat.
Answer:
Silver and copper are good conductors of heat.

Question 6.
Name two metals which are good conductors of electricity.
Answer:
Copper and aluminium are good conductors of electricity.

Question 7.
Name two non-metals which are in solid state at room temperature.
Answer:
Carbon and sulphur are solids at room temperature.

Question 8.
Name two non-metals which are in gaseous state at room temperature.
Answer:
Hydrogen and oxygen are in gaseous state at room temperature.

Question 9.
Name the non-metal having electrical conductivity.
Answer:
Graphite has electrical conductivity.

Question 10.
Name two non-metals which are lustrous.
Answer:
Iodine and diamond are lustrous in nature.

Question 11.
Name two non-metals having high melting points.
Answer:
Carbon and boron melt at high temperature.

Question 12.
State two metals which can be cut easily with a knife.
Answer:
Sodium and potassium are soft metals and can be cut easily with a knife.

Question 13.
Which is the hardest natural substance?
Answer:
Diamond, which is a form of carbon is the hardest natural substance.

Question 14.
State the property of the metals for which they can be drawn into wires.
Answer:
The property of the metals due to which they can be drawn into wires is called ductility.

Question 15.
State the property of the metals due to which they can be beaten into thin sheets.
Answer:
The property of the metals due to which they can be beaten into thin sheets is called malleability.

Question 16.
Which of the following metals react with cold water? Sodium, iron, copper, potassium.
Answer:
Sodium and potassium metals react with cold water.

Question 17.
Which of the following metals do not react with cold water or hot water?
Answer:
Aluminium and iron do not react with cold water or hot water.

Question 18.
What are the constituents of stainless steel?
Answer:
Iron with carbon, chromium and nickel are the constituents of stainless steel.

Question 19.
State the term used to express the purity of gold.
Answer:
The purity of gold is expressed in carat.

Question 20.
Name two metals having low melting points and boiling points.
Answer:
Sodium and potassium having low melting points and boiling points.

Question 21.
Name a non-metal which is in liquid state at room temperature.
Answer:
Bromine is in liquid state at room temperature.

Question 22.
State the atomic number and electronic configuration of sodium.
Answer:
Atomic number of sodium: 11.
Electronic configuration of sodium (Na): 2, 8, 1.

Question 23.
State the atomic number and electronic configuration of aluminium.
Answer:
Atomic number of aluminium: 13.
Electronic configuration of aluminium (Al): 2, 8, 3.

Question 24.
Give two example of metalloids.
Answer:
Metalloids: Arsenic (As),
germanium (Ge), antimony (Sb).

Question 25.
State the atomic number and electronic configuration of nitrogen.
Answer:
Atomic number of nitrogen: 7.
Electronic configuration of nitrogen (N): 2, 5.

Answer the following questions:

Question 1.
A metal can be drawn into a wire. Explain why.
Answer:

1. The property due to which a substance can be draw into a thin wire is called ductility.
2. Metals are ductile. Thus, a metal can be draw into a wire.

Question 2.
A metal can be hammered into a i thin sheet. Explain why.
Answer:

1. The property due to which a substance can be hammered (or rolled) into a thin sheet without cracking is called malleability.
2. Metals are malleable. Thus, a metal can be hammered to form a thin sheet.

Question 3.
State the properties of metals.
Answer:

1. Metals have a lustre.
2. Metals are malleable. They can be beaten into thin sheets.
3. Metals are ductile. They can be drawn into wires.
4. They are good conductors of heat and electricity.
5. At ordinary temperature, metals are generally solid. (Exception: Mercury is liquid.)
6. Metals usually have high density.

Question 4.
State the properties of non-metals.
Answer:

1. Non-metals lack lustre.
2. As non-metals are brittle, they are not malleable.
3. They are not ductile.
4. Non-metals are poor conductors of heat and electricity.
5. At ordinary temperature, non-metals are in the solid or gaseous state. (Exception: Bromine is liquid.)
6. Non-metals have low density in the solid state.

Question 5.
How are metal ions formed? Give two examples.
Answer:
Metals have a tendency to lose their valence electrons to form positively charged ions, called cations.
Examples:

Question 6.
Classify the following elements into metals and non-metals :
Silicon, gold, silver, sulphur, carbon, aluminium, copper and phosphorus.
Answer:
Metals: Gold, silver, aluminium and copper.
Non-metals: Silicon, sulphur, carbon and phosphorus.

Question 7.
How are metal oxides formed? Explain with an example.
Answer:
Metals combine with oxygen in the ’ air to form their oxides.
Metal + Oxygen → Metal oxide
[Note: Reaction/explanation given in the screen are for better understanding of students. The same are not given in the textbook.]
When magnesium burns in air, it combines with oxygen to form magnesium oxide.
2Mg + O₂ → 2MgO
Magnesium Oxygen Magnesium oxide

Question 8.
How will you show that metal oxides are basic in nature?
Answer:
1. Take magnesium oxide in a test S tube. Add water in the test-tube. Shake the test-tube. Test the solution with red and blue litmus paper. Blue litmus paper remains as it is while red litmus paper turns blue. This shows that metal oxides are basic in nature.

2. Metal oxides react with an acid to form salt and water. Therefore, metal oxides are basic in nature.
Metal oxide + Acid → + Water

Question 9.
How do metals react with acid? Explain with an example.
Answer:
The metals react with dilute acids to form metal salts. Metals liberate hydrogen gas from dilute acids.
Metal + Dilute acid → Metal salt + Hydrogen gas.
Magnesium reacts with dilute HCl to form magnesium chloride and hydrogen gas.

Question 10.
How do metals react with water? Explain with an example.
Answer:
Most metals do not react with cold water. But some metals like sodium and potassium react with cold water to form their hydroxides and hydrogen gas.
Sodium reacts with water to form sodium hydroxide.

Question 11.
How do non-metals react with oxygen? Explain with an example.
Answer:
Non-metals combine with oxyen to form their oxides.
Non-metal + Oxygen → Non-metal oxide
When carbon burns in air, it combines with oxygen to form carbon dioxide.

Question 12.
How will you show that non- metal oxides are acidic in nature?
Answer:
1. Take non-metal oxide in a test tube. Add water in the test tube. Test the solution with blue and red litmus paper. Red litmus paper remains as it is while blue litmus paper turns red. This shows that non¬metal oxides are acidic in nature.

2. The oxides of non-metals are acidic in nature. They react with bases to form soluble salt and water. Therefore, non-metal oxides are acidic in nature.
C + O₂ → CO₂
CO₂ + 2NaOH → Na2CO₃ + H₂O

Question 13.
How are non-metal ions formed? Give two examples.
Answer: Non-metals have a tendency to accept electrons in their valence shell to form negatively charged ions called anions.
Examples:

Question 14.
What is meant by noble metals? Give two examples.
Answer:
Some metals like gold, silver, platinum, paladium and rhodium are found in nature in the form of elements. They are not affected by air, water, acids and heat etc. Hence, they are called noble metals.

Give scientific reasons:

Question 1.
Ornaments are generally not made from 24-carat gold.
Answer:

1. 24-carat gold is considered as gold of 100% purity.
2. 24-carat pure gold is very soft. 100% pure gold breaks due to pressure or gets bent easily. Therefore, ornaments are generally not made from 24-carat gold.

c

Question 2.
Ships are painted at frequent intervals.
Answer:
1. The metal sheets of ships are made from iron.
2. Due to salty seawater, the metal sheets of ships get rusted and corroded. Paints contain metals like zinc or magnesium. This prevents the metal surface of the ship coming into direct contact with salty seawater. Therefore, ships are painted at frequent intervals.

Question 3.
Gold and platinum are called noble elements.
Answer:

1. The metals such as gold and platinum are found in nature in the form of elements.
2. They are not affected by water, air, acids, heat and also do not take part in chemical reactions. Hence, they are called noble metals.

Explain the following reactions with the help of balanced equations:

Question 1.
Magnesium combines with oxygen.
Answer:
Magnesium combines with oxygen to form magnesium oxide.

Question 2.
Magnesium oxide reacts with dilute HCl.
Answer:
When magnesium oxide reacts with dilute HCl, magnesium chloride (salt) and water are formed.

Question 3.
Magnesium reacts with dilute HCl.
Answer:
When magnesium reacts with dil.
hydrochloric acid, magnesium chloride is formed and hydrogen gas is evolved.

Question 4.
Sodium reacts with water.
Answer:
When sodium reacts with water, sodium hydroxide and hydrogen gas are formed.

Question 5.
Carbon burns in air.
Answer:
When carbon burns in air, carbon dioxide is formed.

Question 6.
Carbon dioxide reacts with sodium hydroxide.
Answer:
When carbon dioxide reacts with sodium hydroxide, soluble salt sodium ; carbonate and water are formed.

Question 7.
Carbon dioxide reacts with water.
Answer:
When carbon dioxide reacts with water, carbonic acid is formed.

Question 8.
Sulphur dioxide reacts with water.
Answer:
When sulphur dioxide reacts with water, sulphurous acid is formed.

Question 9.
Sulphur trioxide reacts with water.
Answer:
When sulphur trioxide reacts with water, sulphuric acid is formed.

Write short notes:

Question 1.
The noble metals.
Answer:
Some metals like gold, silver, platinum, paladium and rhodium are found in nature in the form of elements. They are not affected by air, water, acids and heat etc. Hence, they are called noble metals.

Uses of Nobel Metals:

1. Gold, silver and platinum are used to prepare ornaments.
2. Silver is used in medicines. (It has antibacterial property).
3. Gold and silver are also used to make metals and few electronic devices.
4. Platinum, palladium metals are used as catalyst. .

Question 2.
The purity of gold.
Answer:
1. The purity of gold is measured in carats.
2. Twenty-four carat gold is considered as gold of 100% purity.
3. 24-carat pure gold is very soft. 100% pure gold bends easily or breaks due to pressure. Therefore, copper or silver is added to gold in the necessary proportion (while making ornaments).
4. Generally, 22-carat gold is used in making ornaments.

Percentage of gold	Carat
100%	24
91.66%	22
75%	18
58.33%	14
50%	12
41.66%	10

Question 3.
Corrosion.
Answer:
Corrosion: Gases in the air react with metals in presence of moisture to form metal compounds on its surface, this destroys the metal gradually and is called corrosion.

By applying a layer of paint, oil, grease or varnish on the surface of a metal to prevent corrosion. Also plating with noncorroding metal is done. Iron is coated with thin layer of zinc. Due to these processes, the contact of a metal surface with air is lost and corrosion is prevented.

1. A reddish coloured deposit (rust) is formed on iron by reaction with oxygen gas.
2. A greenish coloured deposit (copper carbonate) is formed on copper by reaction with carbon dioxide.
3. A blackish coloured deposit is formed (silver sulphide) on silver.
4. Corrosion causes damages to car bodies, bridges, iron railings, ships specially those of iron, silver articles and copper vessels.

Question 4.
Alloys.
Answer:

1. A homogeneous mixture of two or more metals or of metals and non-metals is called an alloy.
2. They contain metals in specific proportions.
3. The physical properties of an alloy are different from those of its constituents, but chemical properties remain the same.
4. Copper and tin are used to make an alloy called bronze. It is hard and corrosion resistant.
5. When iron and carbon are mixed, an alloy steel is formed. It is a stronger material.
6. The alloy, stainless steel is made from iron, carbon, nickel and chromium. It is more durable, clean and does not rust.

Distinguish between the following:

Question 1.
Metals and Non-metals
Answer:

Metals	Non-metals
1. Metals have lustre.	1. Non-metals do not have lustre.
2. Metals are malleable.	2. Non-metals are not malleable.
3. Metals are ductile.	3. Non-metals are not ductile.
4. Metals are good conductors of heat and electricity.	4. Non-metals are poor conductors of heat and electricity.
5. At room temperature, metals are in the solid state. (Exception : Mercury is liquid.)	5. At room temperature, non-metals are in the solid, or gaseous state. (Exception Bromine. It is in liquid state.)
6. Generally, metals have high densities.	6. Generally, non-metals have lower densities.

8th Std Science Questions And Answers:

• Living World and Classification of Microbes Class 8 Questions And Answers
• Health and Diseases Class 8 Questions And Answers
• Force and Pressure Class 8 Questions And Answers
• Current Electricity and Magnetism Class 8 Questions And Answers
• Inside the Atom Class 8 Questions And Answers
• Composition of Matter Class 8 Questions And Answers
• Metals and Nonmetals Class 8 Questions And Answers
• Pollution Class 8 Questions And Answers
• Disaster Management Class 8 Questions And Answers
• Cell and Cell Organelles Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 18 Ecosystems Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 18 Ecosystems Question Answer Maharashtra Board

Class 8 Science Chapter 18 Ecosystems Question Answer Maharashtra Board

1. Complete the following by using the correct option:

Question a.
Air, water, minerals, soil are ………….. factors of an ecosystem. (physical, organic, inorganic)
Answer:
Air, water, minerals, soil are physical factors of an ecosystem.

Question b.
River, ponds, ocean are …………. ecosystems, (land, aquatic, synthetic)
Answer:
River, ponds, ocean are aquatic ecosystems.

Question c.
Man is ………….. in an ecosystem. (producer, consumer, decomposer)
Answer:
Man is consumer in an ecosystem.

2. Match the following:

Question a.

Producer	Ecosystem
1. Cactus	a. Forest
2. Aquatic plants	b. Creek
3. Mangroves	c. Aquatic
4. Pine	d. Desert

Answer:

Producer	Ecosystem
1. Cactus	d. Desert
2. Aquatic plants	c. Aquatic
3. Mangroves	b. Creek
4. Pine	a. Forest

3. Give my information:

Question a.
Ecosystem.
Answer:
Ecosystem is the structure formed by the interrelationships between the living organisms and non-living components in their surrounding habitats. Living organisms are also called biotic factors while the non-living components are called abiotic factors. The abiotic and biotic factors have a reciprocal relationship with each other. The ecosystem has plants as producers, animals as consumers and microbes as decomposers.

The decomposers return the substances back to the nature by process of decomposition in which the organic complex molecules are reduced to their inorganic constituents. Thus the dead remains of plants and animals are once again recycled back to the nature in any healthy ecosystem.

Question b.
Biome.
Answer:
Biomes are a larger collection of small ecosystems which are similar in their structure. In a particular biome there are same climatic, abiotic factors and similar living organisms. Biomes are of two main types, land biomes and aquatic biomes. The land biomes exist only on land. Land biomes are of following subtypes based on the abiotic factors. E.g. Grasslands, evergreen forests, deserts, Tundra ecosystem, ecosystems in Taiga, tropical rainforests, etc. Aquatic biomes include freshwater ecosystem, marine ecosystem and brackish water ecosystem found in the creeks.

Question c.
Food web.
Answer:
The complex feeding relationships form a food web. In nature in any ecosystem, the food chains are not straight and linear. But these are quite complex. A prey is consumed by a predator but this predator can itself be a prey to some other predator. E.g. Frog is a predator for variety of insects but frog is preyed upon by snake and the snake can be eaten by a bird. The same bird can also feed on insects or frog. The interrelationships between different biotic components of any ecosystem thus form the complicated food web.

4. Give scientific reasons: 

Question a.
Plants in an ecosystem are called producers.
Answer:
are cut down causing deforestation. The wild animals lose their habitat. Many of them become extinct. There is great loss of flora and fauna due to construction of large dams. There is undue pressure on the land surface caused due to stored water column. This may cause earthquakes. The human settlements are removed as their houses and farms are immersed under water. In all the above ways large dams destroy the ecosystems.

Question b.
Large dams destroy ecosystems.
Answer:
When a large dam is constructed, the original landscape is changed. The trees are cut down causing deforestation. The wild animals lose their habitat. Many of them become extinct. There is great loss of flora and fauna due to construction of large dams. There is undue pressure on the land surface caused due to stored water column. This may cause earthquakes. The human settlements are removed as their houses and farms are immersed under water. In all the above ways large dams destroy the ecosystems.

Question c.
Rhinos were restored in Dudhwa forest.
Answer:
Long ago in last century, Dudhwa forest was the natural habitat of the one-horned Rhino. However, due to hunting and poaching, the Rhino became almost extinct. In order to restore it back the breeding in captivity was attempted and the young ones were released in the natural habitat. Saving the precious wildlife is important and hence these efforts were done.

5. Answer the following:

Question a.
What are the effects of increased population (of human beings) on ecosystems?
Answer:

1. Human population growth rate is very big as compared to other animals.
2. Moreover, they are apex consumers in an ecosystem. Just like any other organisms, ecosystems provide all the basic needs to humans, too.
3. However, man demands much more due to excessive population. The natural resources are utilized on the large scale only by man.
4. Changing lifestyles, consumerism, degradation of nature and pollution are certain aspects which are created only by human beings.
5. All these cause increased stress on the ecosystems. Large human population also produces large-scale solid wastes.

Question b.
How is urbanization responsible for destruction of ecosystems?
Answer:
When people do not get sufficient food and other amenities, they migrate to the cities. The cities provide employment for many people due to industries, factories and other facilities. Every day many people migrate to cities. This causes urbanization. Due to increased population in cities, there is more need for housing. This results in loss of agricultural lands, marshlands, wetlands, forests and grasslands, etc.

The natural ecosystems are lost in an attempt to have developmental work. Human interference causes changes in the ecosystems. When land usage is changed many naturally occurring species of plants and animals are exterminated. Animal- human conflicts are very common in newly established urban areas which are near the natural forests.

Question c.
What are the reasons for war?
Answer:
There are difference of opinions and competition over land, water, mineral resources, etc. among neighbouring countries. Moreover, there can be economic and political causes which lead to war among different countries. Many times the l religions and ethnicity are also caused for dispute. Due to international conflicts, the peace is disturbed and it results in war.

Question d.
Explain the interactions among the factors of an ecosystem.
Answer:

1. The abiotic factors such as air, water, soil, sunlight, temperature, humidity, etc. affect the biotic factors, i.e. the surviving plants, animals and microbes in the ecosystem.
2. The type of abiotic factors in an ecosystem determine the survival of biotic factors. The population of these biotic factors is also dependent on the quality of abiotic factors.
3. The proportion of abiotic factors in an ecosystem fluctuates as biotic factors use up or release particular abiotic factors.
4. Every biotic factor also interacts with its necessary abiotic factors as well as other biotic factors around it.

Question e.
Differentiate between evergreen forests and grasslands.
Answer:

Evergreen forests	Grasslands
1. About seven percent of the Earth’s surface is occupied by evergreen forests.	1. About 30 percent of the Earth’s surface is covered by grasslands.
2.  More than half of the earth’s terrestrial plants and animals are seen in evergreen forests.	2. Grazing animals are seen in grasslands.
3. Tropical evergreen forests are dense, multi-layered.	3. Grasslands consist of tall grass. There are wild grasses and there may be some trees.
4. These forests are found in the areas receiving more than 200 cm annual rainfall i.e. very heavy rainfall.	4. There is an average of 20 – 35 inches of rainfall a year in grasslands.
5. Tropical regions show evergreen forests.	5. Several parts of the world have grasslands.

6. Describe the following pictures:

Question a.
Describe the following pictures

Answer:
The picture no. 1 is showing desert ecosystem, while the picture no. 2 is showing a forest ecosystem where a wetland (a lake) is also shown. In desert ecosystem, the producers are green cactus and a palm tree. Since in desert there are scanty rains, the vegetation is very less. The soil is sandy soil. The consumers of the desert are also limited.

In the above picture camel is shown which is a primary consumer in the desert ecosystem. Camel is very well adapted for desert life. There is also the scarcity of water in desert and hence cactus shows xerophytic (suitable to survive in desert) adaptations.

In second picture, the forest ecosystem is shown because we can see elephant and tiger in the picture. Also, giant hornbill is sitting on the tree. This forest must be tropical evergreen forest. The rainfall is heavy in such areas and hence different types of wetlands can be seen in forests. Wetlands support variety of life. A separate aquatic ecosystem does exist here which is closely interconnected with the terrestrial ecosystems through food chains and food web.

The birds utilize the fish as their prey. The wild animals come to wetlands to quench their thirst. In this picture the plants, grass, etc. are producers, small fishes in the lake are primary consumers. They will be eaten by larger fishes which are secondary consumer in the aquatic ecosystem. The elephant is herbivorous animal and hence it is primary consumer. Snake is the secondary consumer while the birds of prey (eagle that is flying) and tiger are tertiary consumers.

Can you recall?

Question 1.
Which factors do you find in your surroundings?
Answer:
In our surroundings there are abiotic and biotic factors. The biotic factors are animals and plants in the surroundings. The invisible bacteria and fungi also have important role in the ecosystem as decomposers. Important abiotic factors are air, water, land, sunlight, etc.

Question 2.
Are these factors directly or indirectly related to you?
Answer:
We are directly or indirectly connected to every single factor in the surrounding in which we reside. For the requirement of food, we are dependent on plants and animals. For our survival we continuously need oxygen, water and other minerals, etc.

Classify:

Question 1.
Classify the following into living and non-living factors. Sunlight, Sunflower, Elephant, Lotus, Algae, Stone, Grass, Water, Ant, Soil, Cat, Fern, Air, Lion.
Answer:

Living Factors	Non-living Factors
Sunflower, Elephant, Lotus, Algae, Grass, Ant, Cat, Fern, Air, Lion.	Sunlight, Stone, Water, Soil, Air.

Can you tell?

Question 1.
What is the role of microbes in above interaction?
Answer:
Microbes play the role of decomposers in the above interactions.

Question 2.
How do producers obtain abiotic factors?
Answer:
Producers obtain solar energy from sunlight. The other nutrients are obtained from soil, water and air.

Question 3.
From where do the consumers get their food?
Answer:
Consumers get their food from producers and other consumers.

Project:

Question 1.
Visit an ecosystem nearby, List the biotic and abiotic factors in it. Show with pictures or sketches, how they are dependant on each other.

Question 2.
With the help of internet find out the loss of ecosystems due to wars or atomic explosions. Describe in your words.

Class 8 Science Chapter 18 Ecosystems Important Questions and Answers

Complete the following by using the correct option:

Question 1.
Biotic community is another name for ………. (ecology, ecosystem, population)
Answer:
Biotic community is another name Biotic for ecosystem.

Question 2.
Microbes in an ecosystem are ………… (producers, consumers, decomposers)
Answer:
Microbes in an ecosystem are decomposers.

Question 3.
Many types of ecosystems are found in ………… (oceans, deserts, biosphere)
Answer:
Many types of ecosystems are found in biosphere.

Question 4.
………….. is a type of grassland ecosystem. (Taiga, Tundra, Desert)
Answer:
Taiga is a type of grassland ecosystem.

Question 5.
Gir forest in Gujarat is the only habitat/shelter for the spectacular ………….. (Asiatic lion, Royal Bengal tiger, One-horned rhino)
Answer:
Gir forest in Gujarat is the only habitat shelter for the spectacular Asiatic lion.

Correct the following statements by changing one or two words:

Question 1.
Primary, secondary, tertiary consumers are the abiotic factors of the ecosystem.
Answer:
Primary, secondary, tertiary consumers are the biotic factors of the ecosystem.

Question 2.
Most of the ecosystems are simple.
Answer:
Most of the ecosystems are complex.

Question 3.
The word ecosystem was brought in the use by Ray Claffam.
Answer:
The word ecosystem was brought in the use by A.G. Tansle.

Question 4.
Specific ecosystem developed in a vast area is called a biosphere.
Answer:
Specific ecosystem developed in a vast area is called a biomes.

Question 5.
Longer summer and limited rain develop very tall plants in the grassland ecosystem.
Answer:
Longer summer and limited rain develop dwarf plants in the grassland ecosystem.

Question 6.
29 % of the earth surface is covered by water and only 71 % has land on it.
Answer:
71 % of the earth surface is covered by water and only 29 % has land on it.

Question 7.
Dams increase the water current in lower area.
Answer:
Dams lessen the water current in lower area.

Match the following:

Question 1.

Column ‘A’	Column ‘B’
1. The Great Himalayan National Park	a. Asiatic lion
2. Kaziranga	b. Tigers
3. Bharatpur	c. Aquatic birds
4. Ranthambore	d. One-horned Rhino
5. Gir forest	e. White panther

Answer:

Column ‘A’	Column ‘B’
1. The Great Himalayan National Park	e. White panther
2. Kaziranga	d. One-horned Rhino
3. Bharatpur	c. Aquatic birds
4. Ranthambore	b. Tigers
5. Gir forest	a. Asiatic lion

Find the odd one out:

Question 1.
Typha, Pistia, Eichhornia, Dipterocarpus
Answer:
Dipterocarpus. (All others are aquatic plants.)

Question 2.
Teak, Parthenium, Pine, Sandalwood
Answer:
Parthenium. (All others are forest trees.)

Question 3.
Giraffe, Zebra, Cheetah, Elephant
Answer:
Cheetah.. (All others are surviving grassland animals, Cheetah is a forest animal that is extinct from Asia.)

Question 4.
Hyena, Cow, Deer, Rabbit
Answer:
Hyena. (All others are primary consumers, Hyena is tertiary consumer.)

Question 5.
Fusarium, Leptocorsia, Aspergillus, Polycarpus
Answer:
Leptocorsia. (All others are decomposers, Leptocorsia is primary consumer.)

Taking into consideration the relationship in the first pair, complete the second pair :

Question 1.
Plants : Producers : : ……… : Consumers
Answer:
Animals

Question 2.
Secondary consumers : Jackals : : Tertiary consumer : ………..
Answer:
Kite/Lion/Hyena

Question 3.
Goat, sheep : Grassland ecosystem : : Tigers : …………..
Answer:
Forest ecosystem

Question 4.
Desert : Land biome : : Creek : …………….
Answer:
Aquatic biome

Question 5.
Limited rain : Grassland ecosystem : : ……….. : Tropical rainforests
Answer:
Excessive rain.

Answer in one sentence:

Question 1.
What is an ecosystem?
Answer:
The structure which is formed due to interactions between abiotic and biotic factors is called an ecosystem.

Question 2.
What is an ecological niche?
Answer:
The position of any living organism with reference to other living organisms and the role played by that organism is called ecological niche.

Question 3.
How are types of ecosystems formed?
Answer:
Types of ecosystems are formed according to size, place, climate, types of plants and animals that survive in that ecosystem.

Question 4.
Give examples of land biomes.
Answer:
Grasslands, evergreen forests, deserts, Tundra or polar ecosystem, ecosystems in Taiga, tropical rainforests, etc. are land biomes.

Question 5.
What are the reasons for tourism among the people?
Answer:
Tourism is mainly because people like to visit scenic places, undertake nature watch, wish to visit to sacred places and for entertainment purpose.

Question 6.
Why we should protect natural ecosystems?
Answer:
Natural ecosystems must be protected as they maintain balance in the biosphere.

Give my information Write short notes:

Question 1.
Dudhwa forest.
Answer:
Dudhwa forest is in Uttar Pradesh. About a century back, Dudhwa forest was the habitat for one-horned Rhino. However, due to hunting and poaching in 20th century, this animal became endangered. Thus to restore their populations, on 1st April 1984 rhino were introduced there again. Rhinos can were bred in captivity followed by releasing them back into natural ecosystem.

For this purpose at first the land was selected. This was 27 square km grassland and forest with continuous water supply through natural resources. Two observatories were established for making these efforts successful. Now Dudhwa is known for one¬horned Rhino population.

Question 2.
Aquatic ecosystems.
Answer:
Earth’s 71 % of surface is covered over by aquatic ecosystems. They are of three main types, viz. fresh water ecosystem, marine ecosystem and brackish water ecosystem. Rivers, lakes and ponds and streams are some of the fresh water ecosystem. Fresh water contains less salt and hence it is potable. Marine water has more salinity, i.e. its salt content is high. Brackish water ecosystem is present in the creeks. Here the saline and fresh water mix together. Hence salinity here is lesser than that of the marine water.

Give scientific reasons:

Question 1.
Microbes are called decomposers.
Answer:
Decomposition is the process in which the organic compounds are broken down to their inorganic constituents. The organic substances such as carbohydrates, proteins and lipids which are locked in the dead remains of plants and animals are converted into hydrogen, oxygen, calcium, iron, sodium, potassium, etc. by microbes. Therefore, microbes are called decomposers.

Use your brain power:

Question 1.
Which biotic factors get affected due to a dam?
Answer:
The biotic factors include plants, animals and humans. Lot of vegetation is submerged from the place where dam is constructed. The forests are wiped out. Large scale deforestation takes place in the proposed dam area. Wildlife is destroyed completely. The resident animals whose habitat are there either migrate away or are killed due to lack of their food and shelter. Many instances of animal-human conflicts arise when the wild animals drift to the neighbouring human settlements.

If human settlements are shifted, people are displaced. They lose their houses and agricultural land. Though compensation is given to them, there is great change for the people. Water current in lower area of the dam is reduced due to construction of dam-walls. Therefore, the original lotic (running 5 water) ecosystems that were present are destroyed after the construction of dam.

Question 2.
What will be the effects on biotic factors in the running water of river?
Answer:
The aquatic flora (plants) and fauna (animals) are well adapted to their habitats. The running water ecosystems are known as lotic ecosystems. The non-moving waters on the contrary create lentic ecosystems. Lotic ecosystems provide special adaptations to the plants and animals living therein. Plants are mostly in the form of freely floating algae and phytoplankton. Animals are well adapted for swimming in the currents of water. The fishes especially show a stream-lined body, fins for swimming, and gills to take up dissolved oxygen in the water. The other animals are also well adapted to the currents of water.

Think about it:

Question 1.
Can we call a tree an independent ecosystem?
Answer:
The huge tree can be an independent ecosystem. On one such tree there may be different habitats. Variety of insects thrive on huge tree by making their habitats in cracks and crevices on the tree. Many birds visit the tree to feed on such insects. There are many bird species that live in the hollow of the tree. Garden lizards, chameleons, snakes are the different reptiles seen associated with the tree ecosystem.

Squirrels, rabbits, bats are the mammals which take shelter in and around the trees, also inhabit a tree variety of fungi, lichens, mosses and epiphytic plants like orchids survive on the huge trees. Many food chains and complicated food webs are formed with the support of the tree. Thus one can say that a tree is an independent ecosystem.

Question 2.
Today Divija visited a hill. Honey: bees were hovering on flowers. One of them stung Divijia’s hand. Divija was hurt due to that sting and said with anger that all the honey-bees from the whole world must get destroyed. Afterwards, she thought what will happen if all honey-bees are destroyed? We will get honey, nothing more. What would be your explanation to Divija?
Answer:
Honey bees provide us with honey and bee-wax. Both of these are commercial products. But more than that the role played by honey bees in the pollination of flowers is very important. Due to their nectar collecting behaviour, they bring about pollination of the flowers.

Presence of honey bees is thus very essential for the agriculture and horticulture. Without honey bees, the growth of fruits and grains will not be possible, if pollination and subsequent fertilization does not take place in nature. This will create famine-like conditions. Therefore, honey bees’ position in the nature is very important.

Chart based questions:

Question 1.
Complete the following chart in context to a grassland.

Answer:

Question 2.
Write the information about various components of forest.

Answer:

Question 3.
Observe such ecosystems around you and complete the following chart.

Answer:

Question 4.
Complete the table.

Answer:

National Park/Sanctuary	State
1. Gir	Gujarat
2. Dachigam	Jammu-Kashmir
3. Ranthambore	Rajasthan
4. Dajipur	Maharashtra
5. Kaziranga	Assam
6. Sunderbans	Bengal
7. Melghat	Maharashtra
8. Periyar	Kerala

Question 5.
Complete the following chart:

Answer:

Diagram based questions:

Question 1.
Correct the following diagram:

Answer:

MCQs based on experiment:

Question 1.
Which one out of the following is not biotic factor?
(a) Humidity
(b) Rabbit
(c) Snake
(d) Bacteria
Answer:
(a) Humidity

Question 2.
Which climatic factor is causing migration of the birds in certain regions of the earth?
(a) Humidity and rainfall
(b) Ambient temperature
(c) Amount of carbon dioxide
(d) Amount of plant matter
Answer:
(b) Ambient temperature

Question 3.
Which abiotic factor mainly distinguishes marine water from the fresh water?
(a) Temperature
(b) Dissolved oxygen:
(c) Salinity
(d) Surface tension
Answer:
(c) Salinity

Question 4.
Why is creek water different from the river water?
(a) Creek water is more polluted than the river water.
(b) Creek water is less salty than the river water.
(c) Creek water is more saline than the river water.
(d) Creek water is less turbulent than the river water.
Answer:
(b) Creek water is less salty than the river water.

Question 5.
Which land biome shows presence of cacti plants more?
(a) Polar regions
(b) Grasslands
(c) Rainforest
(d) Desert
Answer:
(d) Desert

Projects:

Collect information

Question 1.
What are the possible threats to grasslands?
Answer:
Man tries to use grasslands for agricultural purpose. This transformation causes loss of natural ecosystem. Many animals from grasslands are killed by hunting and poaching. Though now Wildlife Act has protected these animals, many wildlife species are already extinct. There are chances of forest fires which is the greatest threat to grasslands. The global warming may also cause grasslands to become a desert.

Question 2.
Why did Asian Cheetah become extinct in last century?
Answer:
When British ruled India, Asian Cheetah was hunted to a great extent. The kings and British rulers did rampant hunting. The forest lands were also reduced due to encroachment by humans to build their settlements. Dams were constructed. Mega highways, bridges and roads caused loss of habitat for Asian Cheetah. Cheetah needs large thickly forested territory. This was lost due to all man-made activities. All these reasons made Asian Cheetah extinct in last century.

Question 3.
Observe Asian Cheetah on internet. Describe it.
Answer:
Cheetah is the fastest running feline animal on the earth. Though from Asian regions it is said to be extinct, in African grasslands, its occurrence is still there. But it is endangered animal. By 2016, the global cheetah population is estimated at approximately 7,100 individuals in the wild. Several African countries have taken steps to improve cheetah conservation measures.

Cheetah has a slender body, deep chest, spotted coat. Its head is small rounded and on the face there is black tear-like streaks. It has long thin legs and a long spotted tail. Cheetah in India were tamed during Moghul period for assisting in hunting. Stories about such episodes are commonly told. However, the last Cheetah in India was spotted in Bastar, M.P. in 1948.

8th Std Science Questions And Answers:

• Human Body and Organ System Class 8 Questions And Answers
• Introduction to Acid and Base Class 8 Questions And Answers
• Chemical Change and Chemical Bond Class 8 Questions And Answers
• Measurement and Effects of Heat Class 8 Questions And Answers
• Sound Class 8 Questions And Answers
• Reflection of Light Class 8 Questions And Answers
• Man-made Materials Class 8 Questions And Answers
• Ecosystems Class 8 Questions And Answers
• Life Cycle of Stars Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 15 Sound Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 15 Sound Question Answer Maharashtra Board

Class 8 Science Chapter 15 Sound Question Answer Maharashtra Board

1. Fill in the blank with appropriate word:

Question a.
The region in a sound wave, with higher pressure and density is called ………….. and that with low pressure and density is called ……….. .
Answer:
The region in a sound wave, with higher pressure and density is called compression and that with low pressure and density is called rarefaction.

Question b.
Medium is …………… for generation of sound.
Answer:
Medium is needed (or necessary) for generation of sound.

Question c.
The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is ………… .
Answer:
The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is 500 Hz.
[Note: Total number: 100. ∴ 500 compressions and 500 rarefactions are produced per second.]

Question d.
Different sound notes have different ………………. .
Answer:
Different sound notes have different frequencies.

Question e.
In a loudspeaker, …………… energy is converted into ………….. energy.
Answer:
In a loudspeaker, mechanical energy is converted into sound energy.

2. Give scientific reasons:

Question a.
It is essential to change the tension in the vocal cords, as we produce different sound notes from our larynx.
Answer:
Different sound notes correspond to different frequencies. The frequency of sound depends on the tension in the vocal cords. Hence, it is essential to change the tension in the vocal cords, as we produce different sound notes our larynx.

Question b.
Astronauts on the moon cannot hear each other directly.
Answer:
Two astronauts on the moon talking to each other directly, will be unable to listen to each other, even if they are very close to each other. The moon does not have atmosphere. Since there is no medium which is necessary for generation and propagation of sound, between the astronauts, direct sound propagation between them is not possible. Therefore, the astronauts use some technology like the one used in our cell-phones to communicate with each other. The waves used in cell-phone do not need any medium for propagation.

Question c.
As the sound wave propagates from one place to the other in air, the air itself is not required to move from one place to the other.
Answer:
Propagation of sound through air occurs due to energy transfer by vibrating air molecules in one region to those in the adjacent region away from the source. The molecules simply vibrate about their mean positions. Hence, the air itself is not required to move from one place to the other.

3. How are different sound notes generated in musical instruments like guitar, which uses strings for sound generation, and flute, which uses blown air for sound generation?

Question a.
How are different sound notes generated in musical instruments like guitar, which uses strings for sound generation, and flute, which uses blown air for sound generation?
Answer:
1. Guitar: It is a string based, flat- backed instrument. It has usually six strings, the strings are plucked or strummed with the fingers or a small piece of plastic, wood, etc. It has a flat sounding board with a circular sound hole in the centre. Also it has a fretted fingerboard. Its frequency range is more than three octaves. The frequency of vibration of the string is changed by changing the tension in the string or changing the vibrating length of the string. As the tension is increased, the frequency increases. As the vibrating length is increased, the frequency decreases. This is how different sound notes are generated.
[Note: The acoustic guitar has hollow body and six or twelve strings while the electric guitar usually has solid body and six strings. The electric bass guitar has four strings.]

2. Flute: It is a wind instrument where air is blown against the edge or rim of the blowing hole. The frequency of the sound produced depends upon the length of the vibrating air column in the tube. The greater the length of the vibrating air column, the less is the frequency of the sound produced. This is how different sound notes are produced. The flute has six or seven or eight holes to generate sounds of different frequencies. Different notes can be generated also by changing the way of air-blowing.

4. How is sound produced in the human larynx and a loudspeaker?

Question a.
How is sound produced in the human larynx and a loudspeaker?
Answer:
1. Sound production in the human larynx:
In the humans, sound is produced in the voice box called the larynx. It is located at the upper end of the windpipe. Two vocal cords (chords) are stretched across it with a narrow slit between them for the passage of air (Fig. 15.5). When the lungs force air through the slit, the cords start vibrating.

The frequency of the sound produced depends upon the length and thickness of the cords, and the tension in the chords. The frequency increases with the increase in tension and the more the length or the thickness of the cord, the less is the frequency. Muscles attached to the cords can make the cords tight (more tension) or loose (less tension).

2. Sound produced in a loudspeaker:
Figure shows the internal construction of a loudspeaker. Here, a coil is wound around a permanent magnet. The conical screen of the loudspeaker is attached to the coil.

When a current is passed through the coil, a magnetic field is produced. Its interaction with the permanent magnet results in the back and forth motion of the coil. The frequency and the amplitude of the motion of the coil depends on the variation in the current through the coil.

As the coil moves, the conical screen also moves back and forth. The vibrations of the screen produce sound waves in air. Very loud sound can be produced by changing the current.
[Note: If you gently touch the vibrating screen, you can feel the vibrations.]

5. Explain the experiment, with0 a neat diagram, to prove the following:
‘Sound needs a material medium for propagation.’

Question a.
Explain the experiment, with0 a neat diagram, to prove the following:
‘Sound needs a material medium for propagation.’
Answer:

Figure shows part of the set up used in this experiment. What is not shown is the electric circuit in which the electric bell is connected. A vacuum tight bell jar contains an electric bell connected to a power supply through the lid of the jar. The jar is placed on a smooth horizontal surface such as that of glass.

Initially the vacuum pump is off and the jar contains air. The circuit containing the bell is completed using the key or the switch so that the bell starts ringing. This can be heard outside the jar.

Then the vacuum pump is switched on so that it starts removing the air from the jar. We find that the level of ringing sound heard goes on decreasing as the quantity of air in the jar becomes less and less.

When the pump is operated for a sufficiently long time interval, the quantity of air in the jar becomes so less that the level of ringing sound becomes very low; sound is hardly audible. But we can see the striker in the bell hitting the gong. By extrapolation, we conclude that sound generation and propagation needs a medium.

6. Match the following:

Question a.

Answer:

Column I	Column II
Human larynx	Vibrations in vocal cords
Loudspeaker	Vibrations of screen
Jaltarang	Vibrations in air column
Tuning fork	Vibrations of metal arms
Sitar	Vibrations in strings

Can you recall?

Question a.
How is sound produced?
Answer:
Sound is produced by the vibrations of an object such as a stretched membrane or a string under tension.

Project:

Question 1.
Take two plastic glasses and tie a thread between them to make a toy phone. Does the voice of your friend reach you through the thread? Take metal wire instead of the thread and also change the length of the thread wire and repeat the activity. Discuss with your friends and teacher about your conclusions.

Question 2.
Take a plastic or tin can or a plastic glass. Remove its bottom. Stretch a piece of rubber balloon across one end and fix it with a rubber band. Put a few grains of say ragi or bajra on the stretched rubber. Now ask your friend to shout ‘Hoorrey ……… Hoorrey ………… ‘ from the open end of the glass. Observe the dancing of the grains up and down. Discuss the reason behind this.

Class 8 Science Chapter 15 Sound Important Questions and Answers

Select the correct option and write the completed statements:

Question 1.
Sound waves cannot travel through a ………… .
(a) solid
(b) liquid
(c) gas
(d) vacuum
Answer:
Sound waves cannot travel through a vacuum.

Question 2.
If the frequency of a sound wave is 512 Hz, the number of rarefactions produced per second is ……………… .
(a) 256
(b) 512
(c) 1024
(d) 128
Answer:
If the frequency of a sound wave is 512 Hz, the number of rarefactions produced per second is 512.

Question 3.
In the experiment to show that a medium is necessary for propagation of sound, as the quantity of air inside the bell jar decreases, the level of ringing sound heard outside ……………… .
(a) increases
(b) decreases
(c) fluctuates randomly
(d) changes at regular intervals
Answer:
In the experiment to show that a medium is necessary for propagation of , sound, as the quantity of air inside the bell jar decreases, the level of ringing sound heard outside decreases.

Find the odd one out and give the reason:

Question 1.
Sound waves, Waves on the surface of water, Waves on a stretched string, Light waves
Answer:
Light waves. Light waves can travel in a vacuum. Other waves need a medium for propagation.

Question 2.
Sitar, Violin, Guitar, Flute.
Answer:
Flute. It is not a string based instrument. Others are string based instruments.

State whether the following statements are True or False. (If a statement is false, correct it and rewrite it.)

Question 1.
If the vibrating length of a stretched string is kept constant and the tension in the string is increased, the frequency of the sound produced increases.
Answer:
True.

Question 2.
In Jaltarang, the frequency of sound produced is independent of the height of the air column inside the glass cup.
Answer:
False. (In Jaltarang, the frequency of sound produced depends on the height of the air column inside the glass cup; the more the height of the air column, the less is the frequency of the sound produced.)

Answer the following questions in one sentence each:

Question 1.
What is sound?
Answer:
Sound is a form of energy which produces sensation of hearing in our ears.

Question 2.
What is vibration?
Answer:
Vibration is a rapid, to and from periodic motion of an object about its mean position.

Question 3.
What is compression?
Answer:
As a sound wave travels through air, the region in which air is at high pressure and high density is called compression.

Question 4.
What is rarefaction?
Answer:
As a sound wave travels through air, the region in which air is at low pressure and low density is called rarefaction.

Question 5.
What is the relation between the frequency of vibration and the vibrating length (or height) of the air column?
Answer:
The greater the vibrating length (or height) of the air column, the less is the frequency of vibration of the column.

Question 6.
For a particular string (given material and radius), what are the factors on which its frequency of vibration depends?
Answer:
For a particular string (given material and radius), the frequency of vibration of the string depends upon its vibrating length and the tension in the string.
[Note: in the above case, frequency is proportional to the square root of the tension and inversely proportional to the length.]

Question 7.
Name an instrument in which a stretched skin is used to produce musical sound.
Answer:
Tabla.

Answer the following questions:

Question 1.
Explain the production and propagation of sound in air.
Answer:
Let us consider production of sound by using a tuning fork. A tuning fork is made of a metal. It has a stem and two prongs (Fig). Figure (a) shows the state of air in the absence of propagation sound through it. Equally spaced vertical lines show that the average distance between the air molecules is the same everywhere. It means average pressure and average density in the regions A, B and C are the same.

Suppose the tuning fork is held vertical, its stem is fixed in a stand (not shown in the figure) and its prongs are struck lightly using a light hammer with a piece of rubber at the top. The prongs then start vibrating.
Figure (b) shows what happens when the prongs move away from each other. The air outside the prongs is compressed (layers in the region A). The pressure and density increase in this region.

Figure (c) shows what happens when the prongs move close to each other. The air molecules near the prongs move away from each other resulting in lower pressure and lower density.

This region is called rarefaction. Meanwhile, the compression produced earlier moves forward, i.e., away from the prongs because the air molecules in this region transfer their energy to the air molecules in the region B producing a compression there. The periodic formation of compression and rarefaction results in propagation of sound waves away from the prongs.

Eventually, these sound waves reach our ears, the ear-drum vibrates, and we get a sense of hearing a sound as the specific signals reach the brain.

Use your brain power:

Question a.
If sound waves are generated in air, what moves away from the source? Is it the air itself or the state of compression and rarefaction created in the air?
Answer:
If sound waves are generated in air, the state of compression and rarefaction created in the air moves away from the source.
[Note: The air does not move away from the source because air is elastic and the force due to the tuning fork varies periodically with time.]

Question 2.
Explain the term frequency of a sound wave.
Answer:
Sound travels in the form of waves. A wave consists of cycles of alternate compression and rarefaction. The number of cycles produced in the air (or any other medium) per second is called the frequency of the sound wave. It is expressed in hertz (Hz). 1 hertz = 1 cycle per second or 1 vibration per second or 1 oscillation per second.

[Note: The SI unit of frequency, the hertz, is named in honour of Heinrich Hertz (1857-94), German physicist.]

Question 3.
State the factors on which the frequency of a tuning fork depends.
Answer:
The frequency of a tuning fork depends on the dimensions of the prongs, i.e., length and thickness, and the material used for making the fork.

Try this:

Question a.
Take 6 – 7 glass cups. Arrange them in a line and fill them with water with gradually increasing water level from one end to other. Take a pencil and strike the cups sequentially. The sound generated by each cup will be different. Why is it so?
Use of ICT:
Download videos of Jaltarang from youtube and send them to your friends by email.

Answer:
When a cup is struck, waves are set up in the air column above the water level in the cup. The frequency of the generated wave depends on the height of the air column inside the glass cup. Since the water level in each glass is different, the height of the air column in each glass is also different. Therefore, the frequency of sound generated by each glass cup will also be different. So, the sound generated is different.

Question b.
An ‘app’ for measurement of sound frequency may be available on cellphones. With the help of your teacher, use the app to measure the frequency of the sound generated from each glass cup. Do you observe any relation between the frequency of generated sound and the height of the air column in the glass cup?
Answer:
Yes. The greater the height of the air column in the glass cup, the less is the frequency of the sound produced.

Question c.
This is your simple ‘Jaltarang’! Can this experiment be performed with stainless steel pots of different size?
Answer:
Do it. See what happens as the stainless steel pot itself starts vibrating producing a sound. Also see how this sound is damped by water.

Do you know?

Question a.
What are the frequencies of musical notes ‘sa’, ’re’, ‘ga’, ‘ma’, ‘pa’, ‘dha’, ‘ni’ in the ‘madhya saptak’?
Answer:

[Note: The diatonic scale:


Such a simple fraction is pleasing to the ear and is called a musical interval. Frequency ratios 2 : 1, 3 : 2, 4 : 3 are examples of musical interval.

Observe and discuss:

Question 1.
Identify the instruments and discuss sound production in them. [Students should see these musical instruments.]

Answer:

1. Sitar
2. Shehnai
3. Flute
4. Tabla
5. Santoor
6. Bugle
7. Mandoline or Mandolin
8. Saxophone.

Question i.
Sitar, Santoor and Mandoline or mandoline (String instruments):
Answer:
Vibrating strings produce the sound. The greater the tension in the string, the higher is the frequency of the sound. The greater the vibrating length and the diameter of the string, the lower is the frequency of the sound.

Question ii.
Flute, Saxophone and Shehnai (Wind instruments):
Answer:
Vibrating air column produces the sound. The greater the length of the column, the lower is the frequency of the sound.

Question iii.
Tabla (Percussion instrument):
Answer:
Vibrating skin produces the sound. The greater the tension in the skin, the higher is the frequency of the sound. The greater the thickness of the skin, the lower is the frequency of the sound.

Try this:
‘Apps’ for generation of different sound notes (sound note generator app) may be available on cellphones. With the help of your teacher, using such an app, generate sound notes listed in the table.

Vocal cords are 20 cm in length in male, about 15 cm in female and even smaller in children. Therefore the voice of male, female and children are different.
[Note: Vocal cords are 20 mm in length in male, about 15 mm in female and even smaller in children. Reference NCERT textbook Std. VIII, published in 2017.]

Question 1.
Produce a sound ‘bho … bho …’ just like a dog-barking and ‘meow … meow…’ just like a mewing cat. Carefully notice the tension on the vocal cords, when you produce these sounds. Do you feel that the tension on the vocal cords changes when you produce these two different sounds?
Answer:
Yes.

Question 2.
An ‘app’ may be available on cell-phones to measure the loudness of sound in decibel. With the help of your teacher, use the app to measure the sound level of a sound from a loudspeaker at some public place. Measure the sound level at different distances from the loudspeaker. Do you observe some relation between the distance from the loudspeaker and the sound level?
Answer:
Yes. As the distance increases, the sound level decreases.

Always remember:

We should take care that others are not disturbed when we study sound and its production. Sound pollution is a major cause affecting the environment and social health. Hence we should find ways to avoid sound pollution.

8th Std Science Questions And Answers:

• Human Body and Organ System Class 8 Questions And Answers
• Introduction to Acid and Base Class 8 Questions And Answers
• Chemical Change and Chemical Bond Class 8 Questions And Answers
• Measurement and Effects of Heat Class 8 Questions And Answers
• Sound Class 8 Questions And Answers
• Reflection of Light Class 8 Questions And Answers
• Man-made Materials Class 8 Questions And Answers
• Ecosystems Class 8 Questions And Answers
• Life Cycle of Stars Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 9 Disaster Management Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 9 Disaster Management Question Answer Maharashtra Board

Class 8 Science Chapter 9 Disaster Management Question Answer Maharashtra Board

1. Answer the following in your own words.

Question a.
Explain the relation between continuous rains and landslide. Give reasons.
Answer:
Before the actual landslide, many changes occur in the surrounding hilly ground. The hard rocks develop cracks and crevices. These are natural changes but due to man-made activities, the cracks are widened due to erosion. These cracks make the big rocks to break into smaller stones. The cracks widen further due to excessive rainfall. These rocks further get eroded and they fall down along with soil from the slopes. The entire process is speeded up due to rainwater.

Question b.
Prepare a chart showing ‘Dos’ and ‘don’ts’ at the time of earthquake.
Answer:
At the time of earthquake

Dos	Don’ts
1. Go in the open grounds.	1. Don’t wait in the multi-storeyed buildings. Do not use lifts. Use stairs to descend.
2. Keep the electrical appliances and cooking gas closed.	2. Do not stand near buildings, trees or electric wires and poles.
3. Use battery or torch.	3. Do not light candles, lantern or match- sticks.
4. Stand silently at one place. Do not panic or get scared.	4. Do not sit in uncomfortable posture for a long time.
5. Take protection under some hard covering, especially to protect the head and face. Cover the face.
6. If, in vehicle, find a safe place and stop the vehicle, sit in the vehicle than coming out in open.

Question c.
What are the specifications of an earthquake-proof building?
Answer:
The earthquake-proof building is such that even if the earthquake is caused, it should not cause damage to the buildings due to these earth movements. There are some codes of conduct while constructing such buildings. Advanced technology is used for earthquake resistant construction. The foundation of earthquake-proof building is separated from lower land. The walls are of less weight or they are wooden. The house constructed with special light materials are preferred in earthquake-prone regions.

Question d.
Explain the effects of landslide.
Answer:
1. Mostly rivers originate in the hills and mountains. When there is landslide, the rivers naturally get flooded.
2. The paths of the riverine water changes due to landslide.
3. Waterfalls are displaced from their original positions.
4. New and artificial water reservoirs are created.
5. The falling debris, soil and rocks make the trees at the base of hillside uprooted. All the plant life is lost.
6. The constructions done by the villagers on the slope can be totally damaged due to landslide. There is large scale damage to property and life. In few cases like Malin the entire village was buried due to landslide and accompanied rainfall.
7. Transportation is affected as the roads and railway tracks are blocked due to debris.

Question e.
Is there any relation between dam and earthquake? Explain.
Answer:
There is abundant water stored in the dams. This water column puts additional weight on the ground. Initially there may not be any weight, but later due to construction of dam, suddenly the pressure of this weight is so high that this ground experiences the tension. If such area is already earthquake-prone, then there can be chances of earthquake. According to theory of plate tectonics, there are continuous movements in the earth’s surface. If over such fragile plates, the dams are constructed then the chances of earthquake are enhanced.

2. Give Scientific reasons.

Question a.
It is safer to find shelter under things like a bed, table at the time of earthquake.
Answer:
When earthquake takes place, due to the vibrations in the earth’s surface, there is possibility of the roof and walls of the house to fall. This collapse can cause severe head injury which can be fatal. Thus, one must take shelter below the hard-supporting structures such as bed or table. This precaution can save one’s life.

Question b.
In monsoon, don’t take shelter near hillside.
Answer:
The excessive rainfall can cause landslides. The soil and rocks from the hillside can be pushed down along with the flow of rainwater. This debris slides to the lower heights from the hills. This explains us that taking shelter near the base of the hillside can be disastrous as one can be buried in the debris due to sudden landslide. Therefore, in monsoon, one should not take shelter near hillside.

Question c.
Don’t use lifts at the time of earthquake.
Answer:
Lifts or elevators run on electricity. The electricity supply can be hindered due to earthquake. There may be chances of fire due to short circuit. We may get trapped in the elevator at such times. The building may also collapse due to earthquake. It is always better to use stairs at the time of such calamities, so that one can safely come out of the building. Therefore, it is said that lifts should not be used at the time of earthquakes.

Question d.
The foundation of earthquake- proof building is separated from lower land.
Answer:
The surface of the earth trembles at the time of earthquake. These tremors cause seismic waves which are responsible for the movements of the earth’s surface. The ground thus shakes or it goes up-down. These shocks and waves formed in the interior of the earth spread on the surface in all directions. This causes collapse of the building and other structures on the land.
To prevent such disasters the foundation of earth-quake proof building is separated from lower land.

3. If a crowd gathers at the place of earthquake, what would be the difficulties in relief work?

Question a.
If a crowd gathers at the place of earthquake, what would be the difficulties in relief work?
Answer:
If people flock in crowd, the rescue work will not be possible. The ambulances and the fire engines will not reach the spot where the help is needed. The personnel from disaster management cells cannot thus act in time. It will be difficult to manage the situation and thus such crowding should never be done.

4. Make a list of the institutes and organizations who provide help for disaster managment. Collect more information about their work.

Question a.
Make a list of the institutes and organizations who provide help for disaster management. Collect more information about their work.
Answer:
International government organisations working for disaster management are:
a. The United Nations and its organisations:
1. The Food and Agricultural Organisation of the UN (FAO): It gives early warning of impending food crises, and keep track of global food supply problems.
2. The United Nations Development Programme (UNDP): It helps disaster-prone countries with disaster mitigation, prevention and preparedness measures.
3. The World Food Programme (WFP): It is the main supplier of relief food aid.
4. The World Health Organisation (WHO): It gives global public health leadership by setting standards, monitoring health trends, and providing direction on emergency health issues. WHO’s role is to reduce avoidable loss of life and the burden of disease and disability.

b. The International Committee of the Red Cross ( ICRC): It gives physical rehabilitation to people injured by explosive weapons or other types of incident.

c. The International Federation of Red Cross and Red Crescent Societies ( IFRC): It coordinates and gives international help to victims of natural and technological disasters, to refugees and in health emergencies.

International non-governmental agencies working for disaster management are:
a. International Rescue Committee (IRC): It provides lifesaving care and life-changing help to refugees forced to flee from war or disaster.

b. IMA World Health: It, in collaboration with USAID, the World Bank and many other organisations, builds sustainable health care systems.

c. CARE: It is an organisation fighting global poverty. It works for women and puts efforts to improve their basic education, prevent the spread of HIV by providing awareness among them, give them increase access to clean water and sanitation, expand economic opportunity for them. It provides emergency aid to survivors of war and natural disasters, and helps people rebuild their lives.

Indian organisations and institutes working for disaster management are:
a. National Disaster Response Force (NDRF): The multi-disciplinary, multi-skilled, high-tech force of the NDMA are capable of dealing with all types of natural and man-made disasters.

b. National Disaster Management Authority (NDMA): It lay down the policies, plans and guidelines for Disaster Management to ensure timely and effective response to disasters.

c. National Institute of Disaster Management (NIDM): It has been given the responsibilities for human resource development, capacity building, training, research, documentation and policy advocacy in the field of disaster management.

5. Make a survey of your school according to the plan of disaster management an write the pointwise information.

Question a.
Make a survey of your school according to the plan of disaster managment an write the pointwise information.
Answer:

This an activity based questions. Kindly do it yourself. But make sure to involve the following points in your survey:

• Primary information of the school: It should include information such as name and address of the school and Head Master, total number of school staff and name and phone numbers of school management members.
• School disaster management committee: Get the information of the members involved in disaster management committee.
• Detailed information about school building: Note the number of rooms, classroom, age of the building, types of roofs under this point.
• Information about school ground: It should include information like distance of ground from the main road, types of play grounds.
• Daily routine of the school: It should include information like working time of the school, lunch break time for the school.
• Possible hazards in the school: It should information such as record of past disaster happened in school, current planning for overcoming disasters.
• Disaster management map of the school: It should have information regarding all the buildings of the school, entrances and exit gates, place of probable danger, safer place at the time of disaster.

6. Are there any possible places of landslide in your area? Collect information from experts.

Question a.
Are there any possible places of landslide in your area? Collect information from experts.
Answer:
This is an activity based question. kindly do it yourself.

7. With the help of the following pictures, explain your role in disaster management.

Question a.
With the help of following pictures, explain your role in the disaster management.
Answer:

The picture given is not very clear. It does not indicate the condition of disaster. Therefore, there are two alternatives for this answer.
Option 1 : The boy holds a paper on which is written, ‘I am at risk’! The boy is shocked to read this. Someone has given message to him. He must search that person and help him or her. He should therefore take help from some elder or from police force in such matter.

Option 2 : The boy shown in the picture is in danger. So, he is typing in his I – pad, “I am at Risk”. After receiving this message, someone will help him. It depends upon, to whom is he sending this message. If he sends this message to us, we can immediately try to help him. His message “I am at Risk”, shows that he is in danger and has to be saved.
It is not clear about which disaster is shown in the picture.

Project:

Question 1.
Make a collection of news, photos. and cuttings about landslides and rift collapse.

Question 2.
With the help of Internet, collect information about the latest gadgets and technology to forecast earthquake.

Question 3.
Collect information about NDRF, RPF, CRPF, NCC from Internet.

Question 4.
Discuss- Need of CCTV.

Class 8 Science Chapter 9 Disaster Management Important Questions and Answers

Rewrite the sentences after filling the blanks:

Question 1.
Earthquakes cause ……………. waves leading to movements of the earth’s surface.
Answer:
Earthquakes cause seismic waves leading to movements of the earth’s surface.

Question 2.
The central point of earthquake is the point above the ……………. on the earth’s surface.
Answer:
The central point of earthquake is the point above the epicentre on the earth’s surface.

Question 3.
The accentuation of earthquake is measured in …………… .
Answer:
The accentuation of earthquake is measured in ‘Richter Scale’.

Question 4.
If there is earthquake at the bottom of ocean, it may create ……………… waves.
Answer:
If there is earthquake at the bottom of ocean, it may create tsunami waves.

Question 5.
…………….. is the best device to put off small fires.
Answer:
Stirrup pump is the best device to put off small fires.

Question 6.
………………. of waterfalls occurs due to landslides.
Answer:
Displacement of waterfalls occurs due to landslides.

State whether the following statements are True or False:

Question 1.
Every year nearly 2,400 to 4,000 earthquakes occur on the earth.
Answer:
False. (Every year nearly 12,400 to 14,000 earthquakes occur on the earth.)

Question 2.
Potassium, sodium and calcium are the metals that react with water at high room temperature.
Answer:
False. (Potassium, sodium and calcium are the metals that react with water at normal room temperature.)

Question 3.
A fire caused due to electrical components is extinguished by fire extinguishers like carbon dioxide are used.
Answer:
True.

Question 4.
Indiscriminate cutting of the trees results in improvement of soil quality.
Answer:
False. (Indiscriminate cutting of the trees results in soil erosion.)

Question 5.
Landslide results in loss of plant life.
Answer:
True.

Match the column:

Question 1.

Column ‘A’	Column ‘B’
1. Class A fire	a. Electrical components
2. Class B fire	b. Gaseous substances
3. Class C fire	c. Chemical substances
4. Class D fire	d. Liquid substances
5. Class E fire	e. Solid substances

Answer:

Column ‘A’	Column ‘B’
1. Class A fire	e. Solid substances
2. Class B fire	d. Liquid substances
3. Class C fire	b. Gaseous substances
4. Class D fire	c. Chemical substances
5. Class E fire	a. Electrical components

Question 2.

Column ‘A’	Column ‘B’
1. Earthquake	a. Formation of artificial water reservoir
2. Tsunami	b. Wildlife lost
3. Forest fire	c. Loss of coastal regions
4. Landslide	d. Change in the level of groundwater-table

Answer:

Column ‘A’	Column ‘B’
1. Earthquake	d. Change in the level of groundwater-table
2. Tsunami	c. Loss of coastal regions
3. Forest fire	b. Wildlife lost
4. Landslide	a. Formation of artificial water reservoir

Name the following:

Question 1.
Modern equipment to get prior intimation about earthquake:
Answer:

1. Laser ranging
2. Very long baseline
3. Geiger counter
4. Creep meter
5. Strain meter
6. Tide gauge
7. Tilt meter
8. Volumetric strain gauge.

Question 2.
Some code of conduct made by Indian Standard Institute for construction of buildings:
Answer:

1. IS 456
2. IS 1893
3. IS 13920

Question 3.
The subcommittees of School Disaster Management.
Answer:

1. Fire extinguisher
2. Awareness
3. Instructions
4. Traffic management
5. Safety
6. Communication committee.

Answer the following questions in one sentence:

Question 1.
Which is the instrument or machine that records the intensity of earthquake?
Answer:
The machine/Instrument which records the earthquakes is called ‘Seismograph’ or ‘Seismometer’.

Question 2.
Which is the most common and effective solution for extinguishing fire ?
Answer:
Spraying water is the most common and effective solution for extinguishing fire.:

Question 3.
Which type of fires can be extinguished by the method of suppressing?
Answer:
To extinguish the fires caused due to electricity or oil, the method of suppressing the fire can be used.

Question 4.
Which metals react with water at normal room temperature?
Answer:
Combustible metals like potassium, sodium and calcium, react with water at normal room temperature.

Question 5.
Which metals react with water at higher temperatures?
Answer:
Magnesium, aluminum and zinc react with water at high temperatures.

Question 6.
Which institutes have launched a program to forecast the landslides and its effects?
Answer:
The Government of India in collaboration with Indian Mountaineering Institute and International Centre for Integrated Mountain Development has launched a program to forecast the landslides and its effects.

Give scientific reasons:

Question 1.
The electric mains are shut off immediately after the earthquake.
Answer:
There is increased possibility of short circuit after the earthquake. This may result in fire. The earthquake has already – brought the disaster. To prevent further destruction, the electric mains are shut off immediately after the earthquake.

Write short notes:

Question 1.
Types of fire:
Answer:
There are five types of fire. This division is based on two criteria, viz.
(i) Which substance is being burnt, (ii) What is the method of extinguishing it.
1. Class A Fire: Commonly inflammable solid things such as wood, clothes, coal, papers can be burnt by this type of fire. This fire is extinguished by spraying water over it. This is also called cooling out. Water is effectively used to put off class A fire.

2. Class B Fire: Flammable liquid substances such as petrol, oil, varnish, solvents, cooking oil, paints, etc. catch fire and it is called class B fire. Since these substances are lighter than water, they can be extinguished only by foaming fire extinguishers.

3. Class C Fire: The fire caused due to gaseous substances is called class C fire. Domestic gas (L.P.G.) and acetylene can cause such kind of fire.

4. Class D Fire: Combustible metals catch class D fire. Metals such as potassium, sodium and calcium, can react with water at normal room temperature whereas magnesium, aluminium and zinc react with water at high temperature. When both these groups combine with water, it causes explosion.

5. Class E Fire: When electric components are subjected to fire, they form class C fire. Such fires can be caused by short circuit or due to problems in electric fittings. Such fire is extinguished with the help of carbon dioxide and non-conductive fire extinguishers.

Question 2.
Disaster relief – planning.
Answer:
Anyone can face disaster at any time. The only way to tackle with such disasters is to keep preparedness to deal with any calamity. Schools, colleges and various offices need to chalk out a detailed planning in case of possible disasters. With this purpose in mind, disaster relief planning is done.

E.g. In the disaster relief planning for school, primary information of the school, the structure of School Disaster Management Committee, detailed information about school building and information about school ground, daily routine of the school such as at what time the school starts and what time does it end, the possible hazards in the school, and disaster management map of the school is included.

Answer the following questions:

Question 1.
What are the different ways to extinguish fire? Write briefly about them.
Answer:
There are three main methods to extinguish the fire. These methods are used to stop the spread of fire and to avoid the financial and other losses.
1. Cooling out by use of water: Water is easily available and can be used for putting off fire instantly. Due to spraying of water, there is cooling effect produced which helps in reducing the loss by fire. Fire can be easily controlled by water.

2. Suppressing the fire by covering it: When there is a fire due to electricity or oil it has to be controlled by covering the fire by sand or soil. When the froth is spread on the fire, there is no contact between air and fire. This puts off the fire and stops the spread of fire caused due to oil.

3. Keep away Flammable Substances: Care is taken to keep away all flammable substances from the fire. Wooden articles and inflammable substances are kept in such as way the fire will not engulf it. Stirrup pumps are used to put off small fires by which the water is spread in all directions around the fire.

Question 2
What are the safety measures and precautions to stop the fire?
Answer:

1. Switching off the regulator of cooking gas cylinder when not in use. Put off the connections of all electrical appliances when not in use.
2. If there is fire, call others immediately for help. Take help of others by calling them. Also help others who are in need to save their lives from fire.
3. Get help from fire brigade by calling phone number 101.
4. Know details about working of the fire extinguishing apparatus.
5. Give first aid to the victim of fire. Seek immediate medical help,

Question 3.
What are the causes of landslide?
Answer:

1. Various types of natural disasters like earthquakes, tsunami, heavy rains, storms, floods can result into landslide.
2. The indiscriminate cutting down of the trees can result into soil erosion. Soil erosion in turn results into landslide.
3. The construction of roads, bridges, railway tracks, etc. on the mountain slopes result into lot of digging. Such activities make soil cover loose. From the slopes the soil and rocks then can slide easily. This results into landslides.

Draw a well labelled diagrams:

Question 1
Seismometer/Secismograph.
Answer:

Question 2.
Focal point and epicenter or earthquake.
Answer:

Open-Ended Questions:

Question 1.
Have you at any time faced with any disaster? What were your experiences during such condition? How did you get rescued?
Answer:
Students should write their own experiences about such incidents if any.

Question 2.
In Maharashtra where do the traffic jams occur due to landslides? Make a list of such places. Why does the landslide occur at those places only? Discuss in the classroom and suggest preventive measures.
Answer:
In Maharashtra, the landslides are very common in the hilly regions. The traffic is suspended often during rainy season when such calamity strikes. Malshej, Khandala, Kasara are some of the Ghats which are very prone to land-slide. On the tracks of Konkan railway, the landslides are very common during heavy downpour.

The regions where more landslides occur have many constructions and large scale deforestation. The traffic of Konkan railway comes to standstill after such landslide. Moreover, during heavy monsoon days, the speed of the running trains is kept very slow. The soil structure is also responsible for such landslides.

Preventive measures: There should be wire meshing done across the sides of the roads and railway tracks so that the collapsed rocks and other debris does not obstruct the traffic. Tree plantation should be carried out to prevent loosening of the soil and rock. It will also help in preventing the soil erosion.

8th Std Science Questions And Answers:

• Living World and Classification of Microbes Class 8 Questions And Answers
• Health and Diseases Class 8 Questions And Answers
• Force and Pressure Class 8 Questions And Answers
• Current Electricity and Magnetism Class 8 Questions And Answers
• Inside the Atom Class 8 Questions And Answers
• Composition of Matter Class 8 Questions And Answers
• Metals and Nonmetals Class 8 Questions And Answers
• Pollution Class 8 Questions And Answers
• Disaster Management Class 8 Questions And Answers
• Cell and Cell Organelles Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 5 Inside the Atom Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 5 Inside the Atom Question Answer Maharashtra Board

Class 8 Science Chapter 5 Inside the Atom Question Answer Maharashtra Board

1. Answer the following.

Question a.
What is the difference in the atomic models of Thomson and Rutherford?
Answer:

Thomson’s atomic model	Rutherford’s atomic model
1. According to Thomson’s atomic model, the negatively charged electrons are embedded in a gel of positive charge.	1. According to Rutherford’s atomic model the negatively charged electrons revolve around the nucleus.
2. Atom is homogenous sphere of positive charge.	2. The positive charge is in the nucleus of the atom.

Question b.
What is meant by valency of an element? What is the relationship between the number of valence electron and valency?
Answer:
Valency: The capacity of an ; element to combine with another element is known as valency.
Valence electrons: The electrons in the outermost shell of an atom of an element are called valence electrons.
Helium and neon, atoms of both these gaseous element do not combine with any other atom. These elements are chemically inert, i.e. their valency is zero.

Helium atom contains two electrons, indicates that the outermost shell of helium has an electron duplet. The valence shell of neon is completely filled, i.e. neon has an electron octet. Similarly argon contains eight electrons in the valence shell, i.e. argon has an electron octet. It is confirmed that the valency is zero when electron octet (or duplet) is complete.

Atoms of all the elements except inert gases have tendency to combine with other atoms, i.e. they have a non zero valency. The molecules formed by combination with hydrogen (E.g. H₂, HCl) that valency of hydrogen is one. The electronic configuration of hydrogen shows that there is one electron less than the complete duplet state. This number ‘one’ matches with the valency of hydrogen which is also one.

It means that there is relationship between the valency of an element and the number of electrons in its valence shell.

Question c.
What is meant by atomic mass i number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.
Answer:
The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number. The atomic number, i.e. the proton number of carbon is 6 and the mass number is total number of protons and neutrons in the carbon, i.e. 6 protons + 6 neutrons = 12. Therefore, the atomic number and mass number of carbon are 6 and 12 respectively.

Question d.
What is meant by subatomic particle? Give brief information of three subatomic particles with reference to electrical charge, mass and location.
Answer:
A particle which is a constituent of an atom hence smaller than the atom is called subatomic particle.
An atom is formed from the nucleus and the extranuclear part. These contain three types of subatomic particles.
The nucleus contains two types of subatomic particles together called nucleons. Protons and neutrons are the two types of nucleons or subatomic particles and electrons are subatomic particles in the extra nuclear part.

1. Proton (p): Proton is a positively charged subatomic particle in the atomic nucleus. The positive charge on the nucleus is due to the proton in it. A proton is represented by the symbol ‘p’. Each proton carries a positive charge of +1e. (1e = 1.6 × 10^-19 coulomb). When total positive charge on the nucleus is expressed in the unit ‘e’, its magnitude is equal to the number of proton in the nucleus.
The mass of one proton is approximately lu (1 Dalton).
(1u = 1.66 × 10^-27g) (The mass of one hydrogen atom is also approximately lu.)

2. Neutron (n): Neutron is an electrically neutral subatomic particle and is denoted by the symbol ‘n’. The number of neutron in the nucleus is denoted by the symbol ‘N’ Atomic nuclei of all the elements except hydrogen with atomic mass lu, contain neutrons. The mass of a neutron is approximately lu, which is almost equal to that of a proton.

3. Electron (e^–): Electron is a negatively charged subatomic particle and is denoted by the symbol ‘e-’. Each electron carries one unit of negative charge (-1e). Mass of an electron is 1800 times less than that of a hydrogen atom. Therefore the mass of an electron can be treated as negligible. Electron in the extranuclear part revolve in the discrete orbits around the nucleus. The energy of an electron is determined by the shell in which it is present.

2. Give scientific reasons:

Question a.
All the mass of an atom is concentrated in the nucleus.
Answer:

1. The nucleus of an atom contains protons and neutrons.
2. The electrons revolve around the nucleus.
3. The mass of an electron is negligible compared to that of a proton or a neutron.
4. Hence, the mass of an atom depends mainly on the number of protons and neutrons. Therefore, practically all the mass of an atom is concentrated in the nucleus.

Question b.
Atom is electrically neutral.
Answer:

1. An atom is made of two parts, viz. the nucleus and the extranuclear part.
2. The nucleus is positively charged. The positive charge on the nucleus is due to protons.
3. The extranuclear part of an atom is made of negatively charged electrons.
4. In an atom, the number of protons is equal to the number of extranuclear electrons.
5. The magnitude of the positive charge on the nucleus equals the magnitude of the negative charge on the electrons. As the opposite charges are balanced, the atom is electrically neutral.

Question c.
Atomic mass number is a whole number.
Answer:

1. The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number (A).
2. As protons and neutrons are whole numbers, the atomic mass number is also a whole number.

Question d.
Atoms are stable though negatively charged electron are revolving within it.
Answer:

1. The entire mass of the atom is concentrated in the nucleus and the positively charged nucleus at centre of an atom.
2. The negatively charged electrons revolve around the nucleus.
3. The total negative charge on all the electron is equal to positive charge on the nucleus. As the opposite charges are balanced, the atom is stable.

3. Define the following terms.

Question a.
Atom:
Answer:
An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes.

Question b.
Isotope
Answer:
Atoms of the same element having the same atomic number, but different atomic mass numbers are called isotopes.

Question c.
Atomic number
Answer:
The number of electrons or protons in an atom is called the atomic number. It is denoted by Z.

Question d.
Atomic mass number
Answer:
The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number. It is denoted by A.

Question e.
Moderator in nuclear reactor
Answer:
The substance which reduces the speed of fast-moving neutrons produced in a fission is called a moderator.

4. Draw a neal labelled diagram.

Question a.
Explain Rutherford’s scattering experiment.
Answer:

Alpha particles emitted by radioactive element bear a positive charge. Rutherford bombarded alpha particles through a very thin gold foil. He observed the path of α – particles by means of a fluorescent screen around the gold foil. It was expected that

1. Most of the alpha particles passed through the gold foil without any deviation.
2. Some alpha particles were deflected from their path through small angles.
3. A few alpha particles were scattered at large angles.
4. A still smaller number of same sign particles get deflected through a larger angle and one a-particle out of 20000 bounced back in the direction opposite to the original path.

Question b.
Thomson’s atomic model.
Answer:
Thomson’s plum pudding model of atom:

1. According to Thomson’s model the positive charge is distributed throughout the atom and the negatively charged electron: are embedded in a gel of positive charge (a plum pudding model).
2. The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral.

Question c.
Diagrammatic sketch of electronic configuration of magnesium (Atomic number 12).
Answer:

Question d.
Diagrammatic sketch of electronic configuration of argon (Atomic number 18).
Answer:

5. Fill in the blanks.

Question 1.
Electron, proton, neutron are the types of ………… in an atom.
Answer:
Electron, proton, neutron are the types of subatomic particles in an atom.

Question 2.
An electron carries a ……………. charge.
Answer:
An electron carries a negative charge.

Question 3.
The electron shell ………….. is nearest to the nucleus.
Answer:
The electron shell K is nearest to the nucleus.

Question 4.
The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is …………….. .
Answer:
The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is M.

Question 5.
The valency of hydrogen is one as per the molecular formula H₂O. Therefore valency of ‘Fe’ turns out to be ………….. as per the formula Fe₂O₃.
Answer:
The valency of hydrogen is ‘one’ as per the molecular formula H₂O. Therefore valency of ‘Fe’ turns out to be 3 as per the formula Fe₂O₃.

6. Match the pairs.

Question 1.

Column I	Column II
1. Proton	a. Negatively charged
2. Electron	b. Neutral
3.  Neutron	c. Positively charged

Answer:

Column I	Column II
1. Proton	c. Positively charged
2. Electron	a. Negatively charged
3. Neutron	b. Neutral

7. Deduce from the datum provided.

Question 1.

Answer:
1. There are 12 neutrons in the sodium \(\left({ }_{11}^{23} \mathrm{Na}\right)\).
(N = A – Z) 23 – 11 = 12
2. Atomic mass number of \({ }_{6}^{14} \mathrm{C}\) is 14.
3. There are 17 protons in chlorine \(\left({ }_{37}^{17} \mathrm{Cl}\right)\)

Project:

Explain the atomic models using the material such as old C.D., balloon, thread, marbles, etc.

Class 8 Science Chapter 5 Inside the Atom Additional Important Questions and Answers

Rewrite the sentences after filling the blanks:

Question 1.
An atom is electrically ……………. .
Answer:
An atom is electrically neutral.

Question 2.
Except hydrogen, the nuclei of all atoms contain ………….. .
Answer:
Except hydrogen, the nuclei of all atoms contain neutrons.

Question 3.
\({ }_{6}^{12} \mathrm{C}\), …………… and …………. are isotopes of carbon.
Answer:
\({ }_{6}^{12} \mathrm{C}\), \({ }_{6}^{13} \mathrm{C}\) and \({ }_{6}^{14} \mathrm{C}\) are isotopes of carbon.

Question 4.
An atom has 11 protons and …………. neutrons and hence its atomic mass number is 23.
Answer:
An atom has 11 protons and 12 neutrons and hence its atomic mass number is 23.

Question 5.
The element ………….. has two electrons in the K shell, but it is a noble gas.
Answer:
The element helium has two electrons in the K shell, but it is a noble gas.

Question 6.
Isotopes of the element have the same ……….. properties.
Answer:
Isotopes of the element have the same chemical properties.

Question 7.
Electrons must absorb …………….. to transit between orbits.
Answer:
Electrons must absorb energy to transit between orbits.

Question 8.
……….. discovered the electron.
Answer:
J. J. Thomson discovered the electron.

Question 9.
∝ – particles have ………….. charge.
Answer:
∝ – particles have positive charge.

Question 10.
Electrons revolve around the …………….. in certain discrete orbits.
Answer:
Electrons revolve around the nucleus in certain discrete orbits.

Question 11.
The shell-wise distribution of electrons is called the ………………… .
Answer:
The shell-wise distribution of electrons is called the electronic configuration.

Question 12.
Democritus termed the smallest particles of matter as …………… .
Answer:
Democritus termed the smallest particles of matter as atoms.

Question 13.
………………. discovered neutron.
Answer:
James Chadwick discovered neutron.

Question 14.
Electrons revolve around the nucleus in paths called ………….. .
Answer:
Electrons revolve around the nucleus in paths called orbits.

Question 15.
The second shell has the capacity of ……………… .
Answer:
The second shell has the capacity of 8 electrons.

Question 16.
Electrons in the ………….. shell have minimum energy.
Answer:
Electrons in the K or first shell have minimum energy.

Question 17.
Electrons in an atom occupy shells in increasing order of …………… .
Answer:
Electrons in an atom occupy shells in increasing order of energy.

Question 18.
Atomic masses are measured in a unit called …………… .
Answer:
Atomic masses are measured in a unit called dalton.

Question 19.
According to the atomic model of …………….., electrons are embedded in a gel of positive charge.
Answer:
According to the atomic model of Thomson, electrons are embedded in a gel of positive charge.

Question 20.
All atoms, except ………….. contain neutrons in their nuclei.
Answer:
All atoms, except normal hydrogen contain neutrons in their nuclei.

Question 21.
The maximum capacity of the M shell is ………………… electrons.
Answer:
The maximum capacity of the M shell is 18 electrons.

Question 22.
The atom of the element ……………. has eight electrons in the outermost shell.
Answer:
The atom of the element neon has eight electrons in the outermost shell.

Rewrite the following statements selecting the correct options:

Question 1.
The symbol A is used to denote the …………. .
(a) atomic number
(b) atomic radius
(c) atomic mass number
(d) atomic mass
Answer:
(c) atomic mass number

Question 2.
The existence of isotopes is due to the presence of different number of …………….. .
(a) electrons
(b) protons
(c) neutrons
(d) positrons
Answer:
(c) neutrons

Question 3.
In the nucleus of a sodium atom (\({ }_{11}^{23} \mathrm{Na}\)), there are ……………… neutrons.
(a) 11
(b) 12
(c) 10
(d) 9
Answer:
(b) 12

Question 4.
Isotopes of an element have the same number of …………….. .
(a) neutrons
(b) nucleons
(c) electrons
(d) atoms
Answer:
(c) electrons

Question 5.
The great Indian philosopher ………….. proposed that matter is made up of invisible tiny particles.
(a) Aryabhatta
(b) Kanad
(c) Bhaskaracharya
(d) Chanakya
Answer:
(b) Kanad

Question 6.
The maximum capacity of the M shell is …………… electrons.
(a) 2
(b) 8
(c) 18
(d) 32
Answer:
(c) 18

Question 7.
The mass of the electron is …………….. times less than that of a hydrogen atom.
(a) 1800
(b) 8100
(c) 1550
(d) 1600
Answer:
(a) 1800

Question 8.
The L shell is the valence shell in …………. .
(a) hydrogen
(b) chlorine
(c) oxygen
(d) sodium
Answer:
(c) oxygen

Question 9.
The M shell is the valence shell in ……………….. .
(a) fluorine
(b) neon
(c) carbon
(d) chlorine
Answer:
(d) chlorine

Question 10.
The N shell is the valence shell in ………….. .
(a) fluorine
(b) chlorine
(c) bromine
(d) helium
Answer:
(c) bromine

Question 11.
The maximum capacity of the N shell is ……………. electrons.
(a) 2
(b) 32
(c) 18
(d) 8
Answer:
(b) 32

Question 12.
The maximum number of electrons that can be accommodated in the third orbit is ………… .
(a) 3
(b) 8
(c) 32
(d) 18
Answer:
(d) 18

Question 13.
Rutherford alpha-particle scattering experiment was responsible for the discovery of the ………………. .
(a) atomic nucleus
(b) proton
(c) electron
(d) atomic mass
Answer:
(d) atomic mass

Question 14.
Isotopes of element have …………….. .
(a) different atomic numbers and different atomic mass
(b) different atomic numbers but the same atomic mass number
(c) the same atomic number but different atomic mass numbers
(d) the same atomic number and the same atomic mass number
Answer:
(c) the same atomic number but different atomic mass numbers

Question 15.
The nucleus of an atom contains 19 protons and 21 neutrons. The atomic mass number of the element is ………….. .
(a) 19
(b) 21
(c) 40
(d) 39
Answer:
(c) 40

Question 16.
The nucleus of an atom contains 18 protons and 22 neutrons. The atomic number of the element is ……………… .
(a) 18
(b) 40
(c) 22
(d) 4
Answer:
(a) 18

Question 17.
When writing the symbol of its …………… and ………….. are written.
(a) atomic number, atomic mass number
(b) protons, electrons
(c) protons, neutrons
(d) atomic number, electrons
Answer:
(a) atomic number, atomic mass number

Question 18.
The particles in the atomic nucleus are …………….. .
(a) protons and electrons
(b) electrons
(c) electrons and neutrons
(d) protons and neutrons
Answer:
(d) protons and neutrons

State whether the following statements are True or False:

Question 1.
An atom as a whole is electrically neutral
Answer:
True.

Question 2.
The mass of an atom is distributed evenly within it.
Answer:
False. (The mass of atom is not distributed evenly, practically all the mass is concentrated in the nucleus.)

Question 3.
The electron has the same mass as that of the proton.
Answer:
False. (The mass of the electron is much less than that of the proton.)

Question 4.
The electron in the K shell has maximum energy.
Answer:
False. (The electrons in the K shell has minimum energy.)

Question 5.
Isotopes have same atomic number but different atomic mass number.
Answer:
True.

Question 6.
Matter is composed of molecules and molecules are made of atoms.
Answer:
True.

Question 7.
In India, total 22 nuclear reactors in eight places are functioning.
Answer:
True.

Question 8.
The number of electrons in a given orbit is given by the formula n
Answer:
False. (The number of electrons in a given orbit is given by the formula 2n2)

Question 9.
Atomic masses are measured in a unit called the dalton (u).
Answer:
True.

Question 10.
The chemical properties of isotopes are different.
Answer:
False. (The chemical properties of isotopes are the same.)

Question 11.
The maximum capacity of the N shell is 18 electrons.
Answer:
False. (The maximum capacity of the N shell is 32 electrons.)

Question 12.
Deuterium is an isotope of hydrogen.
Answer:
True.

Question 13.
Isotopes are used in the treatment of cancer.
Answer:
True.

Question 14.
Atoms of all elements, except normal hydrogen contain neutrons.
Answer:
True.

Question 15.
Electrons moving in different orbits possess the same amount of energy.
Answer:
False. (Electrons moving in different orbits possess different amounts of energy.

Question 16.
Rutherford discovered the neutron.
Answer:
False. (Chadwick discovered the neutron.)

Question 17.
The nth orbit contains at the most 2n² electrons.
Answer:
True.

Question 18.
Electrons have different energies according to their orbits.
Answer:
True.

Question 19.
The capacity of the second orbit is 18 electrons.
Answer:
False. (The capacity of the second orbit is 8 electrons.)

Question 20.
The radioactive isotope Sodium-24 is used in the medical treatment of cancer.
Answer:
False. (The radioactive isotope Sodium-24 is used for detection of cracks in the underground pipes.)

Question 21.
Uranium-235 is used in the production of electricity.
Answer:
True.

Consider the relation between I the items in the first pair and write the correlation for second pair:

Question 1.
K : 2 : M : …………. .
Answer:
18

Question 2.
Carbon : 2, 4 : : Fluorine : …………….. .
Answer:
2, 7

Question 3.
Nitrogen : Valency three : : Fluorine : …………….. .
Answer:
valency one

Question 4.
Atomic radius : pm : Atomic mass : ……………. .
Answer:
u

Question 5.
NaH : Valency of Na : 1 : : MgCl₂ : Valency of Mg …………… .
Answer:
2

Question 6.
\(35 \mathrm{Cl}\) : number of neutrons 18 : : \({ }^{37} \mathrm{Cl}/latex] : …………….. .
Answer:
number of neutrons 20

Question 7.
Protons : Positive : : ……………. : Neutral.
Answer:
Neutrons.

Match the following:

Question 1.

Column I	Column II
1. Thomson	a. Well defined orbits
2. Rutherford	b. Neutron
3. Chadwick	c. Scattering experiment
4. Bohr	d. Electron

Answer:

Column I	Column II
1. Thomson	d. Electron
2. Rutherford	c. Scattering experiment
3. Chadwick	b. Neutron
4. Bohr	a. Well defined orbits

Question 2.

Column I	Column II
1. Atom	a. Treatment of goitre
2. Isotopes of iodine	b. Protons + Neutrons
3. Atomic mass number	c. Different number of neutrons
4. Isotopes	d. Electrically neutral

Answer:

Column I	Column II
1. Atom	d. Electrically neutral
2. Isotopes of iodine	a. Treatment of goitre
3. Atomic mass number	b. Protons + Neutrons
4. Isotopes	c. Different number of neutrons

Question 3.

Column I	Column II
1. Alpha particles	a. Inert element
2. Uranium-235	b. Negatively charged particles
3. Helium	c.   Positively charged particles
4. Isotopes	d.   Krypton – 92, Barium 141
	e.   U-233, Th-232
	f.    12C, 13C, 14C

Answer:

Column I	Column II
1. Alpha particles	c. Positively charged particles
2. Uranium-235	d. Krypton – 92, Barium 141
3. Helium	a. Inert element
4. Isotopes	f.    12C, 13C, 14C

Distinguish between the following:

Question 1.
Proton and Neutron:
Answer:

Proton	Neutron
1. The proton is positively charged particle	1. The neutron does not have any charge.
2. In an atom, the number of protons is always equal to the number of electrons.	2. In general, in an atom, the number of neutrons is not equal to the number of electrons.

Question 2.
Neutron and Electron:
Answer:

Neutron	Electron
1. The neutron is an electrically neutral particle.	1. The electron is a negatively charged particle.
2. Neutrons are present in the nucleus of an atom.	2. Electrons revolve around the nucleus of the atom in specific orbits.

Question 3.
Proton and Electron:
Answer:

Proton	Electron
1. The proton is a positively charged particle	1. The electron is a negatively charged particle.
2 Protons are present in the nucleus of an atom.	2. Electrons revolve around the nucleus of the atom in specific orbits.

Question 4.
Atomic number and Atomic mass number:
Answer:

Atomic number	Atomic mass number
1. The number of electrons or protons in an atom is called the atomic number.	1. The sum of the number of protons and neutrons in the nucleus of an atom is called the atomic mass number.
2 All the isotopes of an element have the same atomic number.	2. Different isotopes of the same element have different mass numbers.
3. The atomic number is represented by the letter Z.	3. The atomic mass number is represented by the letter A.

Answer the following questions in one sentence each:

Question 1.
Name the particles which are present in the nucleus of an atom.
Answer:
Protons and neutrons are present in the nucleus of an atom.

Question 2.
State the relation between the number of protons, the number of neutrons and the atomic mass number (A) of an element.
Answer:
Atomic mass = Number of + Number of number (A) protons (p) neutrons (n)

Question 3.
Chlorine contains 17 protons and 18 neutrons. What is its atomic mass number?
Answer:
The atomic mass number of chlorine is 35.

Question 4.
Carbon contains 6 protons and 6 neutrons. State its atomic number and atomic mass number.
Answer:
The atomic number of carbon is 6 and the atomic mass number is 12.

Question 5.
State one use of isotopes of cobalt.
Answer:
Isotopes of Cobalt-60 are used in the treatment of cancer.

Question 6.
State one use of isotopes of i uranium.
Answer:
Isotopes of uranium are used for production of electrical energy.

Question 7.
Write the electronic configuration of oxygen.
Answer:
Electronic configuration of oxygen is 2, 6.

Question 8.
Write the electronic configuration of chlorine.
Answer:
Electronic configuration of chlorine is 2, 8, 7.

Question 9.
State the number of electrons in the L shell of lithium.
Answer:
There is one electron in the L shell of lithium.

Question 10.
State the number of electrons in the M shell of argon.
Answer:
There are 8 electrons in the M shell of argon.

Question 11.
State the number of electrons in the K shell of helium.
Answer:
There are two electrons in the K shell of helium.

Question 12.
Name isotopes of hydrogen.
Answer:
Hydrogen, Deuterium and Tritium are the isotopes of hydrogen.

Question 13.
Name two isotopes of carbon.
Answer:
[latex]{ }_{6}^{12} \mathrm{C}\) and \({ }_{6}^{14} \mathrm{C}\) are the isotopes of carbon.

Question 14.
Name two elements in which the K and L shells of an atom are completely filled with electrons.
Answer:
The elements are neon and argon.

Question 15.
From the symbol \({ }_{8}^{16} \mathrm{O}\), state the electronic configuration of oxygen and the atomic mass number of isotope oxygen.
Answer:
Electronic configuration of oxygen : 2, 6. Atomic mass number of oxygen : 16.

Question 16.
The atomic mass number of an element is 18, and the element contains 8 electrons. What is the number of protons and neutrons in it?
Answer:
Number of protons : 8, Number of neutrons : 10.

Question 17.
An atom contains 2 protons, 2 electrons and 3 neutrons. State its atomic number and atomic mass number.
Answer:
Atomic number: 2, Atomic mass number: 5.

Question 18.
How many electrons could there be in the outermost orbit of an element whose valency is 3?
Answer:
If the valency of an element is 3 then there is a possibility of 3 or 5 electrons in the outermost orbit.

Question 19.
Which element is used as fuel in atomic reactors?
Answer:
Isotopes of uranium are used as fuel in atomic reactors.

Question 20.
Name the place and the first nuclear reactor in India.
Answer:
Apsara at Bhabha Atomic Research Centre in Mumbai is the first nuclear reactor in India.

Answer the following questions:

Question 1.
Explain Dalton’s atomic theory.
OR
Write the postulates of Dalton’s atomic theory.
Answer:
Dalton’s atomic theory:

1. According to Dalton’s atomic theory matter is made up of atoms and atoms are indivisible and indestructible.
2. All atoms of an element are alike while different element have different atom with different mass.

Try this:

Question a.
Take a solid ball and a Bundi Laddu. Press both these spheres with your palms. What did you find?
Answer:
Solid ball does not break and Bundi Laddu breaks into pieces if you press hard.

Question b.
Cut the solid ball with a sharp knife. What did you find?
Answer:
There is no cavity inside the ball.

Question 2.
How will you think about atomic mass distribution according to Thomson’s I model? Whether this distribution is uniform or non uniform as per Dalton’s atomic theory?
(Use your brainpower!
Answer:
In Thomson’s model the distribution of atomic mass is uniform. Dalton’s atomic theory does not say anything about atomic mass distribution.

Question 3.
If the striker flicked by you misses the coin that you aimed at, where would the striker go?
Can you tell?
Answer:
The striker will continue to move practically with the same velocity till it strikes some other coin or the edge of the board or enter the pocket.

Question 4.
If the striker hits the coin, in which direction would it go? Straight forward to a side or in the reverse direction?
Answer:
Striker may go straight forward or it may be deflected depending upon the exact direction of the motion of the striker.

Question 5.
Draw a neat labelled diagram of Thomson’s atomic model.
Answer:

Question 6.
What were the conclusions drawn from the alpha particle experiment performed hy Rutherford?
Answer:
On the basis of the alpha particle experiment the following conclusions were drawn by Rutherford:

1. An atom has tiny, dense positively charged nucleus at centre of an atom.
2. Most of the mass of the atom is concentrated in the nucleus.
3. Negatively charged particles called electrons revolve around the nucleus.
4. The total negative charge on all the ; electron is equal to the positive charge on ‘ the nucleus. As the opposite charges are balanced, the atom is electrically neutral.
5. There is an empty space between the i revolving electron and the atomic nucleus.

Question 7.
Explain Rutherford’s atomic model.
Answer:

1. An atom has tiny, dense, positively charged core called the nucleus.
   
2. Most of the mass of the atom is concentrated in the nucleus.
3. Electrons bear negative charge. They revolve around the nucleus in orbits.
4. The structure of an atom may be regarded as a miniature of the solar system, the nucleus as the sun and the electrons as the planets.

Question 8.
Which discovery did point out that an atom has internal structure?
(Use your brainpower!
Answer:
The discovery of radioactivity (1896) pointed out that an atom has internal structure.

Question 9.
What is the difference between the solid atom in Dalton’s atomic theory and Thomson’s atomic model?
(Use your brain power!
Answer:
The solid atom in Dalton’s atomic theory is a single particle it does not have any structure and also does not have any constituents. In Thomson’s model, atom is made of positive and negative charges. Thus it is not a single particle. It has constituents and has internal structure.

Question 10.
Explain the difference between the distribution of positive charge in Thomson’s atomic model and Rutherford’s atomic model.
(Use your brain power!
Answer:
According to Thomson’s atomic model, the atom is a homogeneous sphere of positive charge. According to Rutherford’s atomic model, the positive charge is concentrated in the nucleus.

Question 11.
What is the point difference between the place of electron in the atomic models of Thomson and Rutherford?
(Use your brain power!
Answer:
According to Thomson’s atomic model, the negatively charged electrons are embedded in a gel of positive charge and they are stationary. According to Rutherford’s atomic model, the negatively charged electrons revolve around the nucleus.

Question 12.
What is the thing which is present in Rutherford’s atomic model and not present in Dalton’s and Thomson’s atomic models?
(Use your brain power!
Answer:
1. Dalton’s atomic model:
Dalton’s atomic theory does not tell anything about the structure of the atom. There is no mention of electron in Dalton’s atomic theory.

Rutherford’s atomic model:
Rutherford’s atomic model tells about the structure of the atom, i.e. the electron revolving around the nucleus.

2. Thomson’s atomic model:
The electrons are embedded in a gel of positive charge. Here the electrons are stationary and there is no concept of nucleus.

Rutherford’s atomic model:
In Rutherford’s atomic model, the electrons are not stationary, they revolve around the nucleus which is positively charged.

Question 13.
Explain Niels Rohr’s atomic model.
OR
Write the postulates of Bohr’s atomic model.
Answer:
The important postulates of Bohr’s atomic model are as follows:

1. The electrons revolving around the atomic nucleus lie in the concentric circular orbits at certain distance from the nucleus.
2. Energy of an electron is constant while it is in a particular orbit.
3. When an electron jumps from an inner orbit to an outer orbit it absorbs specific amount of energy, and when it jumps from an outer orbit to an inner orbit it emits specific amount of energy.
4. The energy emitted or absorbed during these transitions is equal to energy difference between the initial state and the final state of the electron.

Question 14.
How many types of subatomic particles are found in atom?
Use your brainpower!
Answer:
There are three types of subatomic particles found in atom, viz. the proton, neutron and electron.

Question 15.
Which subatomic particles are electrically charged?
Use your brain power!
Answer:
The proton is a positively charged subatomic particle and the electron is a negatively charged particle.

Question 16.
Which subatomic particles are present in the nucleus?
Use your brain power!
Answer:
The proton and neutron are present in the nucleus.

Question 17.
Where are electrons revolving around the nucleus placed?
Use your brain power!
Answer:
Electrons revolve in the discrete orbits around the nucleus.

Question 18.
State the characteristics of the neutron.
Answer:

1. The neutron is electrically neutral and is denoted by the symbol ‘N’.
2. It is present in the nucleus along with the proton.
3. The mass of the neutron is almost equal to lu which is almost equal to that of the proton.

Question 19.
State the characteristics of protons.
Answer:

1. Protons are positively charged particles and are denoted by the symbol ‘P’.
2. They are present in the nucleus of an atom.
3. The mass of the proton is very nearly equal to lu which is almost equal to that of the hydrogen atom.
4. All the elements contain protons.

Question 20.
State the characteristics of electrons.
Answer:

1. Electrons are negatively charged particles and are denoted by the symbol ‘e’.
2. They have negligible mass.
3. They revolve around the nucleus in certain discrete orbits.
4. In an atom, the number of electrons is equal to the number of protons in its nucleus (Z).

Question 21.
The symbol used for oxygen is ‘O’. There are 8 protons and 8 neutrons in its nucleus. From this determine the 1 atomic number (Z) and mass number (A) of oxygen and arrange these in a 1 conventional symbol.
Use your brain power!
Answer:
Atomic mass number
= Protons + Neutrons
= 8 + 8
Atomic mass number = 16
Atomic number, i.e. the proton number = 8
The conventional symbol = \(\mathrm{A} \mathrm{O}\)
Z = Atomic number
i.e. number of protons is 8.
A = Atomic mass number = 16.
The conventional symbol = \({ }_{8}^{16} \mathrm{O}\).

Question 22.
Atomic number of carbon is 6. How many electrons are there in a carbon atom?
Use your brainpower!
Answer:
The atomic number of carbon, i.e. the proton number of carbon is 6.
The number of electron in the extranuclear part is equal to the number of protons in the nucleus.
∴ The number of electrons in a carbon atom = 6.

Question 23.
A sodium atom contains 11 electrons. What is the atomic number of sodium?
Use your brain power!
Answer:
The number of electron in the extranuclear part is equal to the number of protons, i.e. atomic number. The atomic number of sodium is 11.

Question 24.
The atomic number and mass number of magnesium are 12 and 24 respectively. How will you show this by the convention symbol?
Use your brainpower!
Answer:
The atomic number, atomic mass number and the symbol of an element are written as:

\({ }_{12}^{24} \mathrm{Mg}\) the conventional symbol for magnesium.

Question 25.
The atomic number and mass number of calcium are 20 and 40 respectively. Deduce the number of neutron present in the calcium nucleus.
Use your brain power!
Answer:
There are 20 neutrons in the nucleus of calcium.
[Note: The number of neutrons N = A – Z = 40 – 20 = 20]

Question 26.
Write a note on distribution of electrons in orbits.
OR
What is the maximum number of ( electrons that can be accommodated in each of the orbits (shells) K, L, M, N, etc.?
Answer:
The number of maximum electrons in different orbits of the atom are fixed. The orbit (shell) closest to the nucleus is: given the number 1, the next orbit the ; number 2… etc. The orbits one are designated ; by letters K, L, M, N,… corresponding to the shell numbers n = 1, 2, 3, 4, … etc. The maximum number of electrons in a given orbit is 2n², when n is the number of orbit (shell), thus the first shell has the capacity of 2 electrons, the second 8, the third 18, the fourth 32 and so on. The electrons in the K shell have minimum energy. The electrons in the subsequent shells possess higher energy.

Question 27
There is a similarity in atomic structure and solar system. The planets revolve around the sun due to the gravitational force. Which force might be acting in the atomic structure?
Can you tell?
Answer:
The electric force might be acting in the atomic structure. In the atomic structure, electric force is much stronger than gravitational force.
[Note: Here the gravitational force is very very small compared to the electric force, therefore, the gravitational force can be ignored.]

Question 28.
Positively charged proton are together in the nucleus. What might be, one of the function of the neutrons in the nucleus?
Can you tell?
Answer:
One of the function of the neutrons in the nucleus is to keep the protons and neutrons together by nuclear force.

Question 29.
What do you understand by electronic configuration?
Answer:
The shellwise distribution of the electron in an atom of an element is called the electronic configuration.?

Question 30.
Write the electronic configuration of the following elements:
(1) Hydrogen, (2) Magnesium. (3) Aluminium, (4) Phosphorus, (5) Sulphur.
Answer:

Question 31.
Use the following molecular formulae to determine the valencies of H, Cl, O, S, N, C, Br, I, Na.
Molecular formulae – H₂, HCl, H₂O, H₂S, NH₃, CH₄, HBr, HI, NaH.
Can you recall?
Answer:

Question 32.
Draw suitable diagrams to show the electronic configuration of the atoms of the following elements: Hydrogen, helium, carbon, neon, sodium, chlorine.
Answer:

Question 33.
What are the symbols used for the shells which accommodate the electrons in various atoms?
Use your brain power!
Answer:
The symbols used for the shells which accommodate the electrons are K, L, M, N, … respectively.

Question 34
What is the symbol and ordinal number of the innermost shell?
(Use your brain power!
Answer:
The symbol of the innermost shell is K and ordinal number of the innermost shell is 1.

Question 35.
Write symbol of electron distribution in shell of fluorine atom?
Use your brain power!
Answer:
Symbol for fluorine atom = F,
Electronic configuration of fluorine \(\begin{array}{l}\mathrm{K} \\2\end{array}\), \(\begin{array}{l}\mathrm{L} \\7\end{array}\).

Question 36.
Which is the outermost shell of fluorine atom?
Use your brain power!
Answer:
The outermost shell of fluorine atom is L.

Question 37.
Which is the outermost shell of sodium atom?
Use your brain power!
Answer:
The outermost shell of sodium atom is M shell.

Question 38.
Which is the outermost shell of hydrogen atom?
Use your brain power!
Answer:
The outermost shell of hydrogen is K shell.

Question 39.
What is meant by the atomic number (Z) of an element?
(Use your brain power!
Answer:
Atomic number (Z): The number of electrons or protons in an atom is called the atomic number. It is denoted by Z.

Question 40.
Atomic numbers (Z) of some elements are given here. Write down the number of electron present in the outermost shell of each of them.
Use your brain power!
Answer:

Question 41.
The number of electrons of some elements is given here. By using it write the electronic configuration, number of valence electron and valency of the respective elements.
Use your brain power!
Answer:

Question 42.
Why are the atomic numbers and atomic mass numbers always in whole numbers?
Use your brain power!
Answer:
The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number (A). The number of electrons or protons in an atom is called the atomic number (Z). Both protons and neutrons are whole numbers, therefore, atomic numbers and atomic mass numbers are always in whole numbers.

Question 43.
Sulphur contains 16 protons and 16 neutrons. What would be its atomic number and mass number?
Use your brain power.
Answer:
The atomic number of sulphur is 16 and the atomic mass number is 32.

Question 44.
State the uses of isotopes.
Answer:
Isotopes of some elements are radioactive. Isotopes are used in various fields such as industry, agriculture, medicine, research field.

1. Isotopes of uranium is used for nuclear fission and production of electricity.
2. Cobalt – 60 are used in the treatment of cancer.
3. Iodine – 131 is used in the treatment of goitre.
4. The radioactive isotopes, Sodium – 24 are used for detection of cracks (leakage) in the underground pipes.
5. Radioactive isotopes are used for food preservation from microbes.
6. The radioactive C – 14 is used for determining the age of archaeological objects.

Question 45.
Write a note on nuclear reactor.
Answer:
1. A machine that generates electricity on large scale by using atomic energy is called a nuclear reactor. In a nuclear reactor, the nuclear energy in atom is released by bringing about nuclear reactions on the nuclear fuel.

2. When uranium – 235 is bombarded with a slow speed neutron, it undergoes nuclear fission. Various elements are produced. For example: Krypton – 92 and Barium – 141 along with 2 to 3 neutrons are emitted on fission, these neutron have high speed. Their speed is reduced and they are used for bombarding more Uranium – 235 nuclei.

The process is repeated many times. In this way a chain reaction of nuclear fission takes place (See the figure). A large amount of nuclear energy is released during a chain reaction of fission. The chain reaction is controlled to prevent the probable explosion.

Neutrons are slowed down using graphite or heavy water as moderator. The chain reaction is controlled by absorbing neutron with the help of rods of boron, cadmium and beryllium. The heat produced in the fission is taken out by water as coolent. Water is converted into steam. The available heat is used to drive turbines to produce electricity.

Give scientific reasons:

Question 1.
In Rutherford’s experiment, some alpha particles colliding with the thin gold foil are turned back.
Answer:
1. All the positive charges in an atom is concentrated in a very small region at the centre of the atom. This region is called the nucleus. Electrons revolve around the nucleus, but most of the atom is empty. Alpha particles are positively charged.

2. When a thin sheet of gold is bombarded by alpha particles, most of the particles pass through it without deviation as the atom is almost empty. But some alpha particles move directly towards the positive nuclei, collide with them and due to the electric repulsion, turn back. Thus, in Rutherford’s experiment, some alpha particles colliding with the thin gold foil are turned back.

Question 2.
Two electrons in helium atom are placed in only one shell while three electrons in lithium atom occupy two shells.
Answer:
1. The electronic configuration of helium is (2). It indicates that helium atom has two electrons. Both the electrons are accommodated in the K shell. The maximum capacity of the K shell is two electrons and is maintained in all the elements. Hence, two electrons in helium are placed in only one shell.

2. The electronic configuration of lithium is (2, 1). It indicates that lithium atom has three electrons. The first shell K accommodates two electrons, i.e., the maximum capacity of the K shell. The remaining electron is accommodated in the next shell, i.e., the L shell. Thus, three electrons in lithium occupy two shells (K and L).

Complete the following:

Question 1.
Complete the table by putting tick mark in appropriate box.
Answer:
Types of substances:

Question 2.
Complete the table:
Answer:

Write the maximum number of electron in a shell using the above table. K Shell : 2, L Shell : 8, M Shell : 18, N Shell : 32.

Question 3.
Complete the table:
Answer:
Electronic configuration of some elements:

Question 4.
Complete the following table:
Answer:
Relationship between valency and electronic configuration:

Question 5.
In table 4 column you have written of that element? How many electrons identified valency from its molecular are used to complete the octet? formulae. When the number of the valency electrons in an element ‘x’ is 4 or less than 4, does ‘x’ match with the valency of that element?
Use your brain power!
Answer:
When the number of the valence electrons in an element x is 4 or less than 4, then x matches with the valency of that element.

Question 6.
When the number of the valence electrons in an element ‘x’ is 4 or more than 4, does’(8 – x)’ match with the valency of that element? How many electrons are used to complete the octet?
(Use your brain power!
Answer:
When the number of the valence electrons in an element x is 4 or more than 4, then (8-x) matches with the valency of that element. (8-x) electrons are used to complete the octet.

Question 7
Complete the table:
Answer:

Isotopes	Proton number	Neutron number
\({ }_{1}^{1} \mathrm{H}\)	1	–
\({ }_{1}^{2} \mathrm{H}\)	1	1
\({ }_{17}^{35} \mathrm{Cl}\)	1	2
\({ }_{17}^{35} \mathrm{Cl}\)	17	18
\({ }_{17}^{37} \mathrm{Cl}\)	17	20

8th Std Science Questions And Answers:

• Living World and Classification of Microbes Class 8 Questions And Answers
• Health and Diseases Class 8 Questions And Answers
• Force and Pressure Class 8 Questions And Answers
• Current Electricity and Magnetism Class 8 Questions And Answers
• Inside the Atom Class 8 Questions And Answers
• Composition of Matter Class 8 Questions And Answers
• Metals and Nonmetals Class 8 Questions And Answers
• Pollution Class 8 Questions And Answers
• Disaster Management Class 8 Questions And Answers
• Cell and Cell Organelles Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 11 Human Body and Organ System Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 11 Human Body and Organ System Question Answer Maharashtra Board

Class 8 Science Chapter 11 Human Body and Organ System Question Answer Maharashtra Board

1. Find out my partner.

Question a.

Group ‘A’	Group ‘B’
1. Heartbeats	a. 350 ml
2. RBC	b. 7.4
3. WBC	c. 37° C
4. Blood donation	d. 72 per min
5. Normal body temperature	e. 50 – 60 lakh/mm3
6. pH of oxygenated blood	f. 5000 – 6000 per mm3

Answer:

Group ‘A’	Group ‘B’
1. Heartbeats	d. 72 per min
2. RBC	e. 50 – 60 lakh/mm3
3. WBC	f. 5000 – 6000 per mm3
4. Blood donation	a. 350 ml
5. Normal body temperature	c. 37° C
6. pH of oxygenated blood	b. 7.4

2. Complete the following table.

Question a.

Answer:

3. Draw neat and labeled diagrams. 

Question a.
Respiratory system.
Answer:

Question b.
Internal structure of heart.
Answer:

4. Explain with reasons.

Question a.
Human blood is red coloured.
Answer:
The red colour of human blood is due to hemoglobin which is a red coloured conjugated protein with iron that is present on the red blood cells. Therefore, it looks red.

Question b.
Upward and downward movement of diaphragm occurs consecutively.
Answer:
The breathing movements are possible due to contraction and relaxation of the diaphragm. The rib muscles also help in these movements. When the ribs rise and diaphragm is lowered at the same time, then there is a decrease in pressure on lungs.

This causes movement of air into the lungs at the time of inhalation. On the other hand, when ribs come back to their normal position and diaphragm is risen, then pressure on the lungs increases. This causes movement of the air out of the body through the nose in the form of exhalation. These movements are possible only due to consecutive upward and downward movement of the diaphragm.

b

Question c.
Blood donation is considered to be superior of all donations.
Answer:
Blood cannot be manufactured by any artificial chemical process. The only way to obtain blood is by donations of blood from a live donor. Blood is needed at times of emergency. The life of person can be saved if timely blood transfusion is given to the needy victim or a patient. Since such donation can save a valuable human life, it is called superior of all donations.

Question d.
Person with ‘O’ blood group is considered as ‘universal donor’.
Answer:
Person with ‘O’ blood group does not have any antigen on his/her RBCs. The ‘O’ type blood thus cannot cause clotting reactions in the body of the recipients. Such persons with ‘O’ blood group can donate blood to any person having any blood group therefore they are considered as ‘universal donor’.

Question e.
Food must have limited amount of salts.
Answer:
More salt in diet means more sodium ions. These extra sodium salts cause rise in blood pressure. Such condition is called hypertension. This condition can be dangerous and fatal in some cases. Therefore, one must keep control over sodium content of the food.

5. Answer the following questions in your own words.

Question a.
Explain the functional correlation of circulatory system with respiratory, digestive and excretory system.
Answer:
1. Three systems viz. respiratory, digestive and circulatory always work in coordination.
2. Digestive system helps in breaking down complex food molecules into simple soluble nutrients at the end of the digestion process.
3. The soluble nutrients are absorbed in the circulating blood in the villi of the intestine.
4. The blood carries these nutrients to each cell during its circulation.
5. The respiratory system helps the oxygen from the air to be absorbed in the blood.
6. This process takes place in alveolus present in lungs. The oxygen is absorbed in the blood and through haemoglobin it is taken to every cell of the body. At the same time the unwanted carbon dioxide produced in each cell is given out in a process of gaseous exchange.
7. The soluble nutrients, and chiefly glucose is metabolized with the help of oxygen-producing energy. Thus, all the three systems bring about coordinated functions to keep the body alive.

Question b.
Explain the structure and function of human blood.
Answer:
I. Structure, i.e. components of the human blood: Human blood is a fluid connective tissue consisting of blood plasma and blood corpuscles suspended in it.

1. Plasma: Plasma is the fluid part of the blood which is pale yellow in colour. It is slightly alkaline in nature. It has 90-92% water, 6-8 % proteins and 1-2 % inorganic salts.
It contains proteins such as albumin, globulin, fibrinogen, etc. There are inorganic ions such as Ca, Na and K.

2. Blood cells:
a. Blood cells are mainly of three types, viz. RBCs, WBCs and blood platelets. They are produced in the red bone marrow.
b. RBCs are small, circular and enucleated cells. They are full of haemoglobin which is essential in transporting oxygen. RBCs are red blood cells which are 50 to 60 lakh per cubic millimetre. Their life span is 100 to 127 days.
c. WBCs are large, nucleated and colourless. They are of five subtypes, viz. neutrophils, basophils, eosinophils, monocytes and lymphocytes. They are 5 to 10 thousands per millimetre of blood.
d. Platelets are very small disc-shaped blood cells which are 2.5 to 4 lakh per cubic millimetre of blood.

II. Function of human blood:

1. Transport functions:

• Gases: Oxygen is carried via blood from lungs to cells in various parts of body and carbon dioxide from tissues to lungs.
• Nutrients: Simple nutrients like glucose, amino acids, fatty acids are taken up by blood from wall of alimentary canal and transported up to each cell in the body.
• Waste materials: Nitrogenous wastes like ammonia, urea, creatinine are released by tissues into blood which carries those to kidney for excretion.
• Enzymes and hormones: Blood transports the enzymes and hormones from the site of their production to the site of their action.

2. Protection: Antibodies are produced in the blood and they protect the body from microbes and other harmful particles.
3. Thermoregulation: Body temperature is maintained constant at 37 °C by vasodilation and vasoconstriction.
4. Maintaining the balance of minerals like Na, K in the body.
5. If bleeding occurs at the injury, platelets and a protein called fibrinogen of the blood form a clot and seal the injury.
6. Functions of blood cells:

• RBCs: With haemoglobin it carries out transport of respiratory gases.
• WBCs: Soldiers of the body. Produce antibodies and give immunity to body.
• Platelets: Help in blood clotting.

Question c.
Explain the importance and need of blood donation.
Answer:
Blood can never be synthesized artificially. There is no substitute for natural blood. Every healthy person possesses about 5 litres of blood in his or her body. In case of haemorrhage i.e. blood loss, the blood volume may reduce which can result into threat to life. Moreover, the loss of blood should be immediately taken care of, otherwise it may cost the life.

Therefore blood transfusion is very crucial in case of victims of accidents, patients of surgeries or mothers who suffer from blood loss during childbirth (parturition). Some diseases such as thalassemia, blood cancer, etc. also need regular transfusions. Therefore, blood is always needed in many such conditions. Blood donation is only option for such transfusions.

6. Explain the differences.

Question a.
Arteries and veins.
Answer:

Arteries	Veins
1. Arteries carry blood away from the heart to the tissues of the body.	1. Veins carry blood from the tissues of the body back to the heart.
2. Arteries are located deeper within the body.	2. Veins are usually located superficially beneath the surface of the skin.
3. Arteries are thick walled	Veins are thin walled.
4. Arteries do not have valves.	4. Veins have valves.
5. Arteries would generally remain open if blood flow stopped, due to their thick muscular layer.	5. Veins would collapse if blood flow stops.
6. Except pulmonary artery, all arteries carry oxygenated blood.	6. Except pulmonary vein, all veins carry deoxygenated blood.
7. Arteries are more muscular than veins, which helps in transporting blood that is full of oxygen efficiently to the tissues.	7. Veins are less muscular than arteries, but contain valves to help keep blood flowing in the right direction, usually toward the heart.
8. There is maximum blood pressure in the arteries.	8. There is minimum blood pressure in the veins.

Question b.
External and internal respiration.
Answer:

External respiration	Internal respiration
1. Intake of air from the outside into the body and release of air from the body to outside is called external respiration.	1. Exchange of gases between cells and tissue fluid is called internal respiration.
2. External respiration occurs between cells and the external environment.	2. Internal respiration occurs only in the cells of the body.
3. It involves processes of inspiration and expiration.	3. It involves movement of O2 from blood into tissue fluid and movement of CO2 from tissue.
4. External respiration involves breathing and gaseous exchange.	4. Internal respiration involves neither breathing nor gaseous exchange.
5. Oxygen combines with haemoglobin in external respiration.	5. Chemical reactions occur in the cells to form energy.

7. Which health parameters of blood donor should be checked? 

Question a.
Which health parameters of blood donor should be checked?
Answer:
Blood donor should be healthy. He or she should have good haemoglobin’s content. The RBC and WBC count should also be normal. They should not carry any parasites in their blood such as malarial parasite or dengue virus. The donor should not be HIV positive or should not have any infectious diseases. He should not have any addictions such s drug-abuse or alcohol consumption.

8. Fill in the blanks using appropriate words given in the bracket.
(hemoglobin, alkaline, diaphragm, red bone marrow, acidic, voluntary, involuntary,)

Question a.
RBCs of the blood contain ……….., an iron compound.
Answer:
RBCs of the blood contain haemoglobin, an iron compound.

Question b.
……………….. is present between thoracic and abdominal cavity.
Answer:
Diaphragm is present between thoracic and abdominal cavity.

Question c.
Cardiac muscles are …………… .
Answer:
Cardiac muscles are involuntary.

Question d.
pH of oxygenated blood is …………… .
Answer:
pH of oxygenated blood is alkaline.

Question e.
Production of RBCs occurs in ……………… .
Answer:
Production of RBCs occurs in red bone marrow.

8. Find odd one out.

Question a.
A, O, K, AB, B.
Answer:
K (All others are blood groups.)

Question b.
Blood plasma, platelets, blood transfusion, blood corpuscles.
Answer:
Blood transfusion (All others are components of blood.)

Question c.
Trachea, alveoli, diaphragm, capillaries.
Answer:
Capillaries (All others are parts of respiratory system. Capillaries exist throughout the body.)

Question d.
Neutrophils, globulins, albumins, prothrombin.
Answer:
Neutrophils (All others are proteins present in the plasma.)

10. Read the following paragraph and identify the disease.

Today, her child became one and half year old. However, that child does not seem to be healthy and happy. It was continuously crying and gradually becoming weak. It has shortness of breath. Its nails have become blue.
Answer:
The heart of the child is not functioning properly. Bluish nails show lack of oxygen, thus the baby may be suffering also from respiratory problems.

11. Your neighboring uncle has been diagnosed with hypertension. What should he do to keep his blood pressure within normal range?

Question a.
Your neighboring uncle has been diagnosed with hypertension. What should he do to keep his blood pressure within normal range?
Answer:
Hypertension has many causes. Try to find out what is the exact cause. Is it lack of exercise, obesity, lots of fast and junk food consumption, over-intake of salt, mental tension, etc. He should visit a proper physician and take prescribed blood pressure control medicines. He should never miss a single tablet. He should avoid salty and preserved food. He should practice yoga and meditation. He should also undertake some stress- management techniques.

Project:

Question a.
Collect information about various modern treatments on heart diseases.

Class 8 Science Chapter 11 Human Body and Organ System Important Questions and Answers

Rewrite the statements after correcting them:

Question 1.
Cells need supply of insoluble nutrients and oxygen for the energy production.
Answer:
Cells need the supply of soluble nutrients and oxygen for the energy production.

Question 2.
Respiratory system and respiration begins with mouth.
Answer:
Respiratory system and respiration begins with nose.

Question 3.
A lung is present on either sides of heart in abdominal cavity.
Answer:
A lung is present on either sides of heart in thoracic cavity.

Question 4.
Continuous upward and downward movement of diaphragm is necessary to bring about the beating of heart.
Answer:
Continuous upward and downward movement of diaphragm is necessary to bring about the breathing.

Question 5.
Blood vessels which carry the blood away from heart are called veins.
Answer:
Blood vessels which carry the blood away from heart are called arteries.

Question 6.
William Harvey described the blood groups in man.
Answer:
William Harvey described the mechanism of circulation in the body.

Question 7.
Capillaries unite together to form the arteries.
Answer:
Capillaries unite together to form the veins.

Match the columns/Find out my partner:

Question 1.

Group ‘A’	Group ‘B’
1. William Harvey	a. Blood group AB.
2. Carl Landsteiner	b. A, B, O blood group system.
3. Decastello and Sturli	c. Mechanism of circulation of blood.

Answer:

Group ‘A’	Group ‘B’
1. William Harvey	c. Mechanism of circulation of blood.
2. Carl Landsteiner	b. A, B, O blood group system.
3. Decastello and Sturli	a. Blood group AB.

Complete the following table:

Question 1.

Answer:

Type	Systolic pressure	Diastolic pressure
Normal blood pressure	90 – 119 mmHg	60 – 79 mmHg
Primary hypertension	120 – 139 mmHg	80 – 89 mmHg
Hypertension stage – I	140 – 159 mmHg	90 – 99 mmHg
Hypertension stage – II	> 160 mmHg	> 100 mm

Define the following terms:

1. Inhalation: Inhalation is taking in air through the nose from the surrounding
2. Exhalation: Exhalation is giving out the air back to the outer environment.
3. External respiration: The processes of inhalation and exhalation both together are called external respiration.
4. Internal respiration: Exchange of gases between cells and tissue fluid is called internal respiration.
5. Cellular respiration: Production of energy in the form of ATP from oxidation of glucose and other soluble nutrients is called cellular respiration.
6. Thermoregulation: Maintenance of the body temperature to a constant level by performing vasoconstriction or vasodilation is called thermoregulation.
7. Blood pressure: Pressure exerted by the flowing blood on the blood vessel wall is called blood pressure.
8. Systolic pressure: The maximum blood pressure exerted by the flowing blood when the heart is contracting is called systolic pressure.
9. Diastolic pressure: The minimum blood pressure exerted by the flowing blood when the heart is not contracting but receiving (i.e. relaxing) the blood is called diastolic pressure.
10. Hypertension or High Blood Pressure: The blood pressure value of 140 – 159 mm Hg which is more than the normal blood pressure is called hypertension or high blood pressure.
11. Sphygmomanometer: The instru¬ment used for measuring blood pressure is called sphygmomanometer.
12. Hematology: The branch of medical science in which the blood, haemapoietic organs (organs that produce blood cells) and blood disorders are studied is called haematology.

Answer the following questions in one or two sentences each:

Question 1.
What are the heart sounds and why are they produced?
Answer:
There are two types of heart sounds, one is ‘lubb’ and other is ‘dub’. These are produced due to closure of the heart valves.

Question 2.
What is blood circulation?
Answer:
The process of pumping the blood to all the parts of the body and bringing it back again to the heart is called the blood circulation.

Question 3.
Name any four proteins present in the blood plasma.
Answer:
The proteins present in the blood plasma are albumin, globulins, fibrinogen and prothrombin.

Question 4.
Which inorganic ions control the function of muscles and nerves?
Answer:
Calcium, sodium and potassium control the functions of muscles and nerves.

Question 5.
What is the basis on which blood group is determined?
Answer:
The antigen present on the RBCs: and the antibodies present in the plasma of I the blood determine the type of blood group. The genes of parents are responsible for the type of blood group that the child inherits.

Question 6.
Under which conditions, blood is required for donation?
Answer:
Whenever there is hemorrhage, the patient requires blood. Such patients are accident victims, those who excessively bleed, women during childbirth (parturition) and for patients undergoing surgeries.

Question 7.
How much blood is collected from a person during donation?
Answer:
About 350 ml of blood is collected from a person during donation.

Question 8.
When is National Voluntary Blood Donation Day observed?
Answer:
National Voluntary Blood Donation Day is observed on 1st October every year.

Explain the differences:

Question 1.
Atria and ventricles:
Answer:

Atria	Ventricles
1. Atria are the upper chambers of the heart.	1. Ventricles are the lower chambers of the heart.
2. Atria are the smaller receiving chambers.	2. Ventricles are the larger distributing chambers.
3. Atria are thin walled chambers having lesser blood pressure.	3. Ventricles are thick walled chambers with greater blood pressure.
4. Atria do not have inlet valves.	4. Ventricles have inlet valves.
5. Right atrium receives deoxygenated blood from whole body through the inferior vena cava (lower body), superior vena cava (from upper body). It pumps blood into right ventricle.	5. Right ventricle pumps deoxygenated blood received from right atrium to lungs for oxygenation which is known as pulmonary circulation.
6. Left atrium receives oxygenated blood from lungs through pulmonary veins. It pumps blood into left ventricle.	6. Left ventricle pumps oxygenated blood received from left atrium to whole body. This is called systemic circulation.

Question 2.
RBCs and WBCs:
Answer:

RBCs	WBCs
1. RBCs are red in colour due to haemoglobin present in them.	1. WBCs are colourless as there is no pigment in them.
2. RBCs are produced in red bone marrow.	2. WBCs are produced mostly in bone marrow. But they are also produced in lymph nodes, spleen, etc.
3. RBCs are smaller in size and rounded in .shape.	3. WBCs are larger in size and are of different shapes.
4. RBCs have an average lifespan of 120 days.	4. Life span of WBCs vary according to their role. They have a life span from a few days to 3 weeks.
5. Normal RBC count is 50 – 60 lakh RBCs per cubic mm. of blood.	5. Normal WBC count is 5 thousands to 10 thousands per cubic mm. of blood.
6. Their function is to transport the respiratory gases (Oxygen and Carbon Dioxide).	6. Their main function is to produce antibodies and fight against the infections. Thus they are called soldiers of the body.

Give scientific reasons:

Question 1.
A very large number of alveoli is present in lungs, which are covered over by capillary network.
Answer:
Due to very large number of alveoli the surface area of the lungs is increased many a times for the gaseous exchange. The alveoli are covered over by capillary network for rapid gaseous exchange. The oxygen is taken in the body and at the same time carbon dioxide is given out of the body only by the gaseous exchange occurring at the alveolar surface.

Question 2.
Heart is covered by double layered pericardial membrane.
Answer:
Pericardium is the protective double membrane that covers the heart. In between the two layers of this membrane there is protective fluid. The pericardium and the fluid together protect the heart from friction and mechanical shock. Since heart is a vital organ, it is well protected by such pericardial membrane.

Question 3.
Veins are provided with valves.
Answer:
Valves prevent the backflow of the blood. Blood in the veins is not under great pressure so it is likely that it may flow back. But valves prevent such movements. Therefore they are provided with valves.

Write short notes on the following:

Question 1.
Cellular respiration:
Answer:

1. During respiration, the glucose molecules along with some other soluble nutrients are slowly oxidized with the help of oxygen in each cell.
2. In this process the energy is released in the form of ATP, CO₂ and water vapours are produced.
3. These products are not needed for the body and hence given out of the body in exhalation.
4. This process of cellular respiration is shown by the following reaction:
   C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O + Energy (38ATP)

Question 2.
Lung:
Answer:

1. Pair of lungs is the main respiratory organ in the human body. They are located in thoracic cavity.
2. They are present on either sides of heart.
3. Each lung has double layered pleural membranes.
4. Trachea bifurcates into two bronchi. Each bronchus enters lung on its side and S divide and re-divide into fine bronchioles.
5. At the end of each bronchiole there is alveolus. Alveolus is surrounded by capillary network.
6. Each alveolus is extremely thin walled and hence gaseous exchange can occur through diffusion here. Due to thousands of alveoli, the lung surface is increased many a times.
7. Deoxygenated blood coming from heart by pulmonary arteries is purified here in the lungs.
8. It is mixed with oxygen due to gaseous exchange and returned back to the heart by pulmonary veins. Lungs thus continuously help in oxygenation of blood with the help of all of the alveoli.

Question 3.
Diaphragm:
Answer:

1. Diaphragm is a muscular partition that divides the thoracic and abdominal cavity.
2. Located at the base of thoracic cage, it is very important in breathing movements.
3. Diaphragm can undergo consecutive upward and downward movements.
4. These movements along with movements of thoracic cage cause rise and fall of the pressure in the thoracic cavity.
5. Rising up of ribs and lowering of diaphragm causes the decrease in air pressure which makes the air to move into the lungs through nose. This is inhalation.
6. When the pressure rises in the thoracic cavity again, the air is given out. This is exhalation.
7. This is caused due to ribs returning to their original position and rising diaphragm. This simultaneously increases the pressure in thoracic cavity.

Question 4.
Structure of human heart:
Answer:

1. Human heart is four chambered muscular organ.
2. The size of the heart is about one’s own fist and its weight is about 360 gm.
3. For protection, it is covered over by double-layered pericardium.
4. The wall of the heart is made up of cardiac muscles which are involuntary in nature. They have the capacity of rhythmic beating.
5. The upper two chambers are called right and left atrium and lower two chambers are called right and left ventricle.
6. Between right atrium and right ventricle there is tricuspid valve which guards the opening. Similarly between left atrium and left ventricle there is bicuspid valve.
7. On entire right side of the heart there is deoxygenated blood.
8. On entire left side of the heart there is oxygenated blood.
9. Right atrium receives deoxygenated blood by superior and inferior vena cava. These two major veins bring deoxygenated blood from entire body to the heart.
10. Left atrium receives oxygenated blood from lungs by pulmonary vein.
11. Right ventricle sends the deoxygenated blood to lungs for oxygenation, through pulmonary artery.
12. Left ventricle supplies oxygenated blood to entire body through systemic aorta.

Question 5.
Human Blood Groups:
Answer:

1. The A, B, O blood group system is the most popular and medically important blood group system of human beings.
2. The blood group is determined due to antigenic protein present on the RBCs and the antibody present in the plasma.
3. The four main groups of human blood are A, B, AB and O.
4. According to the presence or absence of Rh antigen the blood groups are further said to be Rh-positive or Rh-negative.
5. This makes total eight blood groups which are taken into consideration at the time of blood transfusions.
6. Blood groups are dependent on genes and are thus hereditary in nature.

Answer the following:

Question 1.
How do the organisms like amoeba, earthworm, cockroach, plants, various aquatic animals, bird, respire? Prepare a chart.
Answer:

Diagram – based questions:

Question 1.
Sketch and label structure of alveolus showing gaseous exchange.
Answer:

Question 2.
Sketch and label artery and vein showing structural difference between the two.
Answer:

Question 3.
Draw a diagram to show formation of capillaries. Also show how capillaries form a vein.
Answer:

MCQs based on experiments:

I. Human respiratory system:

Question 1.
From which organ does respiratory system of man begin?
(a) Trachea
(b) Lungs
(c) Alveolus
(d) Nose
Answer:
(d) Nose

Question 2.
Which of the statement given below is correct?
(a) Wind pipe is present in front of food pipe.
(b) Food pipe is present in front of wind pipe.
(c) Wind pipe and food pipe are not near each other.
(d) Wind pipe is same as oesophagus and food pipe is called trachea.
Answer:
(a) Wind pipe is present in front of food pipe.

Question 3.
Which type of respiration includes inhalation and exhalation?
(a) Internal respiration
(b) External respiration
(c) Cellular respiration
(d) None of the above
Answer:
(b) External respiration

Question 4.
How many molecules of ATP are produced from one molecule of glucose ?
(a) 26
(b) 36
(c) 38
(d) 40
Answer:
(c) 38

Question 5.
Where is sound box located in the body?
(a) At the beginning of the wind pipe.
(b) At the beginning of the food pipe.
(c) At the end of the wind pipe.
(d) At the end of the food pipe.
Answer:
(a) At the beginning of the wind pipe.

II. Structure of heart:

Question 1.
What is the location of a bicuspid valve in human heart?
(a) Between right and left ventricle.
(b) Between left and right atrium.
(c) Between left atrium and left ventricle.
(d) Between right atrium and right ventricle.
Answer:
(c) Between left atrium and left ventricle.

Question 2.
Which blood vessels bring deoxygenated blood to the right atrium?
(a) Superior and inferior vena cava
(b) Pulmonary veins
(c) Pulmonary artery
(d) Systemic aorta
Answer:
(a) Superior and inferior vena cava

Question 3.
Which artery carries deoxygenated blood and which vein carries the oxygenated blood respectively?
(a) Dorsal aorta and inferior vena cava respectively.
(b) Pulmonary vein and pulmonary artery respectively
(c) Pulmonary artery and pulmonary vein respectively
(d) Superior vena cava and inferior vena cava respectively.
Answer:
(c) Pulmonary artery and pulmonary vein respectively

Question 4.
Under which of the following condition will rate of heartbeat be faster?
(a) Sleeping
(b) Resting
(c) Running
(d) Sitting
Answer:
(c) Running

Question 5.
Which chambers of the heart are called receiving chamber?
(a) Both atria
(b) Right ventricle
(c) Left ventricle
(d) Only left atria
Answer:
(a) Both atria

8th Std Science Questions And Answers:

• Human Body and Organ System Class 8 Questions And Answers
• Introduction to Acid and Base Class 8 Questions And Answers
• Chemical Change and Chemical Bond Class 8 Questions And Answers
• Measurement and Effects of Heat Class 8 Questions And Answers
• Sound Class 8 Questions And Answers
• Reflection of Light Class 8 Questions And Answers
• Man-made Materials Class 8 Questions And Answers
• Ecosystems Class 8 Questions And Answers
• Life Cycle of Stars Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 4 Current Electricity and Magnetism Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 4 Current Electricity and Magnetism Question Answer Maharashtra Board

Class 8 Science Chapter 4 Current Electricity and Magnetism Question Answer Maharashtra Board

1. Write proper words from the following group of words in the blanks and rewrite the completed sentences:
(magnetism, 4.5V, 3.0V, gravitational attraction, potential difference, potential, higher, lower, 0V)

Question a.
Water in the waterfall flows from a higher level to the lower level because of …………… .
Answer:
Water in the waterfall flows from a higher level to the lower level because of gravitational attraction.

Question b.
In an electric circuit, electrons flow from a point of potential to the point of ……….. potential.
Answer:
In an electric circuit, electrons flow from a point of lower potential to the point of higher potential.

Question c.
The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the …………. of the cell.
Answer:
The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the potential difference of the cell.

Question d.
Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be ……………… V.
Answer:
Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be 4.5 V.

Question e.
An electric current flowing in a wire creates ………………… around the wire.
Answer:
An electric current flowing in a wire creates magnetism around the wire.

2. A battery is to be formed by joining 3 dry cells with connecting wires. Show how you will connect the wires by drawing a diagram.

Question a.
A battery is to be formed by joining 3 dry cells with connecting wires. Show how you will connect the wires by drawing a diagram.
Answer:

3. In an electric circuit, a battery and a bulb have been connected and the battery consists of two cells of equal potential difference. If the bulb is not glowing, then which tests will you perform in order to find out the reason for the bulb not glowing?

Question a.
In an electric circuit, a battery and a bulb have been connected and the battery consists of two cells of equal potential difference. If the bulb is not glowing, then which tests will you perform in order to find out the reason for the bulb not glowing?
Answer:
If you can see the filament of the bulb, check whether it is intact or broken. Check whether the cells are connected in a proper manner:

Or in a wrong way:
If they are connected in a wrong way as shown above, the total potential difference will be 2V + (-2V) = zero.
Figures for reference:

4. Electric cells having potential difference 2V each have been connected in the form of a battery. What will be the total potential difference of the battery in both cases?

Question a.
Electric cells having potential difference 2V each have been connected in the form of a battery. What will be the total potential difference of the battery in both cases?

Answer:
(i) 6V
(ii) 8V.
[Note: In (i), three cells are connected in series.
∴ Total potential difference = 2V + 2V + 2V = 6V.
In (ii), four cells are connected in series.
∴ Total potential difference = 2V + 2V + 2V + 2V
= 8 V]

5. Describe the construction, working and usefulness of a dry cell, with the help of a diagram.
Question a.
Describe the construction, working and usefulness of a dry cell, with the help of a diagram.
Answer:

Dry cell:
Take a lead dry cell and remove its outer coating. Inside you will find a whitish, metal layer. This is the zinc (Zn) metal layer. This is the negative terminal of the cell. Now, carefully break open this layer. There is another layer inside. An electrolyte is filled between these two layers.

The electrolyte contains negatively charged and positively charged ions. These are the carriers of electricity. The electrolyte is a wet pulp of zinc chloride (ZnCl₂) and ammonium chloride (NH₄Cl). There is a graphite rod at the centre of the cell. This is positive terminal of the cell. A paste of manganese dioxide (MnO₂) is filled outside the rod. Because of the chemical reactions of all these chemicals, electrical charge is produced on the two terminals (graphite rod and zinc layer) and an electric current flows in the circuit.

Due to the wet pulp used in this cell, the chemical reaction proceeds very slowly. Hence a large electric current cannot be obtained from this. Compared to the electric cells using liquids, the shelf life of dry cells is longer. Dry cells are very convenient to use as these can be held in any direction with respect to ground and can be used in mobile instruments such as radio sets, wall clocks and torches.

6. Describe the construction and working of an electric bell with the help of a diagram.

Question a.
Describe the construction and working of an electric bell with the help of a diagram.
Answer:
Figure shows the construction of an electric bell and also the electric circuit. The bell consists of an electromagnet, contact screw, iron strip, metal striker and metal gong. A coil of copper wire wound around an iron piece works as an electromagnet and an iron strip along with a striker is fitted near it. A contact screw touches the strip.

The circuit is closed with the key. The current flows in the circuit when the screw is in contact with the iron strip. The current – carrying coil becomes a magnet and attracts the iron strip towards it. As a result, the metal striker hits the metal gong producing sound. At the same time, the screw loses the contact with the strip. The circuit is now incomplete.

Hence, no current flows in the circuit. Therefore, the electromagnet loses its magnetism and the electric iron strip returns to its initial condition, making contact with the screw. As the circuit is now completed, the electromagnet attracts the iron strip and the striker strikes the gong producing sound.

The action repeats itself and the bell continues to ring till the circuit is broken by opening the key in the circuit.
The working of the electric bell is thus based on the magnetic effect of electric current.

Project:

Present all the activities that you performed in this chapter in Science exhibition.

Class 8 Science Chapter 4 Current Electricity and Magnetism Additional Important Questions and Answers

Rewrite the sentences after filling the blanks:

Question 1.
The SI unit of electric charge is the ……………… .
Answer:
The SI unit of electric charge is the coulomb.

Question 2.
The SI unit of electric current is the ……………… .
Answer:
The SI unit of electric current is the ampere.

Question 3.
The SI unit of electric potential is the ………………. .
Answer:
The SI unit of electric potential is the volt.

Question 4.
1 ampere = 1 coulomb per …………… .
Answer:
1 ampere = 1 coulomb per second.

Question 5.
When a glass rod is rubbed on a silk cloth, the glass rod acquires …………… charge.
Answer:
When a glass rod is rubbed on a silk cloth, the glass rod acquires positive charge.

Rewrite the following statements selecting the correct options:

Question 1.
The potential difference between the two electrodes of the lead-acid cell is nearly ………………. .
(a) 1 V
(b) 1.2 V
(c) 1.5 V
(d) 2 V
Answer:
(d) 2 V

Question 2.
The Ni-Cd cell delivers a potential difference of …………….. .
(a) 1 V
(b) 1.2 V
(c) 1.5 V
(d) 2 V
Answer:
(b) 1.2 V

Question 3.
…………….. is a scalar quantity.
(a) Force
(b) Acceleration
(c) Velocity
(d) Electric current
Answer:
(d) Electric current

Question 4.
The working of an electric bell is based on ………….. .
(a) the heating effect of electric current
(b) the chemical effect of electric current
(c) the magnetic effect of electric current
(d) the optical effect of electric current
Answer:
(c) the magnetic effect of electric current

Question 5.
The coulomb is the SI unit of electric ………….. .
(a) current
(b) resistance
(c) potential
(d) charge
Answer:
(d) charge

State whether the following statements are True or False:

Question 1.
The SI unit of electric potential is the ampere.
Answer:
False. (The SI unit of electric potential is the volt. OR The SI unit of electric current is the ampere.)

Question 2.
In the external circuit, the conventional current flows from the positive terminal of the cell to the negative terminal of the cell.
Answer:
True.

Question 3.
Very small current flows when lightning occurs.
Answer:
False. (A large current flows when lightning occurs.)

Question 4.
Sensation is felt by us due to a microscopically small current flowing to the brain.
Answer:
True.

Question 5.
In a car battery, a current is produced by the flow of both negatively and positively charged particles.
Answer:
True.

Identify the odd term:

Question 1.
Pressure exerted by a liquid, Electric current, Electric potential difference, Buoyant force.
Answer:
Buoyant force. It is a vector quantity. Others are scalar quantities.

Question 2.
Electric bulb, Electric heater, Electric bell, Electric iron.
Answer:
Electric bell. Its working is based on the magnetic effect of electric current. The working of other devices is based on the heating effect of electric current.

Answer the following questions in one sentence:

Question 1.
State the relation among the SI units of electric current, electric charge and time.
Answer:
1 ampere = 1 coulomb per second.

Question 2.
Name the positive terminal of the dry cell.
Answer:
The graphite rod at the centre is the positive terminal of the dry cell.

Question 3.
What constitutes an electric current in a metal?
Answer:
A continuous flow of electrons constitutes an electric current in a metal.

Question 4.
What is a battery?
Answer:
A group of two or more cells connected in series to obtain more potential difference is called a battery.

Question 5.
What is a solar cell?
Answer:
A solar cell is a device that converts solar energy into electrical energy.

Question 6.
State one characteristic of the Ni-Cd cell.
Answer:
The Ni-Cd cell is rechargeable.

Question 7.
Give one example in which the magnetic effect of electric current is used.
Answer:
The working of an electric bell is based on the magnetic effect of electric current.

Answer the following questions:

Question 1.
When do we get current electricity?
Answer:
We get current electricity when charged particles such as electrons, positive ions and negative ions are made to flow in a conductor by applying electric force.

Question 2.
Explain the concept of electrostatic potential (electric potential).
Answer:
A liquid flows from a higher level to a lower level. Heat flows from a body at a higher temperature to a body at a lower temperature. Also when different parts of a body are at different temperatures, heat flows from the part at a higher temperature to the part at a lower temperature.

Similarly, a positive charge (free to move) flows from a point at a higher electric level to a point at a lower electric level and a negative charge (free to move) flows from a point at a lower electric level to a point at a higher electric level. The electric level deciding the direction of flow of electric charge is called electrostatic potential (electric potential).

Question 3.
What is the SI unit of electric potential?
Answer:
The SI unit of electric potential is the volt (V).
[Note : This unit is named in honour of Alessandro Volta (1745-1827), Italian physicist, the inventor of the electric battery.]

Question 4.
What is potential difference?
Answer:
Potential difference is the difference
between the potentials at two points. It is similar to the height of a waterfall or the temperature difference between a hot body and a cold body. Potential difference is expressed in volt.

Try this: 

Take connecting copper wires and connect the ‘circuit’ as shown in Fig.(a). No current is seen to flow in the bulb. Now connect in the same ‘circuit’ a 1.5 V dry cell available in the market as shown in Fig.(b). Now it will be realized from glowing of the bulb that a current is flowing in the circuit. Electrons in the wire flow due to the potential difference between the two ends

In Fig.(a), there is no current as ) there is no potential difference in the absence of any cell. Current starts flowing in the circuit as soon as the potential difference is applied. The unit of potential difference in SI system is the volt (V). We will learn about it in the next standard.

Question 5.
What is electric circuit or electrical circuit?
Answer:
A continuous path consisting of conducting wires, a switch or a plug key and other resistances (for example, resistance of an electric bulb) between the terminals of a cell (or a battery) along which an electric current flows is called an electric circuit.

Question 6.
What is electric current?
Answer:
Electric current is the electric charge flowing through a conductor, such as a metal wire, per unit time.

Question 7.
What is one ampere?
Answer:
If one coulomb of charge is passing through any cross section of a conductor in one second, the amount of current flowing through it is called one ampere.
[Note: 1 ampere = 1 coulomb per second OR 1A = 1 C/s. The coulomb is named in honour of Charles Augustin de Coulomb (1736 – 1806), French physicist. The ampere is named in honour of Andre Ampere (1175 – 1836), French physicist and mathematician.]

Think about it:

Question a.
How can we measure water flow emerging from a pipe? We can find it from the amount of water (litres) coming out in a specific time period. How then is the electric current measured?
Answer:
Electric current, I = Q/t. In principle, if we can measure the quantity of charge (Q) flowing through a cross section of a conductor in time t, we can determine the current (i) using this formula (for a steady current). In practice, this is not done. Usually, electric current is measured using the magnetic effect of electric current. One can also use the heating effect and chemical effect of electric current for measurement of electric current.

Question 8.
What is an electric cell? What is its main function?
Answer:
An electric cell is a general device used to produce a uniform flow of charges in a circuit. Its main function is to maintain a constant potential difference between its two terminals.

Question 9.
Name ten devices in which electric cells are used.
Answer:
Electric cells are used in radio sets, wall clocks, wrist watches, torches, toys, mobile phones, cars, invertors, remote controls, ships, submarines, satellites, etc.

Question 10.
Draw a neat labelled diagram to show the design of the lead-acid cell and explain its principle of working.
Answer:
The lead-acid cell contains two electrodes, Pb and PbO₂, dipped in dilute H₂SO₄. Chemical reaction between the substances in the cell produces electric charges on the electrodes. PbO₂ carries a positive charge and Pb carries a negative charge. The potential difference between the electrodes is nearly 2V.

If a load, such as a bulb, is connected externally between the two electrodes, an electric current flows through it. It can deliver a large current. This cell can be recharged after getting electrically discharged.

Question 11.
State applications of the lead-acid cell.
Answer:
Lead-acid cells are connected in series to obtain higher potential difference. The lead-acid battery is used in cars, trucks, motorcycles and uninterrupted power supplies (UPS).

Question 12.
What is the potential difference delivered by the Ni-Cd cell?
Answer:
The potential difference delivered by the Ni-Cd cell is (about) 1.2 V.
[Note: In this cell, nickel oxide is the positive electrode and cadmium is the negative electrode. It is often used as a dry cell and it can be recharged. These cells are used in some portable machines that run on electricity like a drilling machine or a gardening tool.]

Question 13.
Draw a neat labelled diagram of a simple electric circuit containing a cell, an electric bulb and a plug key. What happens when the key is (i) open (ii) closed?
Answer:

(i) When the key is open, there is no current in the circuit. Hence, the bulb does not glow.
(ii) When the key is closed, a current flows in the circuit. Hence, the bulb glows.

Figure for reference:
Here, six cells can be connected in series to obtain more potential difference.

Research:
Lithium ion cells are used in modern equipment for example, a smart phone, laptop, etc. These cells can be recharged. More electrical energy can be stored in these cells as compared to that in Ni-Cd cells.

Question 14.
See Fig. What is the potential difference between A and B if each cell delivers a potential difference of 2V?

Answer:
Potential difference between A and B
= 2V + 2V + (-2V) + 2V = 4V.

Use your brain power!

Question a.
You must have seen the car battery available in the market. It is called a battery and not a cell. Why?
Answer:
In a car battery, a number of cells are connected in series to obtain more potential difference.

Here, two identical cells are connected in parallel. The potential difference between A and B is 2V. This arrangement can be used for a longer time compared to a single cell.

Series and parallel arrangement for higher potential difference and longer life.
Here, the potential difference between A and B is 8V.

Try this:

Magnetic effects of electric current :
Activity 1 :
Take the inside tray of a used up matchbox. Place a small magnetic needle inside the tray. Now take a long connecting wire and wind it around the tray. Complete the electric circuit by connecting in it, this wire, electric cell, plug key and a bulb Figure.

Mark the position of the magnetic needle. Take a bar magnet near to the magnetic needle.
1. What do you observe? Keep looking at the needle and close the plug key. The bulb will light up, and you will realize that the current has started flowing.
2. Does the magnetic needle change its position? Now open the plug key.
3. Does the magnetic needle come back to the original position?
4. What will you conclude from this experiment?
You know that a magnetic needle is indeed a magnet. You have seen that the magnetic needle changes its direction when a bar magnet is taken near the magnetic needle. Also, you have observed that the magnetic needle changes its direction when a current starts flowing in the circuit. This means that magnetic field is created when an electric current flows in a wire. Hans Christian Oersted made this observation first. Briefly we can say that when an electric current passes through a wire, a magnetic field is produced around that wire.
Answer:

1. The magnetic needle is deflected.
2. Yes.
3. Yes.
4. The electric current produces a magnetic field around a current-carrying conductor.

Activity 2 :
Take a metre long flexible copper wire having resistive coating and wind it tightly on a long iron screw. Connect the two ends of the wire in a circuit as shown in the Figure. Also connect an electric cell and a plug key in the circuit. Keep 2-4 iron pins/small nails near the screw. Now start the current in the circuit by plugging the key. It will be noticed that the pins/nails have stuck to the tip of the screw. Will the pins/nails continue to stick when the plug key is opened?

Answer:
No.
When the electric current flows in the wire, magnetism is produced in the coil around the screw and because of that the screw also attains magnetism. As soon as the current is stopped, this magnetism vanishes. The system of the coil and the screw is called an electromagnet. You have seen various uses of the electromagnet in the sixth standard. Electromagnets are used to produce a strong magnetic field useful in scientific research.

Question 15.
What is an electromagnet? State ! its applications.
Answer:
If a coil is wound around an iron screw (or an iron rod) and a current is passed through the coil with a cell (or a battery), the screw behaves as a magnet as long as there is current in the coil. The i system of the coil and the screw is called an electromagnet. It is an example of magnetic effect of electric current.
Applications: Electric bell, cranes for moving heavy loads from one place to another, toys that run on electric cells.

Question 16.
How can the strength of an electromagnet be increased?
Answer:
The strength of an electromagnet can be increased by increasing the current producing the magnetic field.

Question 17.
What is the use of the elastic iron strip in the electric bell?
Answer:
Due to the elastic iron strip, the making and breaking of the circuit occur alternately and the bell continues to ring as long as the key in the circuit is closed.

Question 18.
What will happen if the elastic iron strip in the electric bell loses its flexibility?
Answer:
When the circuit breaks and the electromagnet loses its magnetism, the strip, due to loss of flexibility, will not return to its normal position of contact with the screw. Hence, the circuit will not be completed and the bell will not ring.

8th Std Science Questions And Answers:

• Living World and Classification of Microbes Class 8 Questions And Answers
• Health and Diseases Class 8 Questions And Answers
• Force and Pressure Class 8 Questions And Answers
• Current Electricity and Magnetism Class 8 Questions And Answers
• Inside the Atom Class 8 Questions And Answers
• Composition of Matter Class 8 Questions And Answers
• Metals and Nonmetals Class 8 Questions And Answers
• Pollution Class 8 Questions And Answers
• Disaster Management Class 8 Questions And Answers
• Cell and Cell Organelles Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 6 Composition of Matter Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 6 Composition of Matter Question Answer Maharashtra Board

Class 8 Science Chapter 6 Composition of Matter Question Answer Maharashtra Board

1. Choose the appropriate option and rewrite the following statements:

Question a.
The intermolecular force is ……………. in the particles of solid.
(a) minimum
(b) moderate
(c) maximum
(d) indefinite
Answer:
(c) maximum

Question b.
Solids retain their volume even when external pressure is applied. This property is called …………… .
(a) plasticity
(b) incompressibility
(c) fluidity
(d) elasticity
Answer:
(d) elasticity

Question c.
Matter is classified into the types mixture, compound and element, by applying the criterion …………… .
(a) states of matter
(b) phases of matters
(c) chemical composition of matter
(d) all of these
Answer:
(c) chemical composition of matter

Question d.
Matter that contain two or more constituent substances is called ……… .
(a) mixture
(b) compound
(c) element
(d) metalloid
Answer:
(a) mixture

Question e.
Milk is an example of type of matter called …………. .
(a) solution
(b) homogeneous mixture
(c) heterogeneous mixture
(d) suspension
Answer:
(c) heterogeneous mixture

Question f.
Water, mercury and bromine are similar to each other, because the three are ……….. .
(a) liquids
(b) compounds
(c) nonmetals
(d) elements
Answer:
(a) liquids

Question g.
Valency of carbon is 4 and that of oxygen is 2. From this, we understand that there are …………. chemical bond/bonds between the carbon atom and one oxygen atom in the compound- carbon dioxide.
(a) 1
(b) 2
(c) 3
(d) 4.
Answer:
(b) 2

2. Identify the odd term out and explain.

Question a.
Gold, silver, copper, brass.
Answer:
Brass. (Others are elements.)

Question b.
Hydrogen, hydrogen peroxide, carbon dioxide, water vapour.
Answer:
Hydrogen. (Others are compounds.)

Question c.
Milk, lemon juice, carbon, steel.
Answer:
Carbon. (Others are mixtures.)

Question d.
Water, mercury, bromine, petrol.
Answer:
Petrol. (Others are inorganic compounds.)

Question e.
Sugar, salt, baking soda, blue vitriol.
Answer:
Sugar. (Others are inorganic compounds.)

Question f.
Hydrogen, sodium, potassium, carbon.
Answer:
Carbon. (Others are monovalent elements)

3. Answer the following question.

Question a.
Plants synthesize glucose in sunlight with the help of chlorophyll from carbon dioxide and water and give away oxygen. Identify the four compounds in this process and name their types.
Answer:
Photosynthesis:

Carbon dioxide, water, glucose, and Chlorophyll are compounds.
Types: Organic compounds: Glucose
Inorganic compounds: Carbon dioxide and water
Complex compounds: Chlorophyll.

Question b.
In one sample of brass, the following ingredients were found: copper (70%) and zinc (30%). Identify the solvent, solute and solution from these.
Answer:
Brass is an alloy, it contains 70% copper and 30% zinc. The largest proportion is solvent, i.e. copper. The smaller proportion is solute, i.e. zinc. The solution is Brass.

Question c.
Seawater tastes salty due to the dissolved salt. The salinity (the proportion of salts in water) of some water bodies Lonar lake – 7.9%, Pacific Ocean 3.5%, Mediterranean sea – 3.8%, 5 Dead sea – 33.7%. Explain two characteristics of mixtures from the above information.
Answer:

1. The constituents of a mixture (the proportion of salts in water) do not combine chemically.
2. Their constituents are present in any proportion by weight.
3. The constituent of a mixture can be separated by a physical process.

4. Give two examples each.

Question a.
Liquid element.
Answer:
Mercury (Hg), Bromine (Br₂)

Question b.
Gaseous element.
Answer:
Hydrogen (H₂), Oxygen (O₂).

Question c.
Solid element.
Answer:
Iron (Fe), Copper (Cu), Silver (Ag).

Question d.
Homogeneous mixture.
Answer:
Sea water, blue vitriol dissolved in water.

Question e.
Colloid.
Answer:
Milk, blood.

Question f.
Organic compound.
Answer:
Glucose, urea.

Question g.
Complex compound.
Answer:
Chlorophyll, Haemoglobin.

Question h.
Inorganic compound.
Answer:
Soda, rust, limestone.

Question i.
Metalloid.
Answer:
Silicon, arsenic.

Question j.
Element with valency 1.
Answer:
Sodium (Na), potassium (K), chlorine (Cl).

Question l.
Element with valency 2.
Answer:
Magnesium (Mg), Calcium (Ca).

5. Write the names and symbols of the constituent elements and identify their valencies from their molecular formulae given below:
KCl, HBr, MgBr₂. K₂O, NaH, CaCl₂, CCl₄, HI, H₂S, Na₂S, FeS, BaCl₂.

Question a.
Write the names and symbols of the constituent elements and identify their valencies from their molecular formulae given below:
KCl, HBr, MgBr_2, K₂O, NaH, CaCl₂, CCl₄, HI, H₂S, Na₂S, FeS, BaCl₂.
Answer:

Molecular formula	Constituent element/Name symbol	Valency
1. KCl	Potassium (K) Chlorine (Cl)	1 1
2. HBr	Hydrogen (H) Bromine (Br)	1 1
3. MgBr2	Magnesium (Mg) Bromine (Br)	2 1
4. K2O	Potassium (K) Oxygen (O)	1 2
5. NaH	Sodium (Na) Hydrogen (H)	1 1
6. CaCl2	Calcium (Ca) Chlorine (Cl)	2 1
7. CCl4	Carbon (C) Chlorine (Cl)	4 1
8. HI	Hydrogen (H) Iodine (I)	1 1
9. H2S	Hydrogen (H) Sulphur (S)	1 2
10. Na2S	Sodium (Na) Sulphur (S)	1 2
11. FeS	Iron (Fe) Sulphur (S)	2 2
12. BaCl2	Barium (Ba) Chlorine (Cl)	2 1

6. Chemical composition of some matter is given in the following table. Identify the main type of matter from them.

Question a.
Chemical composition of some matter is given in the following table. Identify the main type of matter from them.

Answer:

Name of matter	Chemical composition	Main type of matter
Sea water	H2O + NaCl + MgCl2 +…	Mixture
Distilled water	H2O	Compound
Hydrogen gas filled in a balloon	H2	Element
The gas in LPG cylinder	C4H10 + C3H8	Mixture
Baking soda	NaHCO3	Compound
Pure gold	Au	Element
The gas in oxygen cylinder	O2	Element
Bronze	Cu + Sn	Mixture
Diamond	C	Element
Heated white powder of blue vitriol	CuSO4	Compound
Limestone	CaCO3	Compound
Dilute hydrochloric acid	HCl + H2O	Mixture

7. Write scientific reason.

Question a.
Hydrogen is combustible, oxygen helps combustion, but water helps to extinguish fire.
Answer:

1. Water is a compound of hydrogen and oxygen.
2. In a compound, the constituents do not retain their individual properties. Hence, hydrogen is combustible and oxygen helps combustion, but water is neither combustible nor supports combustion, it helps to extinguish fire.

Question b.
The constituent substances of a colloid cannot be separated by oridinary filtration.
Answer:

1. A colloidal solution is heterogeneous.
2. The diameters of colloidal particles are of the order of 10^-5 m.
3. The particles of a colloid can easily pass through a filter paper as the pore size of a filter paper is big. Hence, the constituents of a colloidal cannot be separated by filtration.

Question c.
Lemon sherbat has sweet, sour and salty taste and it can be poured in a glass.
Answer:

1. Lemon sherbat is a mixture. It is made up of lemon juice, sugar, salt and water.
2. Formation of lemon sherbat does not involve any chemical reaction.
3. The constituents of sherbat retain their individual properties. Hence, lemon sherbat is sweet, sour and salty to taste and it can be poured in a glass.

Question d.
A solid matter has the properties of definite shape and volume.
Answer:

1. The forces among the constituent particles (atom/molecules) are called intermolecular forces.
2. In solids these forces are strong enough to keep the particles together in fixed positions, as a result solids have a definite shape and volume.

8. Deduce the molecular formulae of the compound obtained from the following pairs of elements by the cross multiplication method.

Question a.
C (Valency 4) & Cl (Valency 1)
Answer:
Step 1: Write the symbols of the constituent elements.

Step 2: Write the valency below the respective elements.

Step 3: Cross multiply the valencies.

The molecular formula: CCl₄

Question b.
N (Valency 3) & H (Valency 1)
Use your brain power!
Answer:
Step 1: Write the symbols of the constituent elements.

Step 2: Write the valency below the respective elements.

Step 3: Cross multiply the valencies.

The molecular formula: NH₃

Question c.
C (Valency 4) & O (Valency 2)
Answer:
Step 1: Write the symbols of the constituent elements.

Step 2: Write the valency below the respective elements.

Step 3: Cross multiply the valencies.

The formula: C₂O₄
The number of constituent atoms in the final molecular formula should be the smallest possible whole number. Divide the formula C₂O₄ by suitable number.
Final molecular formula obtained by dividing by ‘2’.
Molecular formula: CO₂

Question d.
Ca (Valency 2) & O (Valency 2)
Answer:
Step 1: Write the symbols of the constituent elements.

Step 2: Write the valency below the respective elements.

Step 3: Cross multiply the valencies.

The formula: Ca₂O₂
Divide the formula by suitable number ‘2’.
The molecular formula: CaO.

Project:

Question a.
Collect the wrappers of ready-made foodstuff. Use the information given and prepare a chart of foodstuff and the ingredients in it. Procure the ingredients available. Discuss with friends and teacher, test the procured ingredients with combustion test under the supervision of your teacher. Thereby identify the ingredients as organic or inorganic.

Class 8 Science Chapter 6 Composition of Matter Additional Important Questions and Answers

Rewrite the sentences after filling the blanks:

Question 1.
………….. have no definite shape, but have a definite volume.
Answer:
Liquids have no definite shape, but have a definite volume.

Question 2.
………… have neither a definite shape nor a definite volume.
Answer:
Gases have neither a definite shape nor a definite volume.

Question 3.
The properties of a …………… are different than those of the constitute elements.
Answer:
The properties of a compound are different than those of the constitute elements.

Question 4.
The part of matter having uniform composition is called ………………. .
Answer:
The part of matter having uniform composition is called phase.

Question 5.
The heterogeneous mixture of a liquid and a solid is called a ……………. .
Answer:
The heterogeneous mixture of a liquid and a solid is called a suspension.

Question 6.
A colloid is a ………………. .
Answer:
A colloid is a heterogeneous mixture.

Question 7.
Bronze contains atoms of the elements ……………. and ………….. .
Answer:
Bronze contains atoms of the elements copper and tin.

Question 8.
When all the components of a mixture form one phase, it is called ………….. .
Answer:
When all the components of a mixture form one phase, it is called homogeneous mixture.

Question 9.
…………… is a mixture of water, lactose, fats and proteins.
Answer:
Milk is a mixture of water, lactose, fats and proteins.

Question 10.
The intermolecular force is …………….. in the gaseous state.
Answer:
The intermolecular force is very weak in the gaseous state.

Choose the appropriate option and rewrite the following statements:

Question 1.
When a liquid is poured from one container to another, its shape may change, but its volume ………….. .
(a) increases
(b) decreases
(c) remains the same
(d) none of them
Answer:
(c) remains the same

Question 2.
The property by which some solids maintain their shape even when subjected to external forces is called ………. .
(a) elasticity
(b) fluidity
(c) rigidity
(d) plasticity
Answer:
(c) rigidity

Question 3.
…………… is a complex compound.
(a) Glucose
(b) Blue vitriol
(c) Chlorophyll
(d) Soda
Answer:
(c) Chlorophyll

Question 4.
………….. is a homogeneous mixture.
(a) Sand + water
(b) Flour + water
(c) Salt + water
(d) Oil + water
Answer:
(c) Salt + water

State whether the following statements are True or False:

Question 1.
Liquids do not have a definite shape
Answer:
True.

Question 2.
Gases have a definite shape and volume.
Answer:
False. (Gases do not have shape and volume.)

Question 3.
The crystals of blue vitriol form a single phase.
Answer:
True.

Question 4.
Water is a mixture.
Answer:
False. (Water is a compound)

Question 5.
Milk is a colloid.
Answer:
True.

Question 6.
A suspension does not scatters light.
Answer:
True.

Question 7.
Copper sulphate is a mixture.
Answer:
False. (Copper sulphate is a compound.)

Question 8.
An atom forms as many chemical bonds with other atom as its valency.
Answer:
True.

Question 9.
The strength of intermolecular forces is weak in the liquid state.
Answer:
False. (The strength of intermolecular forces is moderate in the liquid state.)

Question 10.
All the gases present together constitute a single phase.
Answer:
True.

Match the column:

Question 1.

Column I	Column II
1. Potassium dichromate	a. Complex compound
2. Cyanocobalamine	b. Element
3. Air	c. Compound
4. Silver	d. Mixture

Answer:

Column I	Column II
1. Potassium dichromate	c. Compound
2. Cyanocobalamine	a. Complex compound
3. Air	d. Mixture
4. Silver	b. Element

Question 2.

Column 1	Column II
1. Milk	a. Alloy
2. Lemonade	b. Colloid
3. Sand and water	c. Solution
4. Brass	d. Suspension

Answer:

Column 1	Column II
1. Milk	b. Colloid
2. Lemonade	c. Solution
3. Sand and water	d. Suspension
4. Brass	a. Alloy

Question 3.

Column I	Column II
1. Sea water	a. Solid in solid
2. Vinegar	b. Solid in liquid
3. Air	c. Liquid in liquid
4. Bronze	d. Gas in liquid
5. Chlorinated water	e. Gas in gas

Answer:

Column I	Column II
1. Sea water	b. Solid in liquid
2. Vinegar	c. Liquid in liquid
3. Air	e. Gas in gas
4. Bronze	a. Solid in solid
5. Chlorinated water	d. Gas in liquid

Answer the following questions in one sentence:

Question 1.
Why does a solid have definite volume ?
Answer:
In solids intermolecular forces are strong enough to keep the particles together in fixed positions, as a result solids have definite volume.

Question 2.
Why does a liquid have indefinite shape?
Answer:
In a liquid intermolecular forces are not strong enough to keep the particles together in fixed positions, as a result liquid has indefinite shape.

Question 3.
Why does a gas have indefinite volume and shape?
Answer:
In a gas intermolecular forces are very weak and the distance between the particles is very large, as a result gas has indefinite volume and shape.

Question 4.
What can you say about the distance between the neighbouring particles of solid?
Answer:
The particles of solid are very close to each other and the distance between them is minimum.

Question 5.
What can you say about the distance between the constituent particles of gas?
Answer:
The intermolecular force is very weak in gases. The distance between the particles of gas is very large. They move freely and occupy all the available space.

Question 6.
State whether the mixture of oil in water is a homogeneous or heterogeneous mixture.
Answer:
The mixture of oil in water is a heterogeneous mixture.

Question 7.
State whether seawater is a homogeneous or heterogeneous mixture.
Answer:
Seawater is a homogeneous mixture.

Question 8.
Give two examples of liquid in liquid.
Answer:
Examples of liquid in liquid:
Vinegar, dilute sulphuric acid.

Question 9.
Give two examples of gas in gas.
Answer:
Examples of gas in gas: Air, cooking gas.

Question 10.
Give two examples of solid in solid.
Answer:
Examples of solid in solid: Brass, stainless steel.

Question 11.
Give two examples of gas in liquid.
Answer:
Examples of gas in liquid: Chlorinated water, hydrochloric acid.

Question 12.
Give molecular formulae of 1. potassium chloride, 2. sodium sulphide.
Answer:

1. Molecular formula of potassium chloride: KCl
2. Molecular formula of sodium sulphide: Na₂S.

Question 13.
State the molecular formula and number of hydrogen atoms in methane.
Answer:
Molecular formula of methane: CH₄
The number of hydrogen atoms in methane is 4.

Question 14.
Identify the valency of chlorine in
(i) KCl and (ii) CaCl₂.
Answer:
i. The valency of chlorine in KCl is 1.
ii. The valency of chlorine in CaCl₂ is 1.

Answer the following questions:

Question 1.
State two principal ways of classifying matter.
Answer:
Two principal ways of classifying matter are:

1. On the basis of physical state as a solid, liquid or gas.
2. On the basis of chemical constitution as an element, compound or mixture.

Question 2.
What are the different states of matter?
Answer:
The different states of matter are solid, liquid and gas.

Question 3.
Define solid.
Answer:
A substance which has a definite shape and volume is called a solid.

Question 4.
State any three characteristics of solids.
Answer:

1. Solids have a definite shape and volume.
2. Most of the solids are rigid while some are plastic and some elastic.
3. Solids have practically negligible compressibility, i.e., their volume is not very much changed by external forces.

Question 5.
What is meant by rigidity of solids?
Answer:
The property by which solids maintain their shapes when subjected to external forces is called rigidity.

Question 6.
What is meant by elasticity of solids?
Answer:
Some solids undergo a change in their shapes and volume when subjected to external forces and regain their original shapes and volume on removal of the forces. This property of solids is called elasticity.

Question 7.
What is meant by plasticity?
Answer:
Some solids are deformed by external forces and do not regain their original shapes on removal of the forces. This property of solids is called plasticity.

Question 8.
Solids have a high density and negligible compressibility. Explain.
Answer:

1. The forces among the constituent particles (atoms/molecules) are called intermolecular forces.
2. In solids these forces are strong enough to keep the particles together in fixed positions, as a result solids have a high density and negligible compressibility.

Question 9.
Define liquid.
Answer:
A substance which does not have a definite shape, but has a definite volume is called a liquid.

Question 10.
State any three characteristics of liquids.
Answer:

1. Liquids have a definite volume.
2. Liquids do not have a definite shape. They take the shape of the container in which they are stored.
3. When they spill on a plane surface, they spread and flow in the surrounding areas, i.e., they have fluidity.
4. They have very small compressibility.

Question 11.
What is meant by fluidity?
Answer:
Liquids flow easily and change their shapes in response to external forces. This property of liquids is called fluidity. It is exhibited by gases also.

Question 12.
Liquids have fluidity and they do not have a definite shape. Explain.
Answer:
1. The forces among the constituent particles (atoms/molecules) are called intermolecular forces.
2. In liquids, the intermolecular forces are strong enough to keep the particles within the bulk in aggregation, but not strong enough to keep them in fixed positions. As a result, they flow easily and change shapes according to that of container.

Question 13.
Define gas.
Answer:
A substance which has neither a definite shape nor a definite volume is called a gas.

Question 14.
State any three characteristics of gases.
Answer:

1. Gases have neither a definite shape nor a definite volume.
2. They spread in all directions.
3. They can be compressed or expanded easily.
4. In gases intermolecular forces are very weak.

Question 15.
Gases have neither a definite shape nor a definite volume. Explain.
Answer:

1. The forces among the constituent particles (atoms/molecules) are called intermolecular forces.
2. In gases the intermolecular forces are extremely weak. As a result, the molecules of a gas move freely and a gas occupies all the available space.

Question 16.
Why are gases more compressible, than liquids?
Answer:
In liquids the distances between the neighbouring molecules are less than those in the gases. The intermolecular forces are extremely weak in gases, but not so weak in liquids. Hence, gases are more compressible than liquids.

Question 17.
What is an element? Give two examples.
Answer:
A type of matter composed of only one kind of substances where each smallest indivisible part of it has the same properties is called an element. It cannot be subdivided into simpler substances by any physical or chemical means.
Examples: Oxygen, nitrogen, silver.

Question 18.
Write any two characteristics of an element.
Answer:

1. All the atoms/molecules of an element are alike.
2. Atoms/molecules of different elements are different.
3. An element cannot be decomposed into simple substances by any chemical reaction or simple physical process.

Question 19.
What is a compound? Give two examples.
Answer:
A substance produced by a chemical reaction of two or more elements combined in a fixed proportion is called a compound.
Examples: Water, common salt, sugar.

Question 20.
Write any two characteristics of a compound.
Answer:

1. The proportion of constituent elements in a compound is constant.
2. The properties of a compound is different than those of the constituent elements.

Question 21.
What is a mixture? Give two examples.
Answer:
When two or more elements or compounds which do not react chemically with each other are mixed in any proportion, a mixture is formed.
Examples: Air, steel, milk.

Question 22.
Write any two characteristics of a mixture.
Answer:

1. The constituent molecules of mixture are different from each other and notoined by chemical bonds.
2. The proportion of constituent substances are retained in the mixture.
3. The constituents of a mixture can be separated by a physical process.

Question 23.
Classify the following into elements, compounds and mixtures:
Sugar, sodium, nitrogen, sodium carbonate, air, sulphuric acid, mercury, stainless steel, cement, potassium dichromate.
Answer:

• Elements: Sodium, nitrogen, mercury.
• Compounds: Sugar, sodium carbonate, sulphuric acid, potassium dichromate.
• Mixtures: Air, stainless steel, cement.

Question 24.
Write the composition of the following materials by means of chemical formulae and classify them accordingly.

Answer:

Name of the material	Chemical formulae/composition	Type of matter
Water	H2O	Compound
Carbon	C	Element
Oxygen	O2	Element
Air	O2 + N2 + CO2 + other gases	Mixture
Aluminium	Al	Element
Brass	Cu + Zn	Mixture
Carbon dioxide	CO2	Compound

Question 25.
What is meant by an organic compound? Give two examples.
Answer:
The compound which when heated strongly gives black coloured carbon residue is called an organic compound or carbon compound.
Examples: Carbohydrates, proteins, hydrocarbons (petrol, cooking gas).

Question 26.
What is meant by an inorganic compound? Give two examples.
Answer:
Compounds which when heated strongly decomposes to give a residue behind are called inorganic compounds.
Examples: Common salt, rust, blue vitriol, limestone.

Question 27.
What is meant by complex compounds? Give two examples.
Answer:
The molecules of compounds which have a complex structure formed by many atoms and in the centre of this structure metal atoms are also included are called complex compounds.
Examples: Haemoglobin, chlorophyll.

Question 28.
Classify the following compounds into organic compounds, inorganic compounds and complex compounds.
Petrol, common salt, haemoglobin, blue vitriol, cyanocobalamine, cooking gas.
Answer:

• Organic compounds: Petrol, cooking gas
• Inorganic compounds: Common salt, blue vitriol.
• Complex compounds: Cyano cobalamine, haemoglobin.

Question 29.
What is a phase?
Answer:
The part of matter with uniform composition is called a phase.

Question 29.
What is a homogeneous mixture? Give two examples.
Answer:
When all the components of a mixture form one phase, it is called homogeneous mixture.
Example: Solution of blue vitriol in water, solution of sugar in water.

Question 30.
What is heterogeneous mixture? Give two examples.
Answer:
When the components of a mixture are distributed into two or more phases, it is called a heterogeneous mixture.
Examples: Oil and water, sand and water.

Question 31.
Define solution or What is meant by a solution? Give two examples.
Answer:
A homogeneous mixture of two or more substances is called a solution.
Examples: Seawater, sugar, syrup.

Question 32.
What is meant by a solvent?
OR
Define solvent. Give two examples.
Answer:
The component which is present in largest proportion in a solution is called a solvent.
Examples:

1. In sea water, water is the solvent.
2. In tincture iodine, alcohol is the solvent.

Question 33.
What is meant by a solute?
OR
Define solute. Give two examples.
Answer:
The component of a solution which is present in smaller proportion than the solvent is called a solute.
Examples:

1. In sea water, salt is a solute.
2. When a small of amount of sugar is dissolved in water, sugar is the solute in the solution.

Question 34.
Define suspension.
OR
What is meant by a suspension?
Answer:
A heterogeneous mixture of a liquid and a solid is called a suspension. The diameter of the solid particles in a suspension is larger than 10^-4 m.

Question 35.
Define colloid.
Answer:
A heterogeneous mixture in which the particles cannot be seen with a naked eye is called a colloid. The diameter of colloidal particles is around 10^-5 m.

Question 36.
State the characteristics of a solution.
Answer:

1. A solution is clear and transparent, it passes through a filter paper.
2. The particles are not visible to the naked eye.
3. A light can easily pass through it without scattering.
4. A solution is homogeneous mixture.

Question 37.
State the characteristics of a suspension.
Answer:

1. In suspension, the solute particles do not dissolve but remain suspended in it. The liquid and solid components of suspension are separated by filtration.
2. The particles are visible to the naked eye.
3. The particles in suspension are of very small size and scatter a beam of light passing through it.
4. It is heterogeneous in nature.

Question 38.
State the characteristics of a colloid.
Answer:

1. In a colloid, the particles are spread uniformly throughout the solution.
2. The size of the particles is less than that of the particles in a suspension.
3. The particles are not visible to the naked eye.
4. The particles scatter a beam of light.
5. A colloidal solution appears to be homogeneous, but actually it is heterogeneous. ‘

Question 39.
Give two examples of each of the following:

1. Phase
2. Homogeneous matter
3. Heterogeneous matter
4. Homogeneous mixture
5. Heterogeneous mixture
6. Solution
7. Suspension
8. Colloid.

Answer:

1. Phase: (i) Solid: Sodium chloride, blue vitriol, sand, (ii) Liquid: Water, alcohol, (iii) Gas: Chlorine, ammonia.
2. Homogeneous matter: Blue vitriol (solid), a solution of blue vitriol in water (a solid in a liquid), chlorinated water (a gas in a liquid).
3. Heterogeneous matter: Sand and water, sand and alcohol, blue vitriol and sand.
4. Homogeneous mixture: A solution of sodium chloride in water, air, a mixture of water and alcohol.
5. Heterogeneous mixture: A mixture of sand and blue vitriol, sodium chloride and iron filings, oil and water.
6. Solution: Seawater, a solution of sugar in water, a solution of blue vitriol in water.
7. Suspension: A mixture of sand and water, sand and alcohol, muddy water, limestone in water.
8. Colloid: Milk, milk of magnesia, blood, printing ink.

Question 40.
State the different types of solutions. Give one example.
Answer:
The different types of solutions:

• Liquid in liquid. Examples: Vinegar, dilute sulphuric acid.
• Gas in gas. Example: Air.
• Solid in solid. Examples: Brass, stainless steel, bronze.
• Gas in liquid. Examples: Chlorinated water.

Question 41.
Classify the following into homogeneous solution and heterogeneous solution?
Soda water, potassium permanganate solution, milk, alcohol in water, ink solution, wheat flour in water.
Answer:

• Homogeneous solution: Soda water, potassium permanganate solution, alcohol in water.
• Heterogeneous solution: Milk, ink solution, wheat flour in water.

Question 42.
Classify the following into solution, suspension and colloids:
Milk, sand in water, lemonade, smoke, oxygen in water.
Answer:

• Solution: Lemonade, oxygen in water.
• Suspension: Sand in water.
• Colloids: Milk, smoke.

Question 43.
What is meant by molecular formula?
Answer:
The number of atom of each of the constituent elements present in one molecule of a compound is called molecular formula.

Question 44.
What is meant by valency of an element?
Answer:
The capacity of an element to combine with another element is known as valency.

Question 45.
Deduce the molecular formulae of the compound obtained from the following pairs of elements by the cross multiplication method.
Answer:
Question a.
H (Valency 1) and O (Valency 2)
Answer:
Step 1: Write the symbols of the constituent elements.

Step 2: Write the valency below the respective elements.

Step 3: Cross multiply the valencies.

∴ The molecular formula: H₂O.

Question b.
Fe (Valency 2) and S (Valency 2)
Answer:
Step 1: Write the symbols of the constituent elements.

Step 2: Write the valency below the respective elements.

Step 3: Cross multiply the valencies.

The molecular formula: FeS

Question c.
The valencies of the atom H, O and N are 1, 2 and 3 respectively. The molecular formulae of these gaseous elements are H₂, O₂ and N₂ respectively. How many chemical bonds are there in each of these molecules?
Answer:
Chemical bond in H₂ is one

Chemical bond in O₂ are two

Chemical bond in N₂ are three

Give scientific reasons:

Question 1.
Hydrogen is an element.
Answer:

1. Hydrogen has only one kind of atoms.
2. Hydrogen cannot be further divided into new substances by any physical or chemical means. Therefore, hydrogen is an element.

Question 2.
Water is a compound.
Answer:

1. The composition of water is same throughout i.e., the elements hydrogen and oxygen whose atoms are combined in the ratio by weight is always 1 : 8.
2. The properties of water are totally different from those of its constituents, i.e. hydrogen and I oxygen. Hence, water is a compound.?

Question 3.
Air is a mixture.
Answer:

1. Air contains nitrogen, oxygen and other gases.
2. These gases retain their original properties. They do not combine with each other. Hence, air is a mixture.

Question 4.
A potassium permanganate solution in water is a homogeneous mixture, while a mixture of oil and water is a heterogeneous mixture.
Answer:

1. In a potassium permanganate solution in water the constituents, i.e. potassium permanganate and water are uniformly mixed throughout.
2. The properties and composition of a homogeneous mixture are the same throughout the mixture. Hence, potassium permanganate solution is a homogeneous mixture.
3. In a mixture of oil and water, the constituents are not uniformly mixed throughout the mixture.
4. The properties and composition of a heterogeneous mixture are not the same throughout the mixture. Hence, a mixture of oil and water is a heterogeneous mixture.

Question 5.
Milk is a colloid.
Answer:

1. In milk, the particles are spread uniformly throughout and due to very small size of the particles, milk appears to be homogeneous.
2. But milk is a colloid solution and it is a heterogeneous mixture. Here, milk is liquid particles of proteins and fats having a diameter around 10^-5 m dispersed in the aqueous medium. Hence, a milk is a colloid.

Distinguish between the following:

Question 1.
Answer:

Solids	Liquids
1.  Solids have a definite shape.	1. Liquids have no definite shape.
2.  They have very low compressibility.	2. They have moderate compressibility.
3. In solids, the intermolecular forces are very strong. E.g. iron, stone, salt.	3. In liquids, the intermolecular forces are moderately strong. E.g., water, alcohol, mercury.

Question 2.
Liquids and Gases:
Answer:

Liquids	Gases
1. Liquids have a definite volume.	1. Gases do not have a definite volume.
2. They have moderate compressibility.	2. They have high compressibility.
3. In liquids, the intermolecular forces are moderately strong. E.g. water, alcohol, mercury.	3. In gases, the intermolecular forces are very weak. E.g. oxygen, air.

Question 3.
Solids and Gases:
Answer:

Solids	Gases
1. Solids have a definite shape and volume.	1. Gases have neither a definite shape nor a definite volume.
2. They have incompressibility.	2. They have high compressibility
3. In solids, the intermolecular forces are very strong. E.g. iron, stone.	3. In gases, the intermolecular forces are very weak. E.g. hydrogen, oxygen.

Question 4.
Element and Compound:
Answer:

Element	Compound
1. An element consists of only one kind of substance.	1. A compound is formed from two or more elements.
2. An element cannot be further divided into simpler substances by any physical or chemical means. E.g. iron, copper, oxygen.	2. A compound can be further divided into simple substances by chemical means. E.g. common salt, sugar, water.

Question 5.
Compound and mixture:
Answer:

Compound	Mixture
1. A compound is made of two or more elements chemically combined in a fixed proportion.	1. A mixture is obtained by just mixing two or more substances in any proportion.
2. A compound can be further subdivided into simpler substances by chemical means only.	2. A mixture can be further subdivided into simpler substances by simple physical processes.
3. The properties of a compound are entirely different from the properties of its constituents. E.g. common salt, sugar, water.	3. The constituents of a mixture retain their original properties. E.g. air, seawater.

Question 6.
Colloidals and Suspension:
Answer:

Colloidals	Suspension
1. The particles in a colloidal solution can pass through a filter paper.	1. The particles in a suspension cannot pass through a filter paper.
2. The colloidal particles cannot be seen with a naked eye.	2. The suspended particles can be seen with a naked eye.
3. The diameter of solid particles is around 10-5 m	3. The diameter of the solid particles is larger than 10-4 m.

Activity-based questions:

Activity 1:
Apparatus: Plastic jar, balloon, thread
Material: Mustard seeds
Take some mustard seeds in a transparent plastic jar. Thread a long thread at the centre of a big balloon by means of a needle and tie it tight. Stretch this rubber diaphragm and fix it on the mouth of the jar by means or a rubber band. Pull the diaphragm up and down with the help of the thread first slowly, then with moderate force and then vigorously. Record your observation in the following table.

Question 1.

Answer:

Activity 2:
Apparatus: Hammer, sand paper
Material: Iron nail, copper wire, aluminium wire, coal.
Take the following objects: iron nail/sheet, copper wire, aluminium wire, a piece of coal. Rub each object on a fresh surface of sandpaper and observe. Hammer each object with force. (Take care not to hurt yourself.) Record your observations in the following table.
The objects in the activity are made of the elements iron (Fe), copper (Cu), aluminium (Al) and carbon (C); respectively. Fill the following table on the basis of the observations obtained on doing the above two tests on each of the objects.

Question 1.

Answer:

Activity 3:
Apparatus: Evaporating dish, tripod stand, burner, etc.
Chemicals: Camphor, washing soda, blue vitriol, sugar, glucose, urea.
Keep the evaporating dish on the tripod stand. Take some camphor in the evaporating dish. Heat the camphor in the dish strongly with the help of a burner. Find out what remains behind in the evaporating dish. Repeat the above procedure using limestone, washing soda, blue vitriol, sugar, glucose and urea instead of camphor. Record your observation in the following table. (Do this activity carefully under the supervision of your teacher, as some of the powders may catch fire.)

Question 1.

Answer:

Activity 4: (Textbook page 43)
Apparatus: Beakers, rods
Chemicals: Blue vitriol, sand, water, etc.
Take three beakers. Take a little sand and water in the first beaker. Take some crystals of blue vitriol and water in the second beaker. Take some blue vitriol and sand in the third beaker. Stir the materials in all the three beakers and observe. Record your observation in the following table.

Question 1.

Answer:

Question 2.
In the above activity after stirring a homogeneous mixture is formed in only one beaker? Which is that?
Answer:
The homogeneous mixture is formed in the second beaker. It is a solution of blue vitriol.

Activity 5:
Apparatus: Beaker, conical flask, funnel, filter paper
Chemicals: Common salt, saw dust, milk.
Procedure: Take three beakers. Take 10 g common salt in the first beaker, 10 g saw dust in the second beaker and 10 ml milk in the third beaker. Add 100 ml water to all the three beakers and stir. Which of the mixtures shows separate water phase ? Place the three beakers in front of a vertically held paper and pass a laser beam through the beakers from the opposite side. (Use the laser beam under the guidance of teacher.) At the same time observe what appears on the paper in front of the beaker. Also look at the beaker from the side. Arrange three filtration assemblies using conical flask, funnel and filter paper for doing filtration. Stir the mixtures in the three beakers and carry out filtration. Record all the observations in the following table.

Question 1.

Answer:

Activity 6: (Textbook page 44)
Apparatus: Horseshoe magnet, evaporating dish, burner. Chemicals: Iron filings, sulphur.
Procedure: Take two evaporating dishes. Take 7 g iron filings in the first dish and 4 g sulphur in the second. Take a horseshoe magnet near the matter in both the dishes and observe. Transfer the entire iron filings from the first dish to the second, stir with a glass rod and observe by taking the horseshoe magnet near the matter. Also observe the colour of the matter. Now heat the matter in the second dish for a while and let it cool. Observe the colour change, if any in the matter and observe whether there is any effect of the horseshoe magnet on it. Record all the observation in the following table.

Question 1.

Answer:

Complete the following:

Question 1.
Complete the table:

Answer:
Molecular formulae of some compounds are given in the following table. Use these to fill in the gaps in the table.

Question 2.
Complete the table:

Answer:
Molecular formulae of various compounds formed by hydrogen with other elements are given in the following table. From that, deduce the valencies of the concerned elements.

Question 3.
Complete the table:

Answer:

8th Std Science Questions And Answers:

• Living World and Classification of Microbes Class 8 Questions And Answers
• Health and Diseases Class 8 Questions And Answers
• Force and Pressure Class 8 Questions And Answers
• Current Electricity and Magnetism Class 8 Questions And Answers
• Inside the Atom Class 8 Questions And Answers
• Composition of Matter Class 8 Questions And Answers
• Metals and Nonmetals Class 8 Questions And Answers
• Pollution Class 8 Questions And Answers
• Disaster Management Class 8 Questions And Answers
• Cell and Cell Organelles Class 8 Questions And Answers

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 14 Measurement and Effects of Heat Notes, Textbook Exercise Important Questions and Answers.

Std 8 Science Chapter 14 Measurement and Effects of Heat Question Answer Maharashtra Board

Class 8 Science Chapter 14 Measurement and Effects of Heat Question Answer Maharashtra Board

1. A. Whom should I pair with?

Question a.

Group ‘A’	Group ‘B’
1. Temperature of a healthy human body	a. 296 K
2. Boiling point of water	b. 98.6 °F
3. Room temperature	c. 0°C
4. Freezing point of water	d. 212 °F

Answer:

Group ‘A’	Group ‘B’
1. Temperature of a healthy human body	b. 98.6 °F
2. Boiling point of water	d. 212 °F
3. Room temperature	a. 296 K
4. Freezing point of water	c. 0°C

B. Who is telling the truth?

Question a.
The temperature of a substance is measured in joule.
Answer:
False. (The temperature of a substance is measured in °C or °F or K.)

Question b.
Heat flows from an object at higher temperature to an object at lower temperature.
Answer:
True.

Question c.
The joule is the unit of heat.
Answer:
True.

Question d.
Objects contract on heating.
Answer:
False. (In general, objects expand on heating. There are some exceptions to this, you will learn about them in Standard X.)

Question e.
Atoms of a solid are free.
Answer:
False. (Atoms of a solid are bound to each other due to the forces acting between them.)

Question f.
The average kinetic energy of atoms in a hot object is less than the average kinetic energy of atoms in a cold object.
Answer:
False. (The average kinetic energy of atoms in a hot object is more than the average kinetic energy of atoms in a cold object.)

C. You will find if you search.

Question a.
A thermometer is used to measure ……………. .
Answer:
A thermometer is used to measure temperature.

Question b.
The apparatus used to measure heat is called a ………… .
Answer:
The apparatus used to measure heat is called a calorimeter.

Question c.
Temperature is the measure of the …………. kinetic energy of the atoms in a substance.
Answer:
Temperature is the measure of the average kinetic energy of the atoms in a substance.

Question d.
The heat contained in a substance is the measure of the ………….. kinetic energy of the atoms in the substance.
Answer:
The heat contained in a substance is the measure of the total kinetic energy of the atoms in the substance.

2. Nishigandha kept a vessel containing all the ingredients for making tea in a solar cooker. Shivani kept a similar vessel on a stove. Whose tea will be ready first and why?

Question a.
Nishigandha kept a vessel containing all the ingredients for making tea in a solar cooker. Shivani kept a similar vessel on a stove. Whose tea will be ready first and why?
Answer:
Shivani’s tea will be ready first.
Reason: In a given time, the amount of heat received by the vessel on a stove is far greater than that received by the vessel kept in a solar cooker.

3. Write brief answers.

Question a.
Describe a clinical thermometer. How does it differ from the thermometer used in a laboratory?
Answer:
A clinical thermometer has a narrow stem and a long bulb filled with mercury (or alcohol). There is a small constriction in the stem above the bulb. When the bulb of the thermometer is held in the armpit or the mouth of a patient, the mercury (or alcohol) in the bulb rises in the stem.

When it is taken out of the patient’s body, the small constriction does not allow the mercury (or alcohol) from the stem to retreat into the bulb. Thus, this arrangement enables us to read the temperature of the patient’s body at ease after the removal of the thermometer from his body.

The clinical thermometer is graduated from 35 °C to 42 °C (or from 95 °F to 108 °F). At 37 °C (98.6 °F), there is a red arrow mark which indicates the temperature of a healthy person.

The thermometer used in a laboratory has wider range and does not have constriction like a clinical thermometer.

Question b.
What is the difference between heat and temperature?
Answer:
Heat is related to the total kinetic energy of the atoms in a substance while temperature is related to the average kinetic energy of the atoms in the substance. Heat flows from a body at higher temperature to a body at lower temperature. Thus, temperature is a quantity that determines the direction of flow of heat. It is a quantitative measure of the degree of hotness or coldness of a body.

Higher temperature does not mean higher heat content. Suppose a vessel A contains 2 litres of water at 90 °C and a vessel B contains 100 ml of water at 91 °C. Then the heat content of water in A is greater than that of water in B, but the temperature of water in B is higher than that of water in A.

Units of heat:
Heat is usually expressed in calorie, kilocalorie and joule. It can also be expressed in erg as heat is a form of energy.

Question c.
Explain the construction of a calorimeter. Draw the necessary figure.
Answer:

Figure shows the construction of a calorimeter. Like a thermo flask, a calorimeter has two vessels. The inner vessel, made of copper, is (practically) thermally isolated from the surroundings. The outer vessel is made of wood and is covered with a heat resistant lid. The lid has two holes, one for the thermometer and the other for the stirrer. The inner and outer surfaces of the inner vessel are polished for minimizing exchange of heat with the surroundings by radiation. A heat resistant ring covers the inner vessel.

Question d.
Explain why rails have gaps at specific distances.
Answer:
The rails expand in summer due to increase in the temperature of the atmosphere. Also, they expand due to rise in temperature caused by the friction between the rails and the wheels of the running train. If there is no gap between successive rails of a railway line, the rails would bend due to expansion. This bending and twisting of the rails would cause accidents. Hence, a gap is kept between successive rails of a railway line to make provision for their expansion in summer.

Do you know:

Have you seen rails? They are not continuous. A small gap is kept between them at regular intervals. This is shown in the figure. This is kept to accommodate the change in the length of the rails with change in temperature. If this gap is not kept, then the rail will get distorted due to expansion in summer which may lead to accidents.

Similar to rails, the length of bridges can also increase due to expansion in summer. The length of the 18 km long great belt bridge in Denmark increases by 4.7 m in summer. Therefore, provision in made in the construction of the bridges to accommodate this expansion.

Question e.
Explain with the help of formulae the expansion coefficients of liquid and gas.
Answer:
1. A liquid is held in a container. When it is heated, both the container and the liquid expand. The expansion of the container is usually very small compared to that of the liquid in it. Often, it can be ignored.
Suppose a liquid is heated so that its temperature rises by ΔT (very small) and its volume increases from V₁ to V₂. Experimentally, it is found that the increase in volume, V₂ – V₁, is proportional to V₁ and ΔT.
Hence, (V₂ – V₁) α V₁ΔT.
∴ V₂ – V₁ = β V₁ΔT, where β is a constant of proportionality called the volumetric expansion coefficient of the liquid.
β = \(\frac{V_{2}-V_{1}}{V_{1} \Delta T}\).
It is expressed in per °C.
We have V₂ = V₁ + βV₁ΔT = V₁(1 + β ΔT).
β is the increase in the volume of a liquid per unit original volume per unit rise in its temperature.

2. A gas is enclosed in a container. When it is heated at constant pressure, both the container and the gas expand. Suppose a gas is heated at constant pressure so that its temperature rises by ΔT (very small) and its volume increases from V₁ to V₂. Experimentally, it is found that the increase in volume, V₂ – V₁ is proportional to V₁ and ΔT. Hence, (V₂ – V₁) a V₁ΔT.
∴ V₂ – V₁ = β V₁ΔT, where β is a constant of proportionality, the volumetric expansion coefficient, called the constant pressure expansion coefficient.
β = \(\frac{V_{2}-V_{1}}{V_{1} \Delta T}\).
It is expressed in per °C.
We have V₂ = V₁ + βV₁T = V₁(1 + β ΔT).
β is the increase in the volume of a gas per unit original volume per unit rise in its temperature when the pressure is kept constant.

4. Solve the following examples.

Question a.
What must be the temperature in Fahrenheit so that it will be twice its value in Celsius?
Solution:
Data: F = 2 C ∴ C = F/2, F = ?

∴ F – 0.9 F = 32
∴ 0.1 F = 32
∴ F = \(\frac{32}{0.1}\) = 320 °F.

Question b.
A bridge is made from 20 m long iron rods. At temperature 18 °C, the distance between two rods is 0.4 cm. Up to what temperature will the bridge be in good shape?
Solution:
Data: l₁ = 20 m, l₂ – l₁ = 0.4 cm
= 4 × 10^-3 m, T_i = 18 °C, λ for iron = 11.5 × 10^-6/°C
l₂ – l₁ = λl₁ Δt

= \(\frac{400}{23}\) °C = about 17.39 °C
Now, ΔT= T_f – T_i ∴ T_f = T_i + ΔT
∴ T_f = 18 °C + 17.39 °C = 35.39 °C.
The bridge will be in good shape up to 35.39 °C.

Question c.
At 15 °C the height of Eiffel Tower is 324 m. If it is made of iron, what will be the increase in length in cm, at 30 °C?
Solution:
Data: ΔT = 30 °C – 15 °C = 15 °C, l₁ = 324 m, λ for iron = 11.5 × 10^-6/°C l₂ – l₁ = λl₁ ΔT
= 11.5 × 10^-6/°C × 324 m × 15 °C
= 55890 × 10^-6 m
= 55890 × 10^-6 × 10² cm
= 55890 × 10^-4 cm
= 5.589 cm (nearly 5.6 cm)
This is the increase in the length, i.e., the increase in the height of Eiffel Tower.

Question d.
Two substances A and B have specific heats c and 2 c respectively. If A and B are given Q and 4Q amounts of heat respectively, the change in their temperatures is the same. If the mass of A is m, what is the mass of B?
Solution:
Data: c (A) = c, c (B) = 2c,
Q (A) = Q, Q (B) = 4Q, ΔT same,
m (A) = m, m (B) = ?

∴ m(B) = 2m.
This is the mass of B.

Question e.
When a substance having mass 3 kg receives 600 cal of heat, its temperature increases by 10 °C. What is the specific heat of the substance?
Solution:
Data: m = 3 kg = 3000 g,
Q = 600 cal, ΔT = 10 °C, c = ?
Q = mcΔT

= 0.02 cal/(g.°C)
This is the specific heat of the substance.

Can you recall?

Question a.
Which sources do we get heat from?
Answer:

1. Sun
2. earth
3. fuels like wood, coal, petrol
4. electricity
5. atomic energy
6. air.

Question b.
How is heat transferred?
Answer:
Heat is transferred by conduction, convection and radiation.

Question c.
Which effects of heat do you know?
Answer:
Expansion, change of state, rise in temperature, emission of light, burning.

Question d.
Some effects of heat are shown in Fig. What are they?

Answer:
Rise in temperature/boiling, melting, burning, expansion.

Question e.
What are potential and kinetic energies?
Answer:
The energy stored in a body because of its specific state or position is called its potential energy. The energy possessed by a body because of its motion is called it’s kinetic energy.

Project:

Question a.
Collect information about bimetallic strips and discuss in your class how a fire alarm is made using it.

Class 8 Science Chapter 14 Measurement and Effects of Heat Important Questions and Answers

Rewrite the following statements by selecting the correct options:

Question 1.
32°F is equal to …………….. .
(a) 212 °C
(b) 212 K
(c) 273.15 K
(d) 273.15 K
Answer:
32°F is equal to 273.15 K.

Question 2.
-40°C is equal to ………….. .
(a) -40 °F
(b) 40 °F
(c) -8°F
(d) 40K
Answer:
-40°C is equal to -40 °F.

Question 3.
The boiling point of water is ………… .
(a) 212 K
(b) 212 °F
(c) 273.15 K
(d) 32 °F
Answer:
The boiling point of pure water is 212 °F.

Question 4.
Specific heat is expressed in …………… .
(a) J/(kg. °C)
(b) kg/(J C)
(c) J/kg
(d) cal/g
Answer:
Specific heat is expressed in J/(kg.°C).

Question 5.
The freezing point of water is …………….. .
(a) OK
(b) 212°F
(c) 32°F
(d) 0°F
Answer:
The freezing point of water is 32 °F.

Find the odd one out and give the reason:

Question 1.
0°C, 32°F, 273.15K, 212°F.
Answer:
212 °F. This is the boiling point of water; others correspond to the freezing point of water.

Question 2.
373.15 K, 100 °C, 212 °F, 32 °F.
Answer:
32 °F. This is the freezing point of water others correspond to the boiling point of water.

Answer the following questions in one sentence each:

Question 1.
Name the biggest source of heat received by the earth.
Answer:
The Sun is the biggest source of heat received by the earth.

Question 2.
What is the relation between the temperature in °C and the temperature in °F?
F – 32 °C
Answer:
\(\frac{F-32}{9}\) = \(\frac{C}{5}\), where C denotes temperature in °C and F denotes temperature in °F.

Question 3.
Name the quantity expressed in cal/(g.°C).
Answer:
Specific heat is expressed in cal/(g.°C).

Question 4.
State the formula for the coefficient of linear expansion of a solid substance.
Answer:
λ = (l₂ – l₁]) / (l₁ ΔT).

Question 5.
State the formula for the coefficient of areal expansion of a solid.
Answer:
σ = (A₂ – A₁)/ (A₁ ΔT).

Question 6.
State the formula for the volumetric expansion coefficient of a solid.
Answer:
β = (V₂ – V₁) / (V₁ ΔT).

Question 7.
State the relation between λ and σ (if ΔT is very small).
Answer:
σ = 2 λ.

Answer the following questions:

Question 1.
How is heat generated in the Sun?
Answer:
In the interior of the Sun, at the centre, hydrogen nuclei fuse together to form helium nuclei. A lot of heat is generated in this process.

Question 2.
What is geothermal energy?
Answer:
Heat within the interior of the earth, e.g., that coming from the molten core of the earth, is called geothermal energy.
[Note: Volcanoes, geysers, hot springs are sources of this energy.]

Question 3.
What is atomic energy or nuclear energy?
Answer:
Energy released or obtained in nuclear fission or nuclear fusion is called atomic energy or nuclear energy.

Question 4.
Name three sources of chemical energy.
Answer:
Fuels like wood, coal and petrol are sources of chemical energy.

Try this:

1. Take three similar vessels. Let us call them ‘A’, ‘B’ and ‘C’.
2. Fill A with hot water and B with cold water. Put some water from A and B in C.
3. Dip your right hand in A and left hand in B and keep them immersed for 2 to 3 minutes.
4. Now dip both the hands in C. What do you feel?

Even though, both the hands are dipped in water in the same vessel, i.e., water at the same temperature, your right hand will find the water to be cold while the left hand will find it to be hot. What is the reason for this? Think about it.
Answer:
The right hand finds the water cold because it loses heat to water in C. The left-hand feels the water hot because it gains heat from water in C. (This shows that we cannot determine the temperature of an object accurately by simply touching it)

Question 5.
State the units in which temperature is expressed.
OR
What are the units of temperature?
Answer:
Temperature is expressed in °C (degree Celsius), °F (degree Fahrenheit) and K (kelvin).

Question 6.
Draw diagrams to illustrate motion of atoms in a gas and a solid. Also explain the type of motion.
Answer:
Atoms of a gas are comparatively free and move at random. Figures a and b show the velocities of atoms in a gas at high and low temperature, respectively. The direction and the length of the arrows show respectively the direction and the magnitude of the velocity of the atoms. The velocity of atoms in the gas at higher temperature is larger in magnitude than the velocity of atoms in the gas at lower temperature.

The velocities of atoms in a solid are shown by arrows in Fig. (c). The atoms in a solid are tied to one another because of the forces acting between them. This is indicated by drawing springs between adjacent atoms. Because of heat, they oscillate around their fixed mean positions. The higher the temperature of a solid, the greater is their velocity of oscillation.

Try this:

1. Take two steel vessels A and B of the same size.
2. Fill some water in A and double that amount in B. Make sure that the water in both vessels is at the same temperature.
3. Raise the temperatures of water in both vessels by 10 CC using a spirit lamp. Did it take the same time to increase the temperature in the two vessels?
Answer:
No.
You must have required more time to raise the temperature of water in B. This means that for the same increase in temperature, you had to give more amount of heat to B. Thus, even though the water in A and in B have the same temperature, the amount of heat in B is more than that in A.

Question 7.
How are the different units of temperature-related?
Answer:
1. The SI unit of temperature is the kelvin (K). Temperature is also expressed in °C (degree Celsius) and °F (degree Fahrenheit).

Here, C denotes temperature in °C, F denotes temperature in 0F and K denotes temperature in K (kelvin).
[Notes: (i) The unît degree Celsius is named in honour of Anders Celsius (1701 – 44) Swedish astronomer. He devised a temperature scale in 1742.
(ii) The unit degree Fahrenheit is named in honour of Gabriel Daniel Fahrenheit (1686 – 1736) German physicist. He developed the mercury thermometer and devised the temperature scale.
(iii) The unit kelvin is named in honour of William Thomson, 1st Baron Kelvin (of Largs) (1824 – 1907) British physicist and electrical engineer. He made significant contribution in thermodynamics and electromagnetic theory. He proposed a scale of temperature now known as the Kelvin scale or thermodynamic scale.]

Question 8.
What is a liquid (mercury or alcohol) thermometer?
Answer:
A thermometer in which the change in the volume of a liquid (mercury or alcohol) with temperature is used for measurement of temperature is called a liquid thermometer.

Question 9.
Why has mecury been replaced by alcohol in a thermometer?
Answer:
Because mercury is harmful for us, it has been replaced by alcohol in a thermometer.
[Notes: (i) Mercury thermometers are still widely used in laboratories in schools and colleges.
(ii) A thermometer is a device to measure temperature. A thermometer containing mercury in its bulb is called a mercury thermometer. There are other types of thermometer such as a thermocouple thermometer, a platinum resistance thermometer, a thermistor thermometer, etc.]

Question 10.
Describe the construction of mercury thermometer.
Answer:

1. For constructing a thermometer, a thick-walled capillary tube with a uniform bore is taken. The tube has a thin-walled glass bulb at one end.
2. The bulb and a small part of the tube is filled with mercury. The other end of the tube is sealed after removing air from it.
3. The tube is then calibrated and the temperature of the substance can be read with it.

[Note: The range of an alcohol thermometer is different from that of a mercury thermometer.]

Question 11.
How does a mercury thermometer work?
Answer:
1. When the bulb of the thermometer is brought in contact with the substance whose temperature is to be measured, there is an exchange of heat between the substance and the mercury in the bulb.

2. The mercury expands or contracts depending upon whether it gains heat or loses heat. Accordingly there is a rise or fall of the level of mercury in the tube of the thermometer indicating the temperature of the substance when the mercury and the substance are in thermal equilibrium.

Question 12.
Why is the bore of a thermometer made very small?
Answer:
The bore of a thermometer is made very small so that even a slight variation in the temperature may cause noticeable variation in the mercury level in the tube of the thermometer. As a result, the sensitivity of the thermometer increases and even small changes in the temperature can be recorded.

Question 13.
Why does the bulb of a thermometer have a thin glass wall?
Answer:
The bulb of a thermometer is made of a thin glass so that it can easily conduct the heat from the substance in contact with the mercury in the bulb.

Question 14.
Why does a thermometer usually break at the bulb?
Answer:
The bulb of a thermometer has a thin glass wall. Therefore, a thermometer usually breaks at the bulb.

Question 15.
Explain why the capillary tube of a clinical thermometer has a constriction.
Answer:
When the temperature of a patient is measured with a clinical thermometer, the mercury in the bulb expands and rises in the tube. The small constriction in the thermometer prevents the mercury thread from retreating into the bulb. Thus, the patient’s temperature can be read at ease after removing the thermometer from his body.

Question 16.
Explain why a clinical thermometer should not be washed with hot water.
Answer:
A clinical thermometer is constructed for the purpose of recording the temperature of the human body. Hence, its stem is such that it can allow the mercury to rise up to a temperature of about 42 °C. The temperature of hot water may be more than 42 °C. Therefore, when a clinical thermometer is washed with hot water, mercury will not have enough room for expansion and the thermometer would break.

[Note: (1) A digital thermometer has a sensor that detects the heat coming out from the body directly and displays the temperature.
(2) The maximum-minimum temperature has two scales, one against each arm of the thermometer. One scale indicates the maximum temperature reached (generally during the day) and the other scale indicates the minimum temperature reached (generally during the night).]

Question 17.
Explain what happens when there is exchange of heat between two objects.
Answer:
When there is an exchange of heat between a hot object and a cold object, the temperature of the hot object falls and the temperature of the cold object rises. If this system of two objects is isolated from the surroundings so that no heat enters or leaves the system, then, heat lost by the hot object = heat gained by the cold object. After some time, the average kinetic energies of the atoms in the two objects become equal, i.e., the two objects attain the same temperature.

Question 18.
Define specific heat.
Answer:
The specific heat of an object (substance) is the amount of heat required to increase the temperature of unit mass of that substance through one degree.

Question 19.
Write the formula for specific heat. Hence, obtain its unit.
Answer:
Specific heat of a substance,
c = \(\frac{Q}{m\left(T_{\mathrm{f}}-T_{\mathrm{i}}\right)}\), where m is the mass of the substance and Q is the amount of heat supplied to the substance to increase its temperature from Ti to Tf.

The SI unit of heat is the joule (J), that of mass is kg and that of temperature is kelvin (K).
∴ The SI unit of specific heat = \(\frac{\mathrm{J}}{(\mathrm{kg} \cdot \mathrm{K})}\)
[Note: Specific heat is also expressed in J/(kg°C) and cal/(g°C).]

Question 20.
The specific heat of aluminium is 0.21 cal/(g°C). What do you understand by this statement?
Answer:
The amount of heat required to raise the temperature of 1g of aluminium through 1 °C is 0.21 cal.

Question 21.
The specific heat of gold is 0.03 cal/(g°C). Express it in J/(kg°C).
Answer:
1 cal = 4.18 joules (J)
1 gram = 10^-3 kg
∴ 1 cal/(g°C) = 4.18 J/(10-3 kg°C)
= 4180 J/(kg°C)
∴ 0.03 cal/(g°C) = 0.03 × 4180 J/(kg°C)
= 125.4 J/(kg °C)

Question 22.
Arrange the following materials in increasing order of specific heat: aluminium, gold, iron, water.
Answer:
Gold, iron, aluminium, water.

Use your brain power!

Question 1.
Why does your mother put folded cloth strips soaked in cold water on your forehead when you have high fever?
Answer:
To remove heat quickly from our body and thereby lower the temperature of the body (as water has high specific heat).

Question 2.
Why is the calorimeter made of copper?
Answer:
Copper is a good conductor of heat and has low specific heat. Also copper is durable and not highly reactive.

Question 23.
How will you determine the specific heat of a metal using a calorimeter?
OR
Describe the experiment to determine the specific heat of iron using an iron ball, calorimeter and water.
Answer:
1. Find the mass (m_i) of the iron ball.
2. Find the total mass (m_c) of the calorimeter (metal container) and the stirrer.

3. Fill the calorimeter to two-thirds of its capacity with water and find its mass (m’_c) along with the stirrer. Hence, find the mass (m_w) of the water in the calorimeter (m_w = m’_c – m_c).
4. Keep the calorimeter in the wooden box and note the temperature (T₁) of the water in the calorimeter with the thermometer.

5. Suspend the iron ball in water in a beaker. Heat the beaker so that the water starts boiling. Note the temperature (T₂) of the boiling water.
6. Transfer the iron ball quickly to the calorimeter and cover the calorimeter with the lid immediately.

7. Stir the water in the calorimeter gently and continuously for uniformity of temperature and note the maximum temperature (T₃) attained by the mixture.

8. Find the specific heat capacity of iron using the following formula:
heat lost by the iron ball = heat gained by the calorimeter, stirrer and water
[assuming that there is no exchange of heat between the system (calorimeter, stirrer, water and iron ball) and the surroundings].

where cc = specific heat of the material of the calorimeter and stirrer and c_w = specific heat of water.
Hence, the specific heat of iron (c_i) can be determined when other quantities are known.

Question 24.
Why is a calorimeter used in the study of the exchange of heat between a solid and liquid or between two liquids?
Answer:
When a hot body is kept in contact with a cold one, there is an exchange of heat between the two. Hence, the temperature of the hot body decreases while that of the cold body increases till both the bodies attain the same temperature. During this process, if there is no exchange of heat between the surrounding and the bodies, the heat lost by the hot body is equal to the heat gained by the cold body.

As a calorimeter ensures that there is hardly any exchange of heat between the contents of the calorimeter and the surroundings, the calorimeter is used in the study of the exchange of heat between a solid and liquid or between two liquids.

Question 25.
Explain why the inside and outside of a calorimeter are polished.
Answer:

1. A calorimeter is used for the measurement of heat. Hence, it is essential to minimize the exchange of heat between the vessel and the surroundings.
2. A polished surface is a good reflector of heat. Hence, by polishing the inside and outside of a calorimeter, the loss or gain of heat due to radiation is reduced to a considerable extent.

Question 26.
Explain with the help of a formula the coefficient of linear expansion of a solid.
Answer:
Suppose a rod of length l₁ at temperature T₁ is heated to temperature T₂ such that ΔT = T₂ – T₁ is very small. Let l₂ be the length of the rod at temperature T₂.
Experimentally, it is found that the increase in the length of the rod (linear expension), l₂ – l₁, is proportional to l₁ and ΔT. Therefore, (l₂ – l₁) α, l₁ΔT
∴ l₂ – l₁ = λl₁ ΔT, where X is the constant of proportionality, called the coefficient of linear expansion of the solid.
λ = \(\frac{l_{2}-l_{1}}{l_{1} \Delta T}\) It is expressed in per °C.
We have l₂ – l₁ + λΔT = l₁ (1 + λΔT).

Question 27.
Define coefficient off linear explansion of a solid. Write the formula for it and obtain its unit.
Answer:
Coefficient of linear expansion of a solid is defined as the increase in the length of a rod of the solid per unit initial length per unit rise in its temperature.
Coefficient of linear expansion of a solid,
λ = \(\frac{l_{2}-l_{1}}{l_{1} \Delta T}\), where l₁ and l₂ are respectively the initial and final length of the rod of the solid and ΔT is the rise in its temperature.
Unit of λ = \(\frac{\text { unit of length }}{\text { unit of length } \times \text { unit of temperature }}\)

Question 28.
The coefficient of linear expansion of silver is 18 ×10^-6 per °C. What do you understand by this statement?
Answer:
If the temperature of a silver rod of length lm is increased by 1 °C, the length of the rod increases by 18 × 10^-6 m.

Question 29.
Explain with the help of a formula the coefficient of areal expansion of a solid.
Answer:
Suppose a sheet of a solid with surface area A₁ at temperature T₁ is heated to temperature T₂ such that ΔT = T₂ – T₁ is very small. Let A₂ be the surface area of the sheet at temperature T₂. Experimentally, it is found that the increase in the surface area of the sheet (areal expansion), A₂ – A₁, is proportional to A₁ and ΔT. Therefore,
(A₂ – A₁)α A₁ ΔT
∴ A₂ – A₁ = σ Al₁ΔT, where a is the constant of proportionality, called the coefficient of areal expansion of the solid.
σ = \(\frac{A_{2}-A_{1}}{A_{1} \Delta T}\). It is expressed in per °C.
We have A₂ = A₁ + σA₁ΔT = A₁ (1 + σΔT).
σ is the increase in the area of a solid per unit original area per unit rise in its temperature.
[Note: Consider a thin square metal plate of length l. Area of one face of the plate = A = l₂. Suppose the plate is heated so that the rise in its temperature is ΔT (assumed to be very small). Then in the usual notation, Δl = l λΔT and ΔA =
AσΔT = l² σΔT. Also, ΔA = (l + Δl)² – l² = l² + 2l.Δl + Δl² – l2 = 2l.Δl + Δl² . As Δl² << 2l.Δl, we can write ΔA = 2l.Δl(approximately)
∴ ΔA = 2l(l λΔT) = 2l²λΔT but ΔA = l²σΔT
∴ σ = 2.λ]

Question 30.
Explain with the help of a formula the volumetric expansion coefficient of a solid.
Answer:
Suppose a solid with volume V₁ at temperature T₁ is heated to temperature T₂ such that T₁ = T₂ – T₁ is very small. Let V₂ be the volume of the solid at temperature T₂. Experimentally, it is found that the increase
in the volume of the solid (volumetric expansion), V₂ – V₁, is proportional to V₁ and ΔT. Therefore, (V₂ – V₁)α V₁ΔT.
∴ V₂ – V₁ = βV₁ΔT, where β is the constant of proportionality, called the volumetric expansion coefficient of the solid.
β = \(\frac{V_{2}-V_{1}}{V_{1} \Delta T}\) It is expressed in per °C.
We have V₂ = V₁ + βV₁ ΔT = V₁ (1 + βΔT).
is the increase in the volume of a solid per unit original volume per unit rise in its temperature.
[Note: It can be shown that β = \(\frac{3}{2}\) σ 3λ.]

Question 31.
Obtain an expression for the variation of the density of a solid with temperature.
Answer:

As the temperature increases, density decreases.

Use your brain power!

Question 1.
Which use of the expansion of liquids in daily life do you know?
Answer:
Use of a thermometer. The effect of heat on water is somewhat different from that for other liquids. This is called anomalous behaviour of water. We are going to learn about it in higher standard.

Can you recall?

Expansion of gases:

A gas does not even have a fixed volume. Gas expands on heating but if the gas is kept in a closed box, its volume cannot increase but its pressure increases. This is shown in Fig. Observe Fig. and find out answers to the questions.

Question 1.
Using the formula density = mass/volume, explain what will be the effect of heat on the gas kept in a closed bottle.
Answer:
The density of the gas will remain the same as there is no change on the mass and volume of the gas.

Question 2.
If the bottle is not closed but has a movable piston attached to its open end (see the figure), what will be the effect of heating the gas in the bottle?
Answer:
The piston will move upwards as the gas expands.
Therefore, the expansion of a gas is measured by keeping its pressure constant.

Use your brain power!

Question 1.
The density of a gas decreases on heating. Which of the pictures in Fig. makes use of this?
Answer:
Students should be able to answer this question.

Write short notes on the following:

Question 1.
The Celsius temperature scale:
Answer:
In this case, the temperature at which pure ice melts at normal atmospheric pressure is taken as zero degree (0°C) and the temperature at which pure water boils at normal atmospheric pressure is taken as hundred degree (100 °C). The interval between them is divided into loo equal parts. Each part corresponds to a temperature difference of 1 °C.

Question 2.
The Fahrenheit temperature scale:
Answer:
In this case, the normal melting point of pure ice is taken as 32 °F and the normal boiling point of pure water is taken as 212 °F. The interval between these two points is divided into 180 equal parts. Each part corresponds to a temperature difference of 1°F. If C is the temperature of a body on the Celsius scale and F is the corresponding temperature on the Fahrenheit scale, the two are related by
C = \(\frac{5}{9}\) (F – 32) or F = \(\frac{9}{5}\) C + 32.

Question 3.
The Kelvin temperature scale:
Answer:
In this case, the melting point of pure ice at normal atmospheric pressure is taken as 273.15 kelvin (273.15 K) and the boiling point of pure water at normal f atmospheric pressure is taken as 373.15 I kelvin (373.15 K). The lowest possible temperature (called the absolute zero) on this scale corresponds to -273.15 °C.

Give scientific reasons:

Question 1.
We heat the neck of a glass bottle in order to remove the firmly fitted stopper.
Answer:
When we heat the neck of the glass bottle, it expands. Due to this, the stopper becomes loose and can be removed with ease.

Question 2.
A clinical thermometer has to be jerked before re-use.
Answer:
The constriction above the bulb of the clinical thermometer prevents the mercury thread once risen from falling down to the bulb. Hence, in order to force the mercury thread back into the bulb, the 1 clinical thermometer has to be jerked before re-use.

Question 3.
Instead of the mercury thermometer, alcohol thermometer is used for measurement of very low temperature.
Answer:
Mercury freezes at – 39 °C and turns to a solid. Hence, the mercury thermometer cannot be used to measure temperature below -39°C. The freezing point of alcohol is -117°C. Hence, the alcohol thermometer is used for i measurement of very low temperature.

Solve the following examples:

Question 1.
The normal armpit temperature in humans is 98.6 °F. What is this temperature in (i) degree Celsius (ii) kelvin ?
Solution:
Data: F = 98.6 °F, C = ?, K = ?

(ii) K = C + 273.15
= 37 + 273.15 = 310.15 K.

Question 2.

Find the heat needed to raise the temperature of 2.5 kg of water from 30 °C to 40 °C. Write the answer in calorie as well as joule.
Solution:
Data: m = 2.5 kg = 2500 g,
ΔT = 40 °C-30 °C, Q = ?
Heat needed to raise the temperature of 1 g of water through 1°C is 1 calorie.
∴ Q = 2500 × 10 = 25000 calories
Now, 1 calorie = 4.18 joules
∴ Q = 25000 × 4.18 joules = 104500 joules
Heat needed, Q = 25000 calories = 104500 joules.

Question 3.
If the temperature of water rises by 5 °C when 500 cal of heat is supplied to it, what is the mass of water?
Solution:
Mass of water = \(\frac{500}{5}\) = 100 g.

Question 4.
How much heat is required to raise the temperature of 500 g of mercury from 20 °C to 100 °C? [Specific heat of mercury = 0.033 kcal/(kg °C)]
Solution:
Data: m = 500 g = 0.5 kg,
T₁ = 20 °C,
T₂ = 100 °C, c = 0.033 kcal/(kg . °C), Q = ?
Q = mc (T₂ – T₁)
= 0.5 kg × 0.033 kcal/(kg . °C) × (100 °C – 20 °C)
= 0.5 × 0.033 × 80 kcal
= 0.033 × 40 kcal
∴ Q = 1.32 kcal
Heat required = 1.32 kcal.
[Note: 1kcal/(kg-°C) = 1 cal/(g.°C)]

Question 5.
A certain mass of water at 84 °C is poured into an equal mass of water at 24 °C. What will be the resulting temperature of the mixture ?
Solution:
Data: mx = m2 = m (say),
T₁ = 84 °C, T₂ = 24 °C, T = ?
Heat lost by the hot water = heat gained by the cold water

Resulting temperature of the mixture = 54 °C.

Question 6.
A bucket contains 8 kg of water at 20 °C. When 4 kg of hot water is mixed with it, the temperature of the mixture becomes 40 °C. Calculate the temperature of the hot water. (Ignore the heat absorbed by the bucket.)
Solution:
Data: m₁ = 8 kg, T₁ = 20 °C, m₂ = 4 kg, T = 40 °C, T₂= ?
Heat lost by the hot water = heat gained by the cold water (ignoring the heat absorbed by the bucket)
∴ m₂c (T₂ – T) = m₁c (T – T₁)
∴ 4 kg × c × (T₂ – 40°C)
= 8 kg × c × (40°C – 20°C)
∴ T₂ – 40°C = 2 × 20°C = 40°C
∴ T₂ = 40 °C + 40 °C = 80 °C
Temperature of the hot water = 80 °C.

Question 7.
A blacksmith plunges a 2 kg horseshoe at 400 °C into 1 kg of water at 20 °C. Find the maximum temperature of the water. [Specific heat of iron = 0.11 kcal/(kg-°C)]
Solution:
Data: mx = 2 kg,
c₁ = 0.11 kcal/(kg.°C), T₂ = 400 °C, m₂ = 1 kg,
c₂ = 1 kcal/(kg.°C), T₂ = 20 °C, T = ?
Heat lost by the horseshoe = heat gained by the water
∴ m₁c₁ (T₁ – T) = m₂c₂(T – T₂)
∴ 2 kg × 0.11 kcal/(kg.°C) × (400 °C – T)
= 1 kg × 1 kcal/(kg.°C) × (T – 20 °C)
∴ 0.22 × (400 °C – T) = T – 20 °C
∴ 1.22 T= 108 °C
∴ T = \(\frac{108}{1.22}\)°C = 88.52 °C
Maximum temperature of the water = 88.52 °C.

Question 8.
A copper sphere of mass 500 g is heated to 100 °C and then introduced into a copper calorimeter containing 100 g of water at 20 °C. Find the maximum temperature of the mixture, if the mass of the calorimeter is 100 g and the specific heat of the calorimeter is 0.1 cal/(g.°C).
Solution:
Data: m = 500 g, c = 0.1 cal/(g.°C), T’= 100 °C, m₁ = 100 g, c₁ = 1 cal/(g.°C), T₁ = 20°C, m₂ = 100 g, c₂ = 0.1 cal/(g.°C), T₂ = 20 °C, T= ?
Heat lost by the sphere = heat gained by the water and the calorimeter.
∴ mc (T’ – T) = m₁c₁ (T – T₁) + m₂c₂ (T – T₂)
∴ 500 g × o.l cal/(g.°C) × (100 °C – T)
= 100 g × 1 cal/(g.°C) × (T – 20 °C) + 100 g × 0.1 cal/(g.°C) × (T – 20 °C)
∴ 50 (100 °C – T) = 100 × (T – 20 °C) + 10 × (T – 20 °C)
∴ 50 (100 °C – T) = 110 × (T – 20 °C)
∴ 500 °C – 5T = 11T – 220 °C
∴ 16T = 720 °C
∴ T = \(\frac{720^{\circ} \mathrm{C}}{16}\) = 45 °C
Maximum temperature of the mixture = 45 °C.

Question 9.
A metal rod 1.8 m long, increases ( in length by 1.4 mm, when heated from 0 °C to 50 °C. Find the coefficient of linear expansion of the metal.
Solution:
Data: l₁ = 1.8 m, l₂ – l₁ = 1.4 mm = 1.4 × 10^-3 m.
T₁ = 0 °C, T₂ = 50 °C.
l₂ – l₁ = l₁ λ (T₂ – T₁)
∴ The coefficient of linear expansion of the metal is

Question 10.
A thin metal disc of surface area 500 cm2 at 0 °C is heated to 40 °C. Find the increase in the surface area of the disc. (σ = 4 × 10^-5 °C ^-1)
Solution:
Data: A₁ = 500 cm², T₁ = 0 °C,
T₂ = 40 °C, σ = 4 × 10^-5 °C^-1, A₂ – A₁ = ?
The increase in the surface area of the disc is
A₂ – A₁ = A₁σ(T₂ – T₁)
= (500 cm²) (4 × 10^-5 °C^-1) (40 °C – 0 °C)
= 500 × 4 × 10^-5 × 40 = 0.8 cm².

Question 11.
The surface area of a metal plate is 2.4 × 10^-2m² at 20 °C. When the plate is heated to 185 °C, its area increases by 0.8 cm². Find the coefficient of areal expansion of the metal.
Solution:
Data: A₁ = 2.4 × 10^-2 m²,
T₁ = 20 °C, T₂ = 185 °C, ΔA = 0.8 cm² = 0.8 × 10^-4 m², σ = ?
ΔA = A₂ – A₁ = A₁ σ(T₂ – T₁)
∴ The coefficient of areal expansion of the metal is

Question 12.
A lead bullet has a volume of 25 cm³ at 0 °C, and 25.21 cm³ at 90 °C. Find the volumetric expansion coefficient of lead.
Solution:
Data: V₁ = 25 cm³, T₁ = 0 °C,
V₂ = 25.21 cm³, T₂ = 90 °C
T₂ – T₁ = 90 °C – 0 °C = 90 °C β = ?

The volumetric expansion coefficient of lead is 9.333 × 10^-5 °C^-1.

Example Questions for practice:

Question 1.
The temperature of a body is 30 °C. Express it in (i) degree Fahrenheit (ii) kelvin.
Answer:
86 °F, 303.15 K

Question 2.
The temperature of a body is 283.15 K. Express it in °C and °F.
Answer:
10 °C, 50 °F

Question 3.
The temperature of a body is 68 °F. Express it in °C and K.
Answer:
20 °C, 293.15 K

Question 4.
Find the heat needed to raise the temperature of 5 kg of water from 20 °C to 25 °C. Write the answer in calorie as well as joule.
Answer:
25 × 10³ cal, 1.045 × 10⁵ J

Question 5.
When a substance having mass 2 kg absorbs 2000 cal of heat, its temperature increases by 10 °C. Find the specific heat of the substance.
Answer:
0.1 cal/(g.°C)

Question 6.
Find the heat needed to raise the temperature of 100 g of a metal through 10 °C if the specific heat of the metal is 0.1 cal/g. °C.
Answer:
100 cal

Question 7.
If water of mass 80 g and temperature 40 °C is mixed with water of mass 20 g and temperature 25 °C, what will be the maximum temperature of the mixture?
Answer:
37 °C

Question 8.
A metal rod 2.5 m long, increases in length by 1.25 mm when it is heated from 10 °C to 60 °C. Find the coefficient of linear expansion of the metal.
Answer:
1 × 10^-5 °C^-1

Question 9.
The surface area of a metal plate is 2 × 10^-2 m² at 10 °C. When the plate is heated to 60 °C, its area increases by 0.2 m². Find the coefficient of areal expansion of the metal.
Answer:
2 × 10^-5 °C^-1

Question 10.
A metal ball has volume 50 cm3 at 0 °C and 50.4 cm3 at 100 °C. Find the volumetric expansion coefficient of the metal.
Answer:
8 × 10^-5 °C^-1

Project:

Ref. Project. Useful information:
A bimetallic strip: A bimetallic strip consists of two strips of equal lengths but of different metals rivetted together. At room temperature the strip is straight.

Question 1.
The principle on which a bimetallic strip works:
Answer:
When two different metal strips of the same length at a given temperature are heated to the same higher temperature, they expand in different proportion.
A bimetallic strip of brass and iron is straight at room temperature. The expansion of brass is nearly 1.5 times that of iron. Hence, when this bimetallic strip is heated, it bends, making the iron side concave.

Question 2.
How a bimetallic strip is used in fire alarm:
Answer:
A bimetallic strip of brass and iron is

connected to a battery and an electric bell as shown in the diagram. One terminal of the bell is connected to a screw which is at a very small distance from the iron side of the strip. In case, there is an accidental fire, the bimetallic strip bends towards iron and touches the screw. Thus, the circuit is completed and the bell rings, thereby warning the people of the accidental fire.

8th Std Science Questions And Answers:

• Human Body and Organ System Class 8 Questions And Answers
• Introduction to Acid and Base Class 8 Questions And Answers
• Chemical Change and Chemical Bond Class 8 Questions And Answers
• Measurement and Effects of Heat Class 8 Questions And Answers
• Sound Class 8 Questions And Answers
• Reflection of Light Class 8 Questions And Answers
• Man-made Materials Class 8 Questions And Answers
• Ecosystems Class 8 Questions And Answers
• Life Cycle of Stars Class 8 Questions And Answers