Class 8

Balbharti Maharashtra State Board Class 8 English Solutions Chapter 1.2 Dick Whittington and his Cat Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 English Solutions Chapter 1.2 Dick Whittington and his Cat

Class 8 English Chapter 1.2 Dick Whittington and his Cat Textbook Questions and Answers

Warming Up:

1. Discuss the follow in groups and write the answers:

Question a.
Name the world famous personalities, who reached great heights despite of humble circumstances.
Answer:

• Dr. A.P.J. Abdul Abraham
• Lincoln Kalam
• Charlie Chaplin
• Kalpana Chawla

Question b.
Guess the types of hardships they must have faced in their childhood and youth.
Answer:

• poverty
•  lack of support
• social disapproval
• lack of opportunities

Question c.
What strong qualities possessed by them, could have helped them achieve success and fame, all over?
Answer:

• strong will power
• optimism
• sincerity
• hard work

Question d.
How do stories / biographies of such famous people help youngsters?
Answer:

• They help youngsters to look at life positively.
• They inspire the youth.
• They help them find answers to questions.
• They prove that we can do many seemingly impossible things if we really wish to.

2. Syllable: A syllable is a unit of pronunciation that has one ‘owe1 sound and may or may not have consonant sounds before I after it.
Examples of words having:
One syllable: a, I, see, at, on, man, with, thread
Two syllables: mother, seated, giraffe, parrot, active, happy
Three syllables: happiness, wonderful, animal, surprising, adventure, ignorance.
More than three syllables: consequently, encouragement, responsibility, identifying, encyclopedia

Write the numcs of any 5 of your classmates and list how many syllables each name has.
Answer:

• Jal: 1 syllable
• Remo: 2 syllables
• Saira: 2 syllables
• Shailaja : 3 syllables
• Mandakini: 4 syllables
• Ali: 2 syllables

1. A. Arrange the following set of words in alphabetical order in your notebook.
ship, small, successful, scoldings, stone, saving, someone, stood, streets, still, screamed, sot, seemed, saint, share.
Answer:
saint, sat, saving, scoldings, screamed, seemed, share, ship, small, someone, still, stone, stood, streets, successful.

1. B. Remove the Affixes (Prefix and Suffix) and write the Root-word.

1. successful
2. immediately
3. retrace
4. mouser
5. unfriendly
6. fallen
7. unkind
8. wooden
9. luckily
10. belonging

Answer:

1. success
2. immediate
3. trace
4. mouse
5. friendly
6. fall
7. kind
8. wood
9. luck
10. belong

2. A. Find from the story’ one word for the following.

Question a.
a small room just below a sloping roof
Answer:
attic

Question b.
a very narrow passage between buildings
Answer:
alley

Question c.
the highest – ranking officer in the Municipality of a city/town.
Answer:
Mayor

Question d.
the highest ranking official who commands a ship.
Answer:
Captain

Question e.
any animal that catches mice
Answer:
mouser

Question f.
a low constant sound made by cats
Answer:
Purr

2. B. Make sentences of your own using the following expressions.

Question 1.
overcome:
Answer:
We must learn to overcome any difilcultics we come across.

Question 2.
put up with:
Answer:
We had to put up wilhi the noisy behaviour of our neighbours son.

Question 3.
bargain:
Answer:
People In India love to bargain with hawkers.

3. Rearrange the following events us they occur in the story. Put the correct number in the boxes.

Answer:

4. Think and answer:

Question 1.
What decision taken by Dick changed his fortune?
Answer:
On hearing the church bells send out a message to him, Dick decided to retrace his steps and go back to Mr. Fitzwarren’s house instead of running away. This decision changed Dick’s fortune.

Question 2.
What message does this story bring j out for youngsters?
Answer:
The message that this story brings out for youngsters is that we must S not run away from difficulties but face > them bravely.

5. A. Break the words below into syllables us shown in the first one.

1. belonging
2. morning
3. unfriendly
4. citizen
5. anything
6. message
7. difficulty
8. mayor
9. captain
10. elected

Answer:

1. be – long – ing
2. mor – ning
3. un – friend – ly
4. ci – ti – zen
5. an – y – thing
6. mess – age
7. di – ffi – cul- ty
8. ma – yor
9. cap – tain
10. e – lec – ted

5. B. Degrees of Comparison of Adjectives.

Read the sentences.

(i) Yatin is strong.
(ii) Amit is stronger than Yatin.
(iii) Pruvin is the strongest of all.

In sentence (i) the Adjective ‘strong’ is in its simple form. It is called Positive Degree.
In sentence (ii) the Adjective stronger’ refers to a higher degree, when there is a comparison of two nouns. It is called Comparative Degree. (-er is
added to the basic Adjective)
In sentence (iii) the adjective ‘strongest’ refers to the highest degree of comparison of one with more than two nouns. It is called Superlative
Degree. (-est is added to the basic Adjective)

Complete the table of degrees of comparison.

Answer:

Positive	Compa­rative	Super­lative
(1) great	greater	greatest
(2) famous	more famous	most famous
(3) easy	easier	easiest
(4) bad	worse	worst
(5) poor	poorer	poorest
(6) lazy	lazier	laziest
(7) little	less	least
(8) worthy	worthier	worthiest
(9) popular	more popular	most popular

6. Select any one of the famous personalities mentioned in ‘Warming up 1′ on page no. 5. From the library or internet find out their success story. Write ¡tin your own words in about 20 to 30 lines of your notebook. Give your write-up a suitable title.

Class 8 English Chapter 1.2 Dick Whittington and his Cat Additional Important Questions and Answers

Simple Factual Activities :

Complete the following :

Question 1.
Dick lived in a ______.
Answer:
small village in the south of England

Question 2.
_____ treated Dick unkindly.
Answer:
Many of the villagers

Question 3.
The innocent belief ________.
Answer:
that the streets of London were paved with gold made Dick decide to go to London

Question 4.
The carter asked Dick _______.
Answer:
if he was running away from home

Complex Factual Activity:

Question 1.
Describe the hardships that Dick I faced on arriving in London.
Answer:
Dick arrived in London just before ; nightfall. He had no place to sleep, and he finally slept in the corner of a narrow lane. The following morning, Dick woke ( up feeling cold, miserable and hungry. He j begged for food, but people shouted at him | and hit him angrily on his head. He finally collapsed and lay there, too weak to look further for food.

Activities based on Contextual Grammar:

Question 1.
He thought people might treat him more kindly if he were rich. (Identify the modal auxiliary and state its function.)
Answer:
might-indicates possibility

Question 2.
All he could see were dirty streets and lots of unfriendly people.
(Rewrite using ‘nothing’.)
Answer:
He could see nothing but dirty streets and lots of unfriendly people.

Personal Response:

When Dick begged for food, people shouted at him and beat him. Do you think this is right?
Answer:
of course it is not right. We ; should have sympathy when people beg for something, not get angry with them. They must be having a strong reason for begging, of course, if they are lazy, do not work and then beg, it is wrong. Even then, we must not beat them or shout at them.

Complete the following:

Question 1.
Mr. Fitzwarren favoured Dick by ______.
Answer:
ordering the cook to take him into the kitchen, feed him and then find some work for him

Question 2.
Dick had to face two difficulties in Mr. Fitzwarren’s house. They were:
Answer:
(i) the attic where he slept was overrun by rats and mice, preventing him from sleeping.
(ii) the cook was bad-tempered and shouted and screamed at Dick, scolding him and hitting him with a wooden spoon even when he was working as hard as he could.

Question 3.
The cat proved to be very useful because _____.
Answer:
it chased away all the rats and mice, allowing Dick to sleep peacefully

Activities based on Contextual Grammar:

Question 1.
Dick had fallen in front of a house belonging to a rich merchant. (Rewrite using the simple past tense of the underlined verb.)
Answer:
Dick fell in front of a house belonging to a rich merchant.

Question 2.
Dick was able to sleep peacefully.
(Rewrite using the verb ‘slept’.)
Answer:
Dick slept peacefully.

Personal Response:

Question 1.
What do Mr. Fitzwarren’s actions tell us about him?
Answer:
Mr. Fitzwarren’s actions tell us that he was a kind-hearted and just man.
He did not abuse Dick or hit him as the others had done. He offered Dick a home and a living in return for helping the cook with all the pots and pans in the kitchen.

Simple Factual Activities:

Who said the following words:

Question 1.
I can put up with a few scoldings from the cook.
Answer:
Dick

Question 2.
No, it must be his.
Answer:
Mr. Fitzwarren

Question 3.
Only my cat.
Answer:
Dick

Question 4.
Let me put something in for him.
Answer:
Alice

Fill in the blanks:

Question 1.
Dick walked as far as ________.
Answer:
Holloway

Question 2.
The first of November is _______.
Answer:
All Saints’ Day

Question 3.
Dick went back to _____.
Answer:
Mr. Fitzwarren’s house

Question 4.
Mr. Fitzwarren was sending a ship to the _____ to trade.
Answer:
African coast

Complex Factual Activity:

Question 1.
’“Explain in your own words what inspired Dick to return to Mr. Fitzwarren’s home.
Answer:
When Dick sat down on a stone to rest, he heard the church bells ringing. As he listened, it seemed that they were ringing a message for him, telling him that he should go back home, and calling him the Mayor of London.

Pick out the subject and predicate in the following sentences and put them in the correct columns in the table given below :

1. You can then share in the profits.
2. Everyone produced something except for Dick.
3. The two of them walked as far as Holloway.
4. Dick and his cat retraced their steps to Mr. Fitzwarren’s house.

Answer:

Subject	Predicate
(1) You	can then share in the profits
(2) Everyone	produced something except for Dick
(3) The two of them	walked as far as Holloway
(4) Dick and his cat	retraced their steps to Mr. Fitzwarren’s house.

Personal Response:

Question 1.
According to you, did Dick do the right thing by running away from Mr. Fitzwarren’s house because of the cook?
Answer:
No, I don’t think that Dick did the right thing. Mr. Fitzwarren was a good man, and Dick was making a decent living. He should have tried to win the cook over and solve his difficulties. If he had run away, he may not have got another job and may have had to beg. Nothing is gained by running away from problems; we must face them and solve them.

Write whether the following statements are True or False:

Question 1.
Dick was happy to send his cat on the ship.
Answer:
False

Question 2.
The cat caught plenty of mice and rats.
Answer:
True

Question 3.
The king was rude to the captain.
Answer:
False

Question 4.
The queen knew all about cats.
Answer:
False

Pick out two adverbs ending in ‘-ly’ from the passage, and write the adjectives in them.
Answer:
Adverbs-immediately, normally, Adjectives in them-immediate, normal

Personal Response:

Do you think that all the animals are 1 important to the environment?
Answer:
Yes, all the animals are certainly j important to the environment. They all have their own place and own duties. ; For example, hawks and eagles control the population of mice, rats and snakes. Lions and tigers keep the number of deer, buffaloes and other such animals in check, j Every animal is important in its own way.

Write the following sentences in the correct order of occurrence in the passage:

1. Dick was knighted by the king and became Sir Richard Whittington.
2. Dick was elected the Mayor of London.
3. The king paid ten times the sum for the cat.
4. “I would give great wealth to own this animal,” said the queen.

Answer:

1. “I would give great wealth to own this animal,” said the queen.
2. The king paid ten times the sum for the cat.
3. Dick was elected the Mayor of London.
4. Dick was knighted by the king and became Sir Richard Whittington.

Give reasons:

Question 1.
The queen was afraid of the cat.
Answer:
The queen was afraid of the cat because she had seen how fiercely it had attacked the rats and the mice.

Question 2.
How much did the king pay for the cat?
Answer:
The king paid ten times the sum for the cat that he had paid for the whole cargo from Mr. Fitzwarren’s ship.

Activities based on Contextual Grammar :

Pick out the verbs from the following sentences and state their tense :

Question 1.
The captain showed Mr. Fitzwarren the gold and jewels he had brought from Barbary.
Mr. Fitzwarren then paid him all: the money he had received for the cat.
Answer:
showed – simple past tense; had brought – past perfect tense
paid – simple past tense; had received – past perfect tense

Personal Response:

Question 1.
What message does this story bring j out for youngsters?
Answer:
The message that this story brings out for youngsters is that we must S not run away from difficulties but face > them bravely.

Do as directed:

Question 1.
Pick out two compound words from the lesson.
Answer:
Two compound words : household, nightfall

Question 2.
Use the adverb ’immediately’ in your own sentence :
Answer:
The teacher immediately asked the tired girl to sit down.

Question 3.
Find out four hidden words from : the given word : (Each word should have at least 3 letters.)
successful
Answer:
successful : success, self, fuel, fuse

Question 4.
Use the following word and its homophone in two separate sentences :ate
Answer:

• The little boy ate the cake very quickly,
• He was eight years old when he got a new cycle.

Question 5.
Underline the adjectives from the following sentence :
He was not only famous, but he was popular too, for he always helped the poor ; with his money.
Answer:
He was not only famous, but he was popular too, for he always helped the poor with his money.

Question 6.
Pick out the conjunction in the following sentence :
Dick work up cold, miserable and very hungry.
Answer:
and

Question 7.
Punctuate :
alas yes came the reply the country is suffering from a plague of rats and mice
Answer:
“Alas, yes!” came the reply. “The country is suffering from a plague of rats and mice.”

Question 8.
Pick out the direct and indirect objects:
He gave me a hundred rupee note.
Answer:
Direct object – a hundred-rupee note
Indirect object – me

Question 9.
Use the following word as a noun and a verb in two separate sentences :
sleep
Answer:

• We should get at least 8 hours of sleep daily, (noun)
• I sleep at 10 pm daily, (verb)

Balbharti Maharashtra State Board Class 8 English Solutions Chapter 1.1 A Time To Believe Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 English Solutions Chapter 1.1 A Time To Believe

Class 8 English Chapter 1.1 A Time To Believe Textbook Questions and Answers

Warming Up:

Question 1.
Think, choose and fill up the labels with what a ‘sunrise’ symbolizes.
(jewellery / art / hope / birth / anger / new opportunities / good manners / inspiration/ new achievements / happiness / misery / bright moments / new aims / money / newsurprises)

Answer:

Question 2.
An acrostic is a poem or a write up in which the first letter of each line forms a word, when it is read vertically. Complete the acrostics of BELIEVE and FAITH.

With your benchmate / group, complete the acrostic of BELIEVE and FAITH.
Answer:

1.A. Rearrange the letters to make meaningful words, occurring in the poem.

1. clearmis
2. sowmid
3. tabyue
4. madres
5. laveu
6. downre

Answer:

1. miracles
2. wisdom
3. beauty
4. dreams
5. value
6. wonder

1.B. Write words from the poem that describe the following.

1. sky
2. heart
3. hand
4. beginning

Answer:

1. stardust sky
2. nurturing heart
3. aging hand
4. new beginning

2. Say WHY. . . . .

Question a.
________ the sky has a magical quality.
Answer:
The sky has a magical quality because it has scurrying clouds, twinkling stars and a beautiful moon.

Question b.
_______ even an aging hand has beauty.
Answer:
because it can teach us how to love.

Question c.
_______we should believe that we are strong and courageous.
Answer:
We should believe that we are | strong and courageous so that we can recover after we have gone through difficulties and shocks, and return to our normal lives again.

3. Make a list of any six things that the poet wants us to accept as true.
Answer:
The poet wants us to accept as j true the following things :

• that we have the strength and ( courage to get back to normal after a setback
• that we are never alone
• that life is a gift we must cherish
• that there are wonderful surprises in store for us
• that all our hopes and dreams are within reach
• that great things will happen if we have belief in life and in ourselves

4. Find from the poem, positive qualities to fill up the web.

Answer:

5. Read the lines below.
(A) ‘To believe is to see angels dancing among the clouds’.
The above line contain a human characteristic ‘dancing’ given to something
non – human, ‘angels’. The Figure of Speech used in the line is called
‘Personification’.

(B) ‘To believe is to know that
Everyday is a new beginning’.

(C) To believe is to know ………. life is a gift
Lines in poetry, such as the above, state a general truth and can be used as sayings or
quotes etc. They contain the Figure of Speech called Epigram.
Epigram closely resembles a proverb.

Find from the poem three other examples of Epigram.
Answer:
(i) ‘To believe is to find the strength and courage that lies within us.’
(ii) ‘To believe is to know we are never alone, that life is a gift.’
(iii) ‘To believe is to know that wonderful surprises are just waiting to happen.’
(These lines state a general truth.)

6. Divide the class into 5 groups. Each group can prepare a decorative chart / poster using an epigrammatic lines from the poem. You may use calligraphy / painting / or Paint programme on a computer. (With permission, put up these charts in your school.)

Class 8 English Chapter 1.1 A Time To Believe Additional Important Questions and Answers

Complete the following:

Question 1.
One should know and trust _______.
Answer:
One should know and trust that every day is a new beginning, that miracles happen and dreams really do come true.

Question 2.
The two supernatural references in the second stanza _______.
Answer:
The two supernatural references in the second stanza are to angels and to the man in the moon.

Activities based on Poetic Devices/Vocabulary:

Question 1.
Find out an example of Personification from the extract.
Answer:
‘To believe is to see angels dancing among the clouds.’ (Here, the angels have been given the human quality of ‘dancing’.)

Question 2.
Pick out examples of Alliteration from the extract.
Answer:
(i) …. and the wisdom of the man in the moon’. (Repetition of the sound of the letter ‘m’.)
(ii) For it is through their teachings, we learn to love. (Repetition of the sound of the letters ‘t’ and T.)

Question 3.
Pick out an example of Repetition from the extract.
Answer:
The words ‘To believe is to ….’ are repeated at the beginning of each stanza. This provides emphasis to the message in the poem and gives rhythm to the stanzas.

Complete the following:

Question 1.
The two qualities that are needed to recover after a shocking event are _________.
Answer:
strength and courage.

Question 2.
The poet advises everybody to ______.
Answer:
have a positive attitude.

Complex Factual Activities:

Question 1.
Explain why ….
…. we should believe that we are strong and courageous,
Answer:
We should believe that we are strong and courageous so that we can recover after we have gone through difficulties and shocks, and return to our normal lives again.

Activities based on Poetic Devices/Vocabulary :

Question 1.
Rearrange the letters to form meaningful words occurring in the extract.

1. gearuoc
2. rissupser
3. gnehtrts
4. irehchs

Answer:

1. courage
2. surprises
3. strength
4. cherish

Question 2.
Write word from the extract that describe the following :
surprises
Answer:
wonderful surprises

Question 3.
Pick out an example of Metaphor from the extract.
Answer:
‘That life is a gift.’ Life is implicitly compared to a gift.

Question 4.
Pick out an example of Repetition from the extract.
Answer:
The words ‘To believe is to ….’ are repeated at the beginning of each stanza. This provides emphasis to the message in the poem and gives rhythm to the stanzas.

Question 5.
Analysis/Appreciation Of The Poem ‘A Time To Believe’
Answer:
Poem and poet: ‘A Time to Believe’ by B.J. Morbitzer.
Theme: To believe in ourselves and to have a positive and optimistic attitude towards life.
Tone: Serious and inspirational.
Structure and stanzas: 4 lines in each stanza, except in the last stanza, which has 5 lines.
Rhyme and Rhythm: No rhyme scheme; the poem is written in free verse.
Language and Imagery: The language is simple and direct. The only instances of Imagery are of ‘angels dancing among the clouds’ and the ‘stardust sky’.
Figures of Speech: Personification, Epigram, Repetition, Metaphor.

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 19 Life Cycle of Stars Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 Science Solutions Chapter 19 Life Cycle of Stars

Class 8 Science Chapter 19 Life Cycle of Stars Textbook Questions and Answers

1. Search and you will find:

Question a.
Our galaxy is called ………… .
Answer:
Our galaxy is called the Milky Way and Mandakini.

Question b.
For measuring large distances, ………… is used as a unit.
Answer:
For measuring large distances, the light year is used as a unit.

Question c.
The speed of light is …………. km/s.
Answer:
The speed of light is 300000 km/s.

Question d.
There are about ………… stars in our galaxy.
Answer:
There are about 100 billion stars in our galaxy.

Question e.
The end stage of the Sun will be …………. .
Answer:
The end stage of the Sun will be a white dwarf.

Question f.
Stars are born out of ………… clouds.
Answer:
Stars are born out of interstellar clouds.

Question g.
Milky way is a ………. galaxy.
Answer:
Milky way is a spiral galaxy.

Question h.
Stars are gigantic spheres of ………… gas.
Answer:
Stars are gigantic spheres of hot gas.

Question i.
The masses of other stars are measured relative to the mass of the …………. .
Answer:
The masses of other stars are measured relative to the mass of the Sun.

Question j.
Light takes ……………. to reach us from the Sun while it takes …….. to reach us from the moon.
Answer:
Light takes about 8 minutes to reach us from the Sun while it takes about 1 second to reach us from the moon.

Question k.
The larger the mass of a star, the faster is its ……….. .
Answer:
The larger the mass of a star, the faster is its evolution.

Question l.
The number of fuels used in the life of a star depends on its …………. .
Answer:
The number of fuels used in the life of a star depends on its mass.

2. Who is telling lies?

Question a.
Light year is used to measure time.
Answer:
False. (Light year is used to measure distance.)

Question b.
End stage of a star depends on its initial mass.
Answer:
True.

Question c.
A star ends its life as a neutron star when the pressure of its electrons balances its gravity.
Answer:
False. (A star ends its life as a neutron star when the pressure of its neutrons balances its gravity.)

Question d.
Only light can emit from the black hole.
Answer:
False. (Not even light can be emitted by a black hole.)

Question e.
The Sun will pass through the supergiant stage during its evolution.
Answer:
False. (The Sun will pass through the red giant stage during its evolution.)

Question f.
The Sun will end its life as a white dwarf.
Answer:
True.

3. Answer the following question:

Question a.
How do stars form?
Answer:
There are huge clouds of gas and dust in the empty spaces between the stars in a galaxy. These clouds are called interstellar clouds. The size of an interstellar cloud is about a few light years.

When an interstellar cloud starts contracting due to some disturbance, its density and temperature increase. This results in formation of a dense sphere of hot gas and nuclear energy generation starts at the centre of the star. Therefore, the gas sphere becomes self-luminous. Thus a star is formed, i.e., a star is born.
A huge interstellar cloud can produce thousands of stars at a time.

Question b.
Why do stars evolve?
Answer:
Although stars appear stable for quite a long period of time, their properties do change, though very slowly. A change in the properties of a star, leading to its passing through different stages, is called evolution of the star. Burning of the fuel at the centre of the star and a gradual decrease in its amount is the main reason of evolution of a star.

Question c.
What are the three end stages of stars?
Answer:

1. Stars having initial mass less than 8 times the mass of the Sun ultimately become white dwarfs.
2. Stars having initial mass between i 8 and 25 times the mass of the Sun ultimately become neutron stars.
3. Stars having initial mass larger than 25 times the mass of the Sun ultimately turn into black holes.

Question d.
Why was the name black hole given?
Answer:
When a star having initial mass larger than 25 times the mass of the Sun reaches its end stage, its gravitational force and density increase exponentially. All nearby objects get attracted towards the star and nothing can come out of it, not even light. All incident light is absorbed by the star. We can probably see a very minute black hole at its place. Hence, the name is given as black hole.

Question e.
Which types of stars end their life as a neutron star?
Answer:
Stars having initial mass between 8 and 25 times the mass of the Sun end up as neutron stars. When these stars pass through ( the supergiant stage, their size increases to 1000 times. Huge explosion that occurs in the last stage of these stars is very powerful and very high energy is given off. After the huge explosion, called the supernova explosion, their central portion contracts in size to about 10 km. In this stage, such stars are completely made up of neutrons and hence are called neutron stars.

4. A. If you are the Sun, write about your properties in your own words.
B. Describe white dwarfs.

Question A.
If you are the Sun, write about your properties in your own words.
Answer:

If you are the Sun, write about your properties in your own words.
Answer:
There are billions of stars in the galaxy called the Milky Way and Mandakini. I am one of the small stars called the Sun. I have my own family called the solar system. There are planets, satellites, asteroids, comets and meteors in my family. My mass is 2 × 1030 kg which is about 3.3 lakh times that of the earth. With a radius of 695700 km, my size is about 100 times that of the earth. My surface temperature is about 5800 K while it is 1.5 × 107 K at the centre.

72 % of my mass consists of hydrogen while 26 % consists of helium. Rest 2 % is made up of elements heavier than helium. I am about 4.5 billion years old and the scientists on the earth have concluded that not much change has taken place in my properties during this period. According to the scientists, in the end stage of my life, I will turn into a red giant star. Thereafter, I will first explode and then contract to become as small as the earth. I will appear small as well as white and hence, I will be called a white dwarf. This will be my last stage for ever.

Question B.
Describe white dwarfs.
Answer:
Depending on the initial mass, stars can reach one of the three end stages. White dwarf is one such stage of stars having initial mass less than 8 times the mass of the Sun. These stars undergo huge expansion and their radius increases by a factor of 100 to 200. These stars appear reddish because of their large size and lower temperature. Hence, these stars are also called red giant stars.

At the end of their evolution, these stars explode, their outer gas envelope is thrown outward and the inner part contracts to the size of the earth. However, the density in the star becomes very high. In this stage, the pressure due to electrons becomes independent of temperature and sufficient to balance the gravitational force for ever.

In this stage, such stars look white and due to their small size they are called white dwarfs.

Can you recall?

Question 1.
What is a galaxy?
Answer:
A system of billions of stars, their planetary systems and interstellar clouds of gas and dust held together by gravitational attraction is called a galaxy.

Question 2.
What are the different constituents of our solar system?
Answer:
The different constituents of our solar system are as follows :

1. Sun as a star.
2. Eight planets, namely, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune; of which Mercury, Venus, Earth and Mars are made up of rocks and minerals, while Jupiter, Saturn, Uranus and Neptune are made up of gases.
3. Natural satellites of some planets revolving around the respective planets.
4. Asteroids located between Mars and Jupiter.
5. Comets made up of ice, dust and gases.
6. Meteors and meteoroids.

Question 3.
What are the major differences between a star and a planet?
Answer:

1. Nuclear/atomic explosions regularly take place at the centre of stars. This causes the discharge of heat and light. Hence, stars shine. Planets do not discharge any light. They are visible due to the light reflected by them.
2. Stars are very big in size and massive relative to planets.
3. Stars twinkle, planets do not.
4. Stars are made up of hydrogen, helium and other light elements.
5. Planets are made up of solid, liquid or gaseous substances or a combination thereof.

Question 4.
What is a satellite?
Answer:
An astronomical object orbiting a planet is called a satellite.

Question 5.
Which is the star nearest to us?
Answer:
The star nearest to us is the Sun.

Project:

Question 1.
Use your imagination and make models of the Milky Way and the solar system.
Answer:
N.B. Students can make these models and display the same in science exhibition. You can use sand, scrap material, stones, pebbles, marbles, pieces of glass, beads, etc.

Question 2.
Write the effects: If the Sun disappears ………..
Answer:
N.B. You can write the effects with the help of the following points:

1. No sunrise, no sunset, no beauty of morning and evening wee hours.
2. In the absence of the gravitational force due to the Sun, the earth will drift away in the galaxy.
3. Darkness, no source of energy, end of life on the earth.
4. Moon will be invisible.
5. The temperature of the earth will fall below the freezing point of water. All oceans will also freeze.
6. Even the atmosphere will freeze and fall on the earth. This will cause more cosmic rays to reach the earth.

Class 8 Science Chapter 19 Life Cycle of Stars Important Questions and Answers

Match the columns:

Question 1.

Column ‘A’	Column ‘B’
1. Gravitational force	a. Alpha Centauri
2. The star nearest to us other than the Sun	b. 2 × 1030 kg
3. Mass of the Sun	c. 10 30 g
	d. towards the centre of a star or a planet

Answer:

Column ‘A’	Column ‘B’
1. Gravitational force	d. towards the centre of a star or a planet
2. The star nearest to us other than the Sun	a. Alpha Centauri
3. Mass of the Sun	b. 2 × 1030 kg

Question 2.
Mass of the star:

Column ‘A’	Column ‘B’
1. < 8 MSun	a. Proton star
2. between 8 and 25 times MSun	b. Black hole
3. > 25 MSun	c. Neutron star
	d. White dwarf

Answer:
Mass of the star:

Column ‘A’	Column ‘B’
1. < 8 MSun	d. White dwarf
2. between 8 and 25 times MSun	c. Neutron star
3. > 25 MSun	b. Black hole

Answer the following questions in one sentence each:

Question 1.
Name the star nearest to the earth.
Answer:
The Sun.

Question 2.
Other than the Sun, which is the star nearest to the earth?
Answer:
Alpha Centauri.

Question 3.
What would be the last stage of the Sun?
Answer:
The last stage of the Sun would be white dwarf.

Question 4.
Name two forces that act on any star.
Answer:
Gravitational force and the force due to the pressure of hot gas are the two forces that act on any star.

Question 5.
What is a black hole?
Answer:
A black hole is the end stage of a quite big star, where due to the extremely high gravitational force, nothing, not even light, comes out.

Answer the following questions:

Question 1.
How is stability of stars maintained?
Answer:
Properties of a star remain unchanged for quite a long time. The gravitational force and the force due to the pressure of the hot gas act together on a star. The gravitational force acts towards the centre of the star and tries to bring the gas particles close together. Hot gas shows the tendency to spread and its force acts away from the centre of the star. This force tries to disperse the gas particles. A balance between the gravitational force and the force due to the hot gas keeps the star stable.

However, if the magnitude of any one force is more than that of the other force, the star either contracts or expands depending upon which force dominates.

Question 2.
Why is the Sun called an ordinary star?
Answer:
The star nearest to the earth is the Sun. Hence, it appears quite big and bright. There are billions of stars in our galaxy called the Milky Way and Mandakini which are greater or lower in mass, size and temperature than the Sun. Hence, the Sun is called an ordinary star.

Do you know?

When the Sun will become a red giant, its diameter will increase so much that it will swallow Mercury and Venus. It is possible that the earth will also be absorbed by the Sun. It will take about 4-5 billion years for the Sun to reach this state.

Question 3.
Describe various stages of evolution of star.
Answer:
Though the properties of a star remain unchanged for quite a long time, this situation is never static. A star passes through different stages. This process is called the evolution of a star.
Important stages of the evolution of a star are as follows:

1. Initial stage of stability: The gravitational force and the force due to the pressure of the hot gas act together on a star. The gravitational force acts towards the centre of the star and tries to bring the gas particles close together. Hot gas shows the tendency to spread and its force acts away from the centre of the star.

This force tries to disperse the gas particles. A balance between the gravitational force and the force due to the hot gases keeps the star stable, as long as the energy generation continues at the centre of the star.

2. Burning of the fuel: As a star continuously emits energy, its energy constantly decreases. When the fuel at the centre of the star is exhausted, the energy generation stops and the temperature of the star starts decreasing. Decreasing temperature causes the gas pressure to decrease and the balance between the gravitational force and the force due to the gas pressure is no more maintained.

As the magnitude of the gravitational force is now more than that of the force due to the gas pressure, the star starts contracting. This causes another fuel to start burning, e.g. on exhausting hydrogen, helium starts undergoing fusion. Availability of multiple fuels depends on the mass of the star.

The higher the mass of the star, the more is the number of fuels used. The star either contracts or expands during the course of using these fuels. This may cause the imbalance between the gravitational force and the force due to the hot gas.

3. Total exhaustion of the fuel: When all fuels are exhausted, the energy generation in the star finally stops completely and the temperature of the star starts decreasing. The balance between the gravitational force and the force due to the gas pressure can no more be maintained. The evolution of the star ends and the star proceeds to its end stage.

4. End stage of a star: Once the fuel in the star is totally exhausted, the energy generation in the star stops and subsequently the gas pressure decreases, the star starts contracting and its density starts increasing. When the density becomes very high, some new types of pressures are generated which are independent of the temperature of the gas.

In such a case, the pressure remains stable despite low temperature and absence of any energy generation and thus the star remains stable for ever. This stage is the end stage of the star. Depending on the initial mass, stars can reach one of the three end stages.

• Stars having initial mass less than 8 times the mass of the Sun ultimately become white dwarfs.
• Stars having initial mass between 8 and 25 times the mass of the Sun ultimately become neutron stars.
• Stars having initial mass larger than 25 times the mass of the Sun ultimately turn into black holes.

Write short notes on the following:

Question 1.
End stages of stars having initial mass less than 8 times the mass of the Sun.
Answer:
These stars undergo huge expansion and their radius increases by a factor of 100 to 200 during their various stages of evolution. These stars appear reddish due to their lower temperature. Hence, they are called red giant stars.

At the end of evolution, these stars explode, their outer gas envelope is thrown out and the inner part contracts roughly to the size of the earth. Hence, the density of the star becomes very high. In this stage, the pressure due to electrons becomes independent of temperature and sufficient to balance the gravitational force for ever. Such stars look white and due to their small size they are called white dwarfs.

Question 2.
End stage of the stars having mass between 8 and 25 times the mass of the Sun.
Answer:
These stars pass through the red giant stage and later super giant stage, during which their size may increase to 1000 times. The huge explosion, called the supernova explosion, occurs in the last stage of the evolution. It is very powerful and very high energy is given off in this case.

As a result, the stars are visible even during the day. Later their central portion contracts to about 10 km. In this stage, the stars are completely made up of neutrons and are called neutron stars. The pressure of these neutrons is independent of temperature and sufficient enough to balance the gravitational force for ever.

Question 3.
End stages of stars having mass larger than 25 times the mass of the Sun.
Answer:
After the supernova explosion, no pressure can balance the gravitational force. Hence these stars contract continuously and their gravitational force and density increase exponentially. All nearby objects get attracted towards these stars and not even light can come out of them. Light falling on these stars is completely absorbed by the star. We cannot see these stars. A very minute black hole is formed at the place of such a star. This is the end stage of these stars.

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

Maharashtra State Board Class 8 Science Solutions Chapter 18 Ecosystems

Class 8 Science Chapter 18 Ecosystems Textbook Questions and Answers

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.

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 17 Man-made Materials Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 Science Solutions Chapter 17 Man-made Materials

Class 8 Science Chapter 17 Man-made Materials Textbook Questions and Answers

1. Try to find it:

Question a.
Plastic shows …………. property, hence it can be moulded to any shape.
Answer:
Plastic shows plasticity property, hence it can be moulded to any shape.

Question b.
Motor cars are coated with ……… .
Answer:
Motor cars are coated with Teflon.

Question c.
Thermocol melts at ………… °C.
Answer:
Thermocol melts at more than 100 °C (it is about 240 °C).

Question d.
…………….. glass dissolves in water.
Answer:
Alkali silicate or water glass dissolves in water.

2. Who is my partner?

Question a.

Column ‘A’	Column ‘B’
1. Lead glass	a. Plates
2. Bakelite	b. Mattresses
3. Thermocol	c. Electric bulb
4. Optic glass	d. Electric switch
5. Polypropylene	e. Lens

Answer:

Column ‘A’	Column ‘B’
1. Lead glass	c. Electric bulb
2. Bakelite	d. Electric switch
3. Thermocol	a. Plates
4. Optic glass	e. Lens
5. Polypropylene	b. Mattresses

3. Answer the following.

Question a.
Thermocol is produced from which material?
Answer:
Thermocol is made from polystyrene which is also a complex thermoplastic substance.

Question b.
Write uses of PVC.
Answer:
PVC or Polyvinyl chloride is used for making bottles, raincoat, pipes, handbags, shoes, electric cable insulation, furniture, ropes, toys, etc.

Question c.
Write the natural or man-made raw material of the following items.
Mattress, beaker, bangle, chair, gunny bag, broom, knife, pen.
Answer:

Items	Natural raw material	Man-made raw materials
Mattress	Cotton, Coir (Jute fibres)	Polypropylene
Beaker	___	Glass (Silicate or borosilicate glass)
Bangle	Gold, silver, lac, copper	Plastic, Glass
Chair	Wood	Plastic (PVC)
Gunny bag	Jute, cotton	Plastic (PVC)
Broom	Plant fibres	Plastic fibres (PVC)
Knife	Metals such as iron	Plastic
Pen	Metals	Plastic

Question d.
Which are the main ingredients of glass?
Answer:
The main ingredients of glass are sand and silica.

Question e.
How the plastic is produced?
Answer:

1. Plastics are derived from natural materials such as natural gas, oil, coal, minerals and plants.
2. The first synthetic plastics were i derived from cellulose, a substance found in plants and trees. This cellulose was heated with chemicals and resulted in a plastic like material.
3. In modern times, the different raw materials are used for making plastics, but most plastics are made from the hydrocarbons present in the natural gas, oil and coal.
4. Plastics are simply chains of like molecules linked together. These chains are called polymers. Thus, many plastics begin with “poly,” such as polyethylene, polystyrene and polypropylene.
5. These polymers are made of carbon and hydrogen and sometimes oxygen, nitrogen, sulphur, chlorine, fluorine, phosphorous or silicon.
6. Plastic is produced in factories by suitable chemical reactions.

4. Distinguish between. 

Question a.
Man-made material and natural material
Answer:

Man-made material	Natural material
1. The man-made materials are obtained from processes in scientific laboratory.	1. Natural materials are obtained from nature.
2. Man-made materials are subjected to rigorous processing to alter the material for serving the intended purpose.	2. Natural materials are subjected to less treatment and processing.
3. Man-made materials are typically much more durable having a very long lifespan.	3. Natural materials have shorter lifespan, because these materials were once alive and so gradually perish over time.
4. Maintaining man-made materials require less care and attention.	4. Maintaining natural materials requires lots of care and continuous attention.
5. Man-made materials can have a negative environmental impact because they are not sustainable. E.g. Glass, plastic, Thermocol, soil, metals, rubber.	5. Natural materials do not have a negative environmental impact because they can be degraded easily. E.g. Cotton, silk, wood.

Question b.
Thermoplastic and Thermosetting plastic:
Answer:

Thermoplastic	Thermosetting plastic
1. The plastic that can be moulded as per our wish is called thermoplastic.	1. The plastic in which a specific shape is given with the help of mould and this shape cannot be changed again on heating is called thermosetting plastic.
2. Thermoplastic substances can be recycled and reused. E.g. PVC – Polyvinyl chloride, PS – Polystyrene, PE – Polyethylene and PP – Polypropylene are types of thermoplastics.	2. Thermosetting plastic cannot be reused again. E.g. Bakelite, Melamine, Polyurethane and polyster are the types of thermosetting plastics.

5. Answer the following in your own words.

Question a.
Explain the effect of following materials on environment and human health.
1. Plastic
2. Glass.
3. Thermocol.
Answer:
1. Plastic:

1. Plastic is non-degradable substance. Hence if thrown in any ecosystem, it remains unchanged for many years.
2. It is one of the worst environmental pollutants as its disposal is a major problem.
3. If thrown in water bodies, it affects the aquatic animals. Many of turtles mistake it for algae and eat the plastic. Eventually such animals die due to choking.
4. In terrestrial environment, the grazing animals like cattle are affected due to plastic.
5. If burnt it emits very toxic gases.
6. In landfill sites, it remains unchanged for thousands of years.

2. Glass:

1. The glass production is carried out at high temperatures of about 1500 °C. This burning emits many hazardous gases like sulphur dioxide, nitrogen dioxide, carbon dioxide. These gases cause the greenhouse effect.
2. Moreover, glass being non-degradable, cause pollution.
3. If broken glass pieces or any waste glass material is disposed into aquatic environment, it affects animals and plants.
4. Similarly, glass pieces block the drainages.
5. The waste glass thrown anywhere cause injury to terrestrial fauna.

3. Thermocol:

1. Thermocol contains carcinogenic ingredients in the form of styrene. If there is prolonged contact with thermocol, there is a possibility of blood cancer like leukemia and lymphoma.
2. Thermocol is non-degradable. It cannot be degraded into harmless substances easily.
3. If it is burnt for destruction, it releases toxic gases in atmosphere.
4. The plates and cups used for food, water, tea, etc. are made up of thermocol. This may affect the health. Reheating the food kept in thermocol releases styrene. This styrene may dissolve in that food, causing health problems like cancer.

Question b.
Which measures will you arrange to minimize the environmental problems arising due to non-degradable plastic?
Answer:

1. The use of plastic should be minimum. Reducing the consumption, reusing the same plastic again and again, recycling the used plastic and making some new products from the used plastic are some of the measures that can be adopted.
2. There are attempts to use plastic in making roads. Therefore, plastic is bought with good price at some places.
3. Therefore, instead of disposing of it anywhere, it should be collected and sold in best possible way.
4. The better alternatives for plastic should be adopted.
5. The awareness programmes about misuse of plastic should be arranged so that common man can understand the dangers of using plastic.

6. Write short notes. 

Question a.
Glass production:
Answer:
The general preparation of the glass is as follows:

1. Mixture of sand, soda, lime and small quantity of magnesium oxide is heated in furnace.
2. At 1700 °C sand or silicon dioxide melts.
3. To make the mixture melt at lesser temperature, pieces of discarded glasses are added to it.
4. This addition makes the mixture to melt at lesser temperature of 850 °C.
5. When all the ingredients of mixture are liquified, then again it is heated up to 1500 °C.
6. This heating is immediately followed by cooling.
7. The sudden cooling causes the mixture to become homogeneous, amorphous and transparent instead of crystalline.
8. For variety of glass types, different proportions of ingredients are used for heating.

Question b.
Optic glass:
Answer:

1. Optic glass or optical glass needs to be very clear and transparent as it is used in spectacles, lenses and other devices like microscopes.
2. Optic glasses are produced from the mixture of sand, soda, limestone, barium oxide and boron.

Question c.
Uses of plastic:
Answer:
Plastic, the man-made material is used in various forms in modern age. According to the type of plastic, its uses are different.

I. Thermoplastic materials are used for manufacturing following articles:

1. Polyvinyl chloride or PVC is used to make bottles, raincoat, pipes, handbags,: shoes, electric cable insulation, furniture, ropes, toys, etc.
2. Polystyrene is used in making thermo insulating parts of electric appliances like refrigerators, gears of machines, toys, protective coverings like covers of CD and DVD, etc.
3. Polyethylene (PE) plastics are used for making milk bags, packing bags, flexible garden pipes, etc.
4. Polypropylene (PP) is used in making parts of loudspeakers and vehicles, ropes, mattresses, laboratory appliances, etc.

II. Thermosetting plastic is used in the manufacturing of the following items:

1. Bakelite for making cabinets of radio, T.V., telephones, electric switches, toys, plastic handles of cookers, etc.
2. Melamine for making domestically useful items like cup – saucers, plates, tray, some spare parts of airplane engines, electric and sound insulating coverings, etc.
3. Polyurethane in making surfing boards, small boats, furniture, seats in vehicles, etc.
4. Polyester in fibreglass, toners of laser printers, textile industry, etc.

Can you tell?

Question 1.
Make a list of 20 different man-made materials present in your home, school and places around and discuss.
Answer:
Schoolbag, books, computer, table lamp, tube lights, benches, shoes, dress, pencil, pen, tiffin box, water bottle, raincoat, umbrella, medicine tablets, pen drive, chalk, duster, utensils, gas burner, biscuits.

Can you recall?

Question 1.
How many plastic carry bags are brought in your home in a day? What happens to those later on?
Answer:
Till May 2018 about one or two plastic bags were brought in our home every day. But now due to prohibition on plastic bags and other plastic items by the Government, the use of plastic has been drastically reduced. The bags brought at home caused lot of plastic pollution. Plastic is non-biodegradable, thus it always created problem of their disposal.

If dumped in a water body, it can be hazardous for aquatic animals. If thrown in the garbage, plastic articles find their way to landfilling areas. If thrown anyhow plastic may be consumed by stray animals. This causes toxic effects on them. If burnt they emit toxic gases.

Question 2.
How are the used up and thrown away carry bags, water bottles, milk bags recycled?
Answer:
The plastic waste is collected by the kabadiwala or sweepers. They sell these articles to the recycling units. In recycling units, the recycling of some of the plastic items is done. This leads to formation of plastic of low quality which may be used in making some newer plastic items.

Question 3.
Which material is wrapped around the items of glass or similar material during transport to prevent from breaking up?
Answer:
The plastic bubble wrap or thermocol is used to protect fragile items from the shocks and breaks during their transport.

Project:

Question 1.
Collect the information about the plastic used in production of utensils used in microwave oven.

Question 2.
Collect the information about the material used in production of denture. (A set of artificial teeth)

Class 8 Science Chapter 17 Man-made Materials Important Questions and Answers

Try to find it:

Question 1.
The plastic that can be moulded as per our wish is called …………… .
Answer:
The plastic that can be moulded as per our wish is called thermoplastic.

Question 2.
…………… is a type of plastic used for manufacturing artificial teeth.
Answer:
Polyacrylic is a type of plastic used for manufacturing artificial teeth.

Question 3.
Thermocol is a form of a complex material called ……………. .
Answer:
Thermocol is a form of a complex material called polystyrene.

Write whether the following statements are True or False: Rewrite the false statements after correcting:

Question 1.
Plastics are inorganic polymers that show plasticity.
Answer:
False. (Plastics are organic polymers that show plasticity.)

Question 2.
Polythene, PVC are thermosetting plastic materials.
Answer:
False. (Polythene, PVC are thermoplastic materials.)

Question 3.
Plastic is bad conductor of heat and electricity.
Answer:
True.

Question 4.
Surfing boards are made up of bakelite.
Answer:
False. (Surfing boards are made up of polyurethane.)

Question 5.
Being bad conductor of electricity, glass is used as insulator in electric appliances.
Answer:
True.

Match the columns:

Question 1.

Column A	Column B
1. Borosilicate glass	a. Oxide of specific metal.
2. Alkali silicate glass	b. Sand, soda, limestone, barium oxide and boron.
3. Lead glass	c. Sand and soda.
4. Optical glass	d. Sand, soda, limestone and lead oxide.
5. Coloured glass	e. Sand, soda, boric acid and aluminium oxide.

Answer:

Column A	Column B
1. Borosilicate glass	e. Sand, soda, boric acid and aluminium oxide.
2. Alkali silicate glass	c. Sand and soda.
3. Lead glass	d. Sand, soda, limestone and lead oxide.
4. Optical glass	b. Sand, soda, limestone, barium oxide and boron.
5. Coloured glass	a. Oxide of specific metal.

Find the odd one out:

Question 1.
Bakelite, Melamine, Polystyrene, Polyester.
Answer:
Polystyrene (All others are thermosetting plastic materials, polystyrene is thermoplastic.)

Question 2.
Polyvinyl chloride, Polystyrene, Polypropylene, Polyurethane.
Answer:
Polyurethane (All others are thermoplastic materials, polyurethane is thermosetting plastic.)

Question 3.
Vegetable peels, Cotton bolls, Wooden scraps, Plastic bag.
Answer:
Plastic bag (All others are degradable materials, plastic is non-degradable.)

Question 4.
Teflon, Polyacrylic, Fibreglass, Polyester.
Answer:
Fibreglass (All others are types of plastic.)

Considering the relationship in the first pair, complete the second pair:

Question 1.
Processed glass: Fen glass : : Alkali silicate glass : ………..
Answer:
Water glass

Question 2.
Ferrous oxide: Bluish green glass : : Copper oxide : ……..
Answer:
Red glass

Question 3.
Lens : Optical glass : : Laboratory glassware : ……….
Answer:
Silica glass

Question 4.
Light bulbs, Tubes: Lead glass : : Medicine storing : ………
Answer:
Borosilicate glass

Question 5.
Glass: Mixture of silica and silicate : : Thermocol : ………..
Answer:
Polystyrene.

Distinguish between:

Question 1.
Degradable and Non-degradable substances:
Answer:

Degradable substances	Non-degradable substances
1. The substances which can be naturally reduced into their inorganic constituents are called degradable substances.	1. The substances that cannot be degraded on their own into their inorganic constituents are called non-degradable substances.
2. Degradable substances are not accumulated in the nature.	2. Non-degradable substances remain accumulated in the nature for a long time.
3. Degradable substances emit foul odour when they are being decomposed.	3. Non-degradable substances may not emit foul odour as they are not degraded. But they make the place look dirty.
4. Usually microorganisms play a part in the degradation process, hence such substances are also called biodegradable. E.g. Vegetables, fruits, wood, cotton or wool fibres, etc.	4. Microorganisms cannot act on non-degradable substances and hence they are also called non-biodegradable substances. E.g. Plastic, thermocol, glass, metals, etc.

Write short notes:

Question 1.
Properties of plastic:
Answer:

1. Plastic is non-corrosive.
2. It is non-degradable as it does not decompose.
3. The factors such as humidity, heat, rain, etc. do not affect plastic.
4. Any coloured item can be manufactured from plastic.
5. Plastic shows property of plasticity. Thus it can be moulded into any shape that is needed.
6. Plastic is a bad conductor of heat and electricity.
7. Plastic is light-weight and thus preferred for carrying.

Question 2.
Properties of Glass:
Answer:
The glass becomes soft on heating and thus can be moulded into desired shape.

1. According to ingredients added at the time of preparation of glass, its density changes accordingly.
2. Glass is slow conductor of heat. If a cold glass is quickly heated, it cracks suddenly. Similarly, the warm glass if exposed to sudden cooling, it too cracks.
3. Glass is a bad conductor of electricity. Therefore, it is used as insulating material in electric appliances.
4. Glass is transparent, allowing most of light to pass through it. If there are chromium, vanadium or iron oxides in the glass, large amount of light is absorbed in glass.

Answer the following questions in one sentence only:

Question 1.
What is the use of plastic in healthcare sector?
Answer:
Saline bottles, pouches, blood bags, syringes and medicine bottles are made up of plastic, making plastic irreplaceable in the healthcare sector.

Question 2.
Which material is used in coating of vehicles? Why?
Answer:
Teflon is used for coating the vehicle as it protects the vehicles from scratches.

Question 3.
What is the use of polyacrylic type of plastic?
Answer:
Polyacrylic plastic used for manufacturing lenses and in the manufacture of artificial teeth.

Question 4.
What is 4R principle?
Answer:
The 4R principle is the way of behaviour that prevents or lessens the environmental pollution. The 4Rs are Reduce, Reuse, Recycle and Recover.

Question 5.
What are the eco-friendly options for a plastic bag?
Answer:
Jute bag, cloth bag and reusable bags of any natural materials are the eco-friendly options for the plastic bag.

Question 6.
How is Borosilicate glass produced?
Answer:
Borosilicate glass is produced by melting the mixture of sand, soda, boric acid and aluminium oxide.

Question 7.
When is large amount of light absorbed in the glass?
Answer:
If there are oxides of either chromium, vanadium or iron in the glass, large amount of light is absorbed in it.

Give scientific reasons:

Question 1.
It is harmful to eat food kept in thermocol.
Answer:
Thermocol is made up from polystyrene. It is carcinogenic (cancer causing) substance. When food kept in r thermocol containers is reheated, this , styrene dissolves in it. This may affect the r health of the person who consumes such food. It has been noted that there is a great possibility of blood cancer like leukaemia r and lymphoma if one remains in contact with thermocol for a longer time. Thus, it is harmful to eat food kept in thermocol.

Question 2.
Vessels used to cook food in the microwave oven are made up of plastic.
Answer:
The vessels made from metals cannot be used in the microwave oven. The microwaves cannot penetrate steel and so ‘ can only heat the contents through the open top. Some metals cause sparking with microwaves. Microwave energy is absorbed differently by different materials. Plastics of only certain kind are more suitable to be used in the microwave oven.

Answer the following:

Question 1.
What are the qualities of thermocol?
Answer:

1. Thermocol is modern man-made material made from polystyrene.
2. It can be transformed into liquid state when heated at more than 100 °C temperature. On cooling, it returns to the solid state.
3. Therefore, any desired shape can be given to it.
4. It acts as a good shock-absorber and hence used as a packing material to transport brittle material.
5. It is a bad insulator of heat and hence used for storing fish and other perishable articles in market.

Think about it:

Question 1.
Why are the plastic tanks used for storage of chemicals?
Answer:
Plastic is non-corrosive. It is comparatively non-reactive and it does not decompose. Moreover, the plastic containers are easier for handling. They are lighter and unbreakable. Hence the plastic tanks are used for storage of chemicals.

Question 2.
Why most of domestically useful items are replaced by plastic?
Answer:
Plastic is lighter and unbreakable. The handling thus becomes easier. It is non-corrosive, hence pickles, spices, etc. can be stored in plastic containers. It is cheaper as compared to the metal and glass articles. Thus, it becomes popular with general public. It is easier to wash and maintain the plastic articles. These are some of the reasons of making plastic a domestically useful material.

Collect information:

Question 1.
To prevent the degradation due to sunlight, some materials are stored in which type of bottles?
Answer:

The amber glass or brown coloured glass bottles are used to store the materials that may get degraded due to sunlight. This colour prevents the ultraviolet rays of the sun to enter the bottle and bring about chemical changes in the materials. Especially, for sensitive medicines, brown glass bottles are used.

Question 2.
Which type of glass is used in vehicles to avoid injuries in accidents?
Answer:
Front and rear doors have tempered glass. The windshield has laminated safety glass. Even if the accident takes place the glass pieces do not injure the passengers. Laminated safety glass combines two curved glass sheets and a plastic lamina between the two layers of the glass.

This glass is tough and protective. Tempered glass on the other hand is toughened glass which is processed by controlled thermal or chemical treatments. Upon breaking, the tempered glasses crumble into small granular chunks instead of splintering into sharp glass pieces. This prevents the injuries.

Sketch and label the diagram of glass manufacturing process:

Open-Ended Questions:

Question 1.
Classify and make a chart of the materials used in various items in house. Make additions to that chart with reference to various materials.

Answer:

Question 2.
Make list about use of thermocol in your daily life.
Answer:

1. Decorative items used at the time of festivals.
2. Packing materials.
3. Insulating boxes to keep food warm.
4. Insulating boxes to keep fish in iced condition to prevent decomposition.
5. Thermocol beads in the bean bags.

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

Maharashtra State Board Class 8 Science Solutions Chapter 15 Sound

Class 8 Science Chapter 15 Sound Textbook Questions and Answers

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.

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 14 Measurement and Effects of Heat Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 Science Solutions Chapter 14 Measurement and Effects of Heat

Class 8 Science Chapter 14 Measurement and Effects of Heat Textbook Questions and Answers

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.

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 16 Reflection of Light Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 Science Solutions Chapter 16 Reflection of Light

Class 8 Science Chapter 16 Reflection of Light Textbook Questions and Answers

1. Fill in the blanks:

Question i.
The perpendicular to the mirror at the point of incidence is called …………. .
Answer:
The perpendicular to the mirror at the point of incidence is called the normal.

Question ii.
The reflection of light from a wooden surface is ……….. reflection.
Answer:
The reflection of light from a wooden surface is irregular reflection.

Question iii.
The working of a kaleidoscope is based on the properties of …………… .
Answer:
The working of a kaleidoscope is based on the properties of reflection of light.

2. Draw a figure describing the following: The reflecting surfaces of two mirrors make an angle of 90° with each other. If a ray incident on one mirror has an angle of incidence of 30°, i draw the ray reflected from the second mirror. What will be its angle of reflection?

Question a.
Draw a figure describing the following: The reflecting surfaces of two mirrors make an angle of 90° with each other. If a ray incident on one mirror has an angle of incidence of 30°, draw the ray reflected from the second mirror. What will be its angle of reflection?
Answer:
For the ray C, the angle of reflection = 60°.

3. How will you explain the statement ‘We cannot see the objects in a dark room’?

Question a.
How will you explain the statement ‘We cannot see the objects in a dark room’?
Answer:
In a room that is completely dark, no light falls on objects. Hence, no light enters our eyes. Hence, there is no sensation of vision, i.e., we cannot see the objects.

4. Explain the difference between regular and irregular reflection of light.

Question a.
Explain the difference between regular and irregular reflection of light.
Answer:
For regular reflection of light, the angles of incidence as well as the angles of reflection are the same for all parallel rays of light incident on the plane and smooth surface. Hence, the reflected rays are also parallel to one another.

For irregular reflection of light, the angles of incidence for parallel rays of light incident on the rough surface are not equal, and hence the angles of reflection are also not equal. Here, the reflected rays are not parallel to one another and spread over a large surface.

5. Draw a figure showing the following:
(a) Incident ray, (b) Normal, (c) Angle of incidence, (d) Angle of reflection, (e) Point of incidence, (f) Reflected ray.

Question a.
Draw a figure showing the following:
(a) Incident ray
(b) Normal
(c) Angle of incidence
(d) Angle of reflection
(e) Point of incidence
(f) Reflected ray.
Answer:

6. Study the following incident.

Swara and Yash were looking in a water-filled vessel. They could see their images clearly in the still water. At that instant, Yash threw a stone in the water. Now their images were blurred. Swara could not understand the reason for the blurring of the images.
Explain the reason for blurring of the images to Swara by answering the following questions:

Question i.
Is there a relation between the reflection of light and the blurring of the images?
Answer:
Yes.

Question ii.
Which types of reflection of light can you notice from this?
Answer:
Regular reflection of light when light is incident on the still water and irregular reflection of light when light is incident on the water as ripples are produced on its surface when a stone is thrown in the water.
Still water behaves as a plane and smooth surface while oscillating water behaves as a rough surface.

Question iii.
Are the laws of reflection followed in these types of reflection?
Answer:
Yes.

7. Solve the following examples.

Question a.
If the angle between the plane mirror and the incident ray is 40°, what are the angles of incidence and reflection?
Solution:
The angle between the plane mirror and the incident ray is 40°. Therefore, the angle of incidence (i) = the angle made by the incident ray with the normal to the plane mirror = 90° – 40° = 50°. The angle of reflection, r – i – 50°.

Question b.
If the angle between the mirror and reflected ray is 23°, what is the angle of incidence of the incident ray?
Solution:
The angle between the mirror and the reflected ray is 23°. Therefore, the angle of reflection (r) = the angle made by the reflected ray with the normal to the plane mirror = 90° – 23° = 67°.
∴ The angle of incidence, i = r = 67°.

Project:

Question a.
Apollo astronauts who stepped on the moon have kept some large mirrors there. Collect information about how the distance to the moon is measured using these.

Class 8 Science Chapter 16 Reflection of Light Important Questions and Answers

Rewrite the following statements by selecting the correct option:

Question 1.
If the angle made by the incident ray with the surface of a plane mirror is 30°, the angle of reflection must be …….. .
(a) 30°
(b) 90°
(c) 60°
(d) 15°
Answer:
If the angle made by the incident ray with the surface of a plane mirror is 30°, the angle of reflection must be 60°.

Question 2.
If the angle of incidence is 40°, the angle made by the reflected ray with the surface of the plane mirror must be ……. .
(a) 40°
(b) 50°
(c) 20°
(d) 80°
Answer:
If the angle of incidence is 40°, the angle made by the reflected ray with the surface of the plane mirror must be 50°.

Question 3.
If the angle of incidence is 20°, the angle made by the reflected ray with the normal to the surface must be ……… .
(a) 20°
(b) 70°
(c) 10°
(d) 40°
Answer:
If the angle of incidence is 20°, the angle made by the reflected ray with the normal to the surface must be 20°.

Question 4.
In a kaleidoscope, the mirrors are inclined to each other at ……. .
(a) 60°
(b) 30°
(c) 45°
(d) 90°
Answer:
In a kaleidoscope, the mirrors are inclined to each other at 60°.

Question 5.
In a periscope, the mirrors are ………….. .
(a) parallel to each other
(b) at right angles to each other
(c) inclined at 45° to each other
(d) inclined at 60° to each other
Answer:
In a periscope, the mirrors are parallel to each other.

Find the odd one out and give the reason:

Question 1.
Plane mirror, Plywood, Wood, Rough tile.
Answer:
Plane mirror. In this case, regular reflection of light takes place. In other cases, reflection of light is irregular.

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

Question 1.
The sense of vision is the most important among our five senses.
Answer:
True.

Question 2.
In a periscope, the angle between the incident ray and the normal to the mirror is 30°.
Answer:
False. (In a periscope, the angle between the incident ray and the normal to the mirror is 45°.)

Answer the following questions in one sentence each:

Question 1.
What is an incident ray?
OR
Define incident ray.
Answer:
A ray of light falling on a surface is called an incident ray.

Question 2.
What is the point of incidence?
OR
Define point of incidence.
Answer:
The point at which the incident ray strikes the surface is called the point of incidence.
[Note: It is also the point of reflection.]

Question 3.
What is the normal?
OR
Define normal.
Answer:
The perpendicular to a surface at the point of incidence is called the normal.

Question 4.
What is the reflected ray?
OR
Define reflected ray.
Answer:
The ray of light that leaves the surface at the point of reflection (the same as the point of incidence) is called the reflected ray.

Question 5.
What is the angle of incidence?
OR
Define angle of incidence.
Answer:
The angle between the incident ray and the normal is called the angle of incidence.

Question 6.
What is the angle of reflection?
OR
Define angle of reflection.
Answer:
The angle between the reflected ray and the normal is called the angle of reflection.

Try this:

Switch off the light in your room at night for some time and then turn it on again.

Question 1.
Could you see the objects in the room clearly when the light was switched off?
Ans.
No.

Question 2.
What did you feel when it was turned on again?
Answer:
We could see the objects clearly. From the above activity you can notice that there is some connection between the sense of vision and light. When we switch off the light at night, the objects in the room cannot be seen and they can be seen as before when the light is switched on again. Thus, we can see objects when the light coming from these objects enters our eyes.

Answer the following questions:

Question 1.
What is reflection of light?
Answer:
When light rays fall on an object, their direction changes and they turn back. This is called the reflection of light.

Try this:

Material:
Torch light, mirror, a stand for hanging the mirror, black paper, comb, white paper, drawing board.
Activity :
1. Fit a white paper tightly over a table or drawing board.
2. Leaving out some portion in the middle of the comb, cover the rest with black paper so that light can only pass through the open central portion.
3. Hold the comb perpendicular to the white paper and throw torch light on its central portion.
4. Adjust the comb and torch so as to get light rays on the white paper. Now keep a mirror in the path of this ray of light as shown in the figure.
5. What do you observe?

Answer:
Light rays which fall on the mirror get reflected and travel in a different direction.

Question 2.
State the laws of reflection of light.
Answer:

1. The angle of reflection is equal to the angle of incidence.
2. The incident ray, the reflected ray and the normal lie in the same plane.
3. The incident ray and the reflected ray are on the opposite sides of the normal.

Try this:

Verification of the laws of reflection of light.
Equipment: Mirror, drawing board, pins, white paper, protractor, scale, pencil.
Activity:

1. Fit a white paper on the drawing board tightly as possible.
2. On the paper draw a line PQ indicating the position of the mirror.
3. Draw a perpendicular ON to PQ at point O.
4. Draw a ray AO making an angle of 30° with ON.
5. Fix two pins S and R along AO.
6. Fix the mirror to a stand and place it along PQ perpendicular to the drawing board.
7. Fix pins at T and U along the line joining the bottom of the reflected images of the pins at S and R.
8. Remove the mirror and join the points T and U and extend it up to O.
9. Measure ZTON.
10. Repeat steps 4 to 9 for angle of incidence equal to 45° and 60° and write down the angles in the following table.

What relation do you find between the angle of incidence and the angle of reflection? If you have done the experiment carefully, you will find that the angle of incidence is equal to the angle of reflection in all three cases. This verifies the laws of reflection.

Question a.
What will happen when a light ray is incident perpendicular to the mirror?
Answer:

Here,
r = i = 90°.
Hence the light ray, on reflection, will retrace the path.

Question 3.
Figures (a) and (b) show three parallel rays, shown in grey, incident on smooth and rough surfaces. The reflected rays drawn using laws of reflection are shown in red.
1. Rays reflected from which surface are parallel to one another?
2. What conclusion can you draw from the figure?
Answer:
1. Rays reflected from the smooth surface are parallel to one another.

2.When the reflecting surface is plane and smooth, the angles of incidence (i) as well as of reflection (r) are the same for all parallel rays incident on the surface. If i₁, i₂, i₃, … are the angles of incidence for incident parallel rays, and r₁, r₂, r₃, …, are the corresponding angles of reflection, then, i₁ = i₂ = i₃ = ……. = r₁ = r₂ = r₃ = ….. This is called regular reflection. Here, the reflected rays are parallel to one another. If the reflecting surface is rough and parallel rays are incident on it, then the angles of incidence are not equal and hence the angles of reflection are also not equal. Here, i₁ ≠ i₂ ≠ i₃ … and r₁ ≠ r₂ ≠ r₃ …, but r₁ = i₁, r₂ = i₂, r₃ = i₃ … as laws of reflection are obeyed. This is called irregular reflection. [Fig.(b)]. Here, the reflected rays are not parallel to one another and spread over a large surface.

Question 4.
What is regular reflection of light?
Answer:
The reflection of light from a plane and smooth surface is called regular reflection of light.

Question 5.
What is irregular reflection of light?
Answer:
The reflection of light from a rough surface is called irregular reflection of light.

Always remember:

1. Laws of reflection are followed in both and regular and irregular reflection.
2. The reflection of light in irregular reflection has not been obtained because the laws of reflection are not followed. They are obtained because the surface is rough (irregular).
3. In irregular reflection, the angles of incidence at different points are different. But at any one point, the angles of incidence and reflection are equal, i.e. i₁ = r₁, i₂ = r₂ …..

Can you recall?

Reflection of reflected light :

Question 1.
How do you see if the barber m a saloon has cut the hair on your neck properly or not?
Answer:
In a saloon, there are mirrors in your front and at back. The image of the back of your head is formed in the mirror at the back. The image of this image is formed in the mirror in front of you. Thus you can see how the hair at the backside of your head is cut.

Question 2.
What type of image do we see in a mirror? What happens to the left and right sides?
Answer:
The image in a plane mirror is upright (erect) and of the same size as the object, but the left and right sides are interchanged. Our right hand appears to be the left hand in the image and the left hand appears to be the right hand in the image. (This is called lateral inversion.)

Question 3.
How do we see the image of the moon in water?
Answer:
The moon is not self luminous. The sunlight falling on the surface of the moon is reflected. This reflected light is again reflected by water to give us the image of the moon.

Try this:

Kaleidoscope:

Activity:

1. Take three rectangular mirrors of the same size.
2. Using sticking tape, stick the mirrors together making a triangle with the reflecting surface facing inwards (see Fig.).
3. Take a white paper of triangular shape and fix it with a tape at one end of the mirrors closing that end.
4. Insert 4 – 5 coloured glass pieces in the hollow of the mirrors.
5. Close the other end also with a paper and make a hole in it.
6. Look through the hole towards light. You will see innumerable images of the glass pieces. These are formed due to reflections by the three mirrors.

You can see different designs in the kaleidoscope. The speciality of a kaleidoscope is that the designs do not easily repeat themselves. Every time the design is different. People making wall papers which are used to decorate walls and cloth designers use a kaleidoscope for making new designs.

Periscope:
Activity:

1. Take a cardboard box. Make slits in the top and bottom sides of the box and place two mirrors so that they make an angle of 45° with the sides of the box and are parallel to each other. Fix them with a sticking tape.
2. Make two windows of 1 inch each near the two mirrors. Now see through the bottom window.
3. Make note of what you see.

From the bottom window, one can see what is in front of the top window. This device is called a periscope. This is used in submarines to see objects above the surface of water. It is also used to observe and keep a watch on the objects or persons on the ground from an underground bunker. The kaleidoscope and periscope both use the properties of reflection of light.

[Note: In a periscope, the angle of incidence is 45° and the two plane mirrors are parallel to each other. Hence, the emergent ray is parallel to the incident ray.]

Example questions for practice:

Question 1.
If the angle between the plane mirror and the incident ray is 20°, what is the angle between the reflected ray
and the plane mirror?
Answer:
20°.

Question 2.
See Fig. In terms of O, what are the angles (i) AON (ii) BON (iii) AOB (iv) BOQ?

Answer:
(i) 90° – θ (ii) 90° – θ (iii) 180° – 2θ

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

Maharashtra State Board Class 8 Science Solutions Chapter 12 Introduction to Acid and Base

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

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.

Balbharti Maharashtra State Board Class 8 Science Solutions Chapter 13 Chemical Change and Chemical Bond Notes, Textbook Exercise Important Questions and Answers.

Maharashtra State Board Class 8 Science Solutions Chapter 13 Chemical Change and Chemical Bond

Class 8 Science Chapter 13 Chemical Change and Chemical Bond Textbook Questions and Answers

1. Complete the statement by filling the gaps using appropriate term from the terms given in the brackets:
(slow, coloured, arrow, fast, smell, milky, physical, product, chemical, reactant, covalent, ionic, octet, duplet, exchange, sharing, equality sign)

Question a.
An ……….. is drawn in between the reactants and products while writing the equation for a chemical reaction.
Answer:
An arrow is drawn in between the reactants and products while writing the equation for a chemical reaction.

Question b.
Rusting of iron is a ……….. chemical change.
Answer:
Rusting of iron is a slow chemical change.

Question c.
The spoiling of food is a chemical change which is recognized from the generation of certain …………. due to it.
Answer:
The spoiling of food is a chemical change which is recognized from the generation of certain smell due to it.

Question d.
A colourless solution of calcium hydroxide in a test tube turns ………….. on blowing in it through a blow tube for some time.
Answer:
A colourless solution of calcium hydroxide in a test tube turns milky on blowing in it through a blow tube for some time.

Question e.
The white particles of baking soda disappear when put in lemon juice. This means that it is a ……….. change.
Answer:
The white particles of baking soda disappear when put in lemon juice. This means that it is a chemical change.

Question f.
Oxygen is a …………….. in respiration.
Answer:
Oxygen is a reactant in respiration.

Question g.
Sodium chloride is …………… compound while hydrogen chloride is compound.
Answer:
Sodium chloride is ionic compound while hydrogen chloride is covalent compound.

Question h.
Electron …………….. is complete in each hydrogen in a hydrogen molecule.
Answer:
Electron duplet is complete in each hydrogen in a hydrogen molecule.

Question i.
Chlorine (Cl₂) molecule is formed by ………….. of electrons between two chlorine atoms.
Answer:
Chlorine (Cl₂) molecule is formed by sharing of electrons between two chlorine atoms.

2. Explain by writing a word equation. 

Question a.
Respiration is a chemical change.
Answer:
Respiration is a biological process, in this process air is inhaled, oxygen present in this inhaled air reacts with glucose present in the cells of the body forming carbon dioxide and water. Moreover, we cannot obtain glucose and oxygen from carbon dioxide and water. Hence, respiration is a chemical change.
Word equation:

Question b.
Hard water gets softened on mixing with a solution of washing soda.
Answer:
Hard water does not form lather with soap and is brackish to taste. This is because hard water contains the chloride and sulphate salts of calcium and magnesium in dissolved state. When a solution of washing soda is added to hard water, it forms a precipitate of calcium carbonate and magnesium carbonate, which is removed by filtration thus water is softened.
Word equation:

Question c.
Limestone powder disappears on adding to dilute hydrochloric acids.
Answer:
In the reaction of dil. HCl and limestone powder (CaCO₃), limestone disappears slowly and carbon dioxide (CO₂) liberates slowly.
Word equation:

Question d.
Bubbles are seen on adding lemon juice to baking soda.
Answer:
When baking soda is added to lemon juice a chemical change takes place in citric acid present in the lemon juice and carbon dioxide gas is formed. Word equation:

This is a neutralization reaction.

3. Match the pairs.

Question a.

Column I	Column II
1. Photosynthesis	a. Tendency to lose electrons
2. Water	b. Reactant in combustion process
3. Sodium chloride	c. Chemical change
4. Dissolution of salt in water	d. Covalent bond
5. Carbon	e. Ionic bond
6. Fluorine	f. Physical change
7. Magnesium	g. Tendency to form anion

Answer:

Column I	Column II
1. Photosynthesis	c. Chemical change
2. Water	d. Covalent bond
3. Sodium chloride	e. Ionic bond
4. Dissolution of salt in water	f. Physical change
5. Carbon	b. Reactant in combustion process
6. Fluorine	g. Tendency to form anion
7. Magnesium	a. Tendency to lose electrons

4. Show with the help of diagram of electronic configuration how the following compounds are formed from the constituent atoms.

Question a.
Sodium Chloride:

Answer:
1. Sodium has atomic number 11 and electronic configuration 2, 8, 1.
2. Sodium atom has 1 electron in its outermost shell.

3. It loses one electron from its outermost shell, i.e., M shell. Then its L shell becomes the outermost shell with a stable octet. The nucleus of sodium atom has 11 protons but the number of electrons in the atom has become 10. So, there is a net unit positive charge giving a sodium cation (Na⁺).

4. On the other hand, chlorine has electronic configuration 2, 8, 7. Chlorine atom has 7 electrons in its outermost shell and requires one electron to complete its octet.
5. Thus, the electron lost by sodium is taken up by chlorine.

6. When chlorine atom gains one electron, octet of chlorine is completed and its K, L, M shells have together 18 electrons and the nucleus has 17 protons. This leads to the formation of an ion (CP).

7. Thus, a chlorine atom accepts one electron from a sodium atom and consequently a chloride ion with one unit negative charge and a sodium ion with one unit positive charge are formed.

8. Sodium and chloride ions, being oppositely charged, attract each other due to the electrostatic force of attraction. An ionic bond is formed and this results in the formation of sodium chloride (NaCl) molecule.

Question b.
Potassium fluoride:
Answer:
1. Potassium has atomic number 19 and electronic configuration 2, 8, 8, 1.
2. Potassium atom has 1 electron in its outermost shell.

3. It loses one electron from its outermost shell, i.e. N shell. Then its M shell becomes the outermost shell with a stable octet. The nucleus of potassium atom has 19 protons but the number of electrons in the atom has become 18. So there is a net unit positive charge giving a potassium cation (K⁺).

4. On the other hand, fluorine has electronic configuration 2, 7. Fluorine has 7 electrons in the outermost shell and requires one electron to complete its octet.

5. Thus, the electron lost by potassium is taken up by chlorine.

6. When fluorine atom gains one electron, octet of fluorine is completed, its K and L shells have together 10 electrons and the nucleus has 9 protons. This leads to the formation of an ion (F^–).

7. Thus, a fluorine atom accepts one electron from a potassium atom and consequently a fluoride ion with one unit negative charge and a potassium ion with one unit positive charge are formed.

8. Potassium and fluoride ions, being oppositely charged, attract each other due to electrostatic force of attraction. An ionic bond is formed and this results in the formation of potassium fluoride (KF) molecule.

Question c.
Water:
Answer:

1. Hydrogen has atomic number 1 and electronic configuration 1.
2. Hydrogen has 1 electron in its K shell.

3. Oxygen has atomic 8 and electronic configuration 2, 6. There are 6 electrons in the valence shell of oxygen atom. It means that the electron octet in oxygen is short of two electrons and the valency of oxygen is two.

4. In the H₂O molecule, the oxygen atom complete its octet by sharing two electrons one each with two hydrogen atoms, thus, forming two covalent bonds. While this happens, the duplets of two hydrogen atoms are completed.

Question d.
Hydrogen chloride:
Answer:
1. Hydrogen has atomic number 1 and electronic configuration 1, that means it has 1 electron in its K shell and its duplet is short of one electron therefore, the valency of hydrogen is one.

2. On the other hand, chlorine has electronic configuration 2, 8, 7. Chlorine atom has 7 electrons in its outermost shell and requires one electron to complete its octet.

3. The two atoms, hydrogen and chlorine share one electron with each other. As a result, the electron duplet of hydrogen and octet of chlorine is complete and a covalent band is formed between them.

Can you recall?

Question a.
What are the methods of classification of changes?
Answer:
The methods of classification of changes: Physical change and chemical change.

Question b.
What is the difference between physical and chemical change?
Answer:

1. In physical change, the composition of substance does not change. No new substance is formed.
2. In chemical change, the composition of compounds change and new compounds are formed.

Question c.
Classify the following changes into physical and chemical change. Ripening of mango, melting of ice, boiling of water, dissolution of salt in water, Ripening of banana, fragrance on ripening fruit, darkening of a cut potato, bursting of an inflated balloon, sound of bursting fire cracker, foul smell from a portion of spoiled food.
Answer:

• Physical change: Melting of ice, boiling of water, dissolution of salt in water.
• Chemical change: Ripening of mango, ripening of banana, fragrance of ripening fruit, darkening of a cut potato, bursting of an inflated balloon, sound of bursting firecracker, foul smell from a spoiled food.

Project:

Question a.
Prepare a list of the chemical changes that occur in your house and surroundings and discuss these in the class.

Class 8 Science Chapter 13 Chemical Change and Chemical Bond Important Questions and Answers

Fill in the blanks:

Question 1.
……………. is a continuously occurring biological process.
Answer:
Respiration is a continuously occurring biological process.

Question 2.
The reaction between citric acid and sodium bicarbonate is a ……………. reaction.
Answer:
The reaction between citric acid and sodium bicarbonate is a neutralization reaction.

Question 3.
Combustion of fuel is a ………… and ……………. chemical change.
Answer:
Combustion of fuel is a fast and irreversible chemical change.

Question 4
Electronic configuration of sodium is …………….
Answer:
Electronic configuration of sodium is 2, 8, 1.

Question 5.
Electronic configuration of chlorine is …………….
Answer:
Electronic configuration of chlorine is 2, 8, 7.

Question 6.
A chemical bond formed by sharing of valence electrons of two atoms with each other is called a …………….
Answer:
A chemical bond formed by sharing of valence electrons of two atoms with each other is called a covalent bond.

Question 7.
Hard water contains the chloride and sulphate salts of ……………. and ……………. in dissolved state.
Answer:
Hard water contains the chloride and sulphate salts of calcium and magnesium in dissolved state.

Question 8.
Electronic configuration of fluorine is …………….
Answer:
Electronic configuration of fluorine is 2, 7.

Question 9.
Green plants perform ……………. in sunlight.
Answer:
Green plants perform photosynthesis in sunlight.

Question 10.
Melting of ice is a ……………. change.
Answer:
Melting of ice is a physical change.

Question 11.
The chloride ion has a ……………. charge.
Answer:
The chloride ion has a negative charge.

Question 12.
A covalent bond between two atoms is also represented by ……………. joining their symbols.
Answer:
A covalent bond between two atoms is also represented by dash joining their symbols.

Rewrite the following statements by selecting the correct options:

Question 1.
Sodium atoms and sodium ions ………….. .
(a) are chemically the same
(b) have the same number of protons
(c) have the same number of electrons
(d) form covalent bond
Answer:
(b) have the same number of protons

Question 2.
An ionic bond is formed when …………….. .
(a) two metallic elements react
(b) two nonmetallic elements react
(c) a metallic element reacts with a non-metallic element
(d) a pair of elements react
Answer:
(c) a metallic element reacts with a non-metallic element

Question 3.
…………….. tries to establish the duplet state in its outermost orbit by sharing electrons.
(a) Sodium
(b) Potassium
(c) Hydrogen
(d) Magnesium
Answer:
(c) Hydrogen

Question 4.
…………… is an electron donor.
(a) Helium
(b) Iodine
(c) Chlorine
(d) Magnesium
Answer:
(d) Magnesium

Question 5.
………………. combine to form an ionic compound.
(a) Hydrogen and chlorine
(b) Hydrogen and oxygen
(c) Potassium and chlorine
(d) Nitrogen and oxygen
Answer:
(c) Potassium and chlorine

Question 6.
……………. is an example of chemical change.
(a) Magnetism of iron
(b) Rusting of iron
(c) Heating of iron till it becomes red hot
(d) Dissolution of salt in water
Answer:
(b) Rusting of iron.

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

Question 1.
The preparation of cold drink soda- lemon is a physical change.
Answer:
False. [The preparation of cold drink soda-lemon is a chemical change.]

Question 2.
Hard water contains the chloride and sulphate salts of calcium and magnesium in dissolved state.
Answer:
True.

Question 3.
Combustion of fuel is a fast and irreversible man-made chemical change.
Answer:
True.

Question 4.
Photosynthesis reaction is a man-made chemical change.
Answer:
False. [Photosynthesis reaction is a natural chemical change.]

Question 5.
The atoms with incomplete electron octet/duplet form chemical bonds.
Answer:
True.

Question 6.
Electronic configuration of chlorine is 2, 8, 6.
Answer:
False. (Electronic configuration of chlorine is 2, 8, 7.)

Question 7.
One ionic bond is formed due to the electrical change +1 or -1 on an ion.
Answer:
True.

Question 8.
H₂O Molecule is an ionic compound.
Answer:
False. (H₂O molecule is a covalent compound.)

Question 9.
The bond between two chlorine atoms is a covalent band.
Answer:
True.

Question 10.
Arrow indicates the direction of the reaction.
Answer:
True.

Consider the correlation between the words of the first pair, match the third word/words with the most appropriate answer:

Question 1.
K : 2, 8, 8, 1 : : Mg : ………………
Answer:
2, 8, 2

Question 2.
MgCl₂ : Ionic bond : : CaO : ……………..
Answer:
Covalent bond

Question 3.
Photosynthesis : Natural chemical change : : Cold drink, soda lemon : ………
Answer:
Man-made chemical change

Question 4.
Respiration: Glucose + Oxygen : : Neutralization : ………….
Answer:
Acid + alkali.

Match the columns:

Question 1.

Column I	Column II
1. Respiration	a. Potassium and fluorine
2. Acid + Base	b. Glucose and oxygen
3. Photosynthesis	c. Carbon dioxide and water
4. Ionic bond	d. Salt and water

Answer:

Column I	Column II
1. Respiration	c. Carbon dioxide and water
2. Acid + Base	d. Salt and water
3. Photosynthesis	b. Glucose and oxygen
4. Ionic bond	a. Potassium and fluorine

Define the following:

• Chemical change: In a chemical change, the chemical composition of the original matter changes and new substances having different properties and different chemical composition are formed.
• Ionic bond: The chemical bond formed due to an electrostatic force of attraction between the oppositely charged cation and anion is called an ionic bond or an electrovalent bond.
• Covalent bond: The chemical bond formed by sharing of valence electrons of two atoms with each other is called a covalent bond.

Answer the following questions in one sentence each:

Question 1.
Name the reactants in respiration.
Answer:
Glucose and oxygen are the reactants in respiration.

Question 2.
Name the products of respiration.
Answer:
Carbon dioxide and water are the products of respiration.

Question 3.
Name the reactants in photosynthesis.
Answer:
Carbon dioxide and water are the reactants in photosynthesis.

Question 4.
Name the products of photosynthesis.
Answer:
Glucose and oxygen are the products of photosynthesis.

Question 5.
Name the salts present in hard water.
Answer:
Salts present in hard water are calcium chloride, magnesium chloride, calcium sulphate and magnesium sulphate.

Question 6.
Give two examples of ionic compounds.
Answer:
Ionic compounds: Sodium chloride (NaCl), Potassium fluoride (KF).

Question 7.
Give two examples of covalent compounds.
Answer:
Covalent compounds: Hydrogen (H₂), Water (H₂O).

Question 8.
Name an acid used for cleaning Shahabad tile.
Answer:
An acid used for cleaning Shahabad tile is dilute hydrochloric acid (HCl).

Question 9.
Write a chemical equation for combustion of fuel.
Answer:
Chemical equation:
C + O₂ → CO₂

Question 10.
Write a chemical equation for photosynthesis.
Answer:
Chemical equation:

Answer the following questions:

1. Explain by writing a word equation.

Question 1.
Combustion of fuel is a fast and irreversible chemical change.
Answer:
Wood, coal, petrol or cooking gas are burnt for producing energy. Carbon is the common substance that burns in all these fuels. The product carbon dioxide is formed when carbon combines with oxygen in the air during the combustion process. We cannot obtain fuel from carbon dioxide by employing any other method. Properties of carbon dioxide are altogether different from those of fuel. Hence, this change is a irreversible chemical change.
Word equation:
Carbon + Oxygen → Carbon dioxide

Question 2.
Dilute hydrochloric acid is used for cleaning Shahabad tiles.
Answer:
The main constituent of Shahabad tile is calcium carbonate. During its cleaning with hydrochloric acid, the upper layer of the tile reacts with hydrochloric acid and three products are formed. One of them is calcium chloride, which being soluble in water, gets washed away with water. The second product is carbon dioxide, it mixes with air. The third product is water.
Word equation:

2. Write a chemical equation (unbalanced) for the following reactions:

Question 1.

Answer:
Chemical equation:
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Question 2.

Answer:
Chemical equation:
CaCO₃ + HCl → CaCl₂ + CO₂ + H₂O

Question 3.
Magnesium salts during the softening of hard water.

Answer:
Chemical equation:
MgCl₂ + Na2CO₃ → MgCOg + 2NaCl

3. Show with the help of diagram of electronic configuration how the following compounds are formed from the constituent atoms.

Question 1.
Hydrogen molecule:
Answer:
1. Hydrogen has atomic number 1 and electronic configuration 1. That means it has 1 electron in its K shell and its duplet is short of one electron therefore, the valency of hydrogen is one.

2. The two atoms of hydrogen are identical, they share their electrons with each other.
3. As a result, the electron duplet of both the hydrogen atoms is complete and a covalent band is formed between them.

4. There is one covalent bond between the component atoms H and Cl of the molecule HCl. Use this information to represent the formation of HCl molecules from H and Cl atoms diagrammatically.
(Use your brainpower)
Answer:
Hydrogen chloride:
1. Hydrogen has atomic number 1 and electronic configuration 1, that means it has 1 electron in its K shell and its duplet is short of one electron therefore, the valency of hydrogen is one.

2. On the other hand, chlorine has electronic configuration 2, 8, 7. Chlorine atom has 7 electrons in its outermost shell and requires one electron to complete its octet.

3. The two atoms, hydrogen and chlorine share one electron with each other. As a result, the electron duplet of hydrogen and octet of chlorine is complete and a covalent band is formed between them.

5. Show the formation of the following ionic compounds from the corresponding elements using two methods namely, numerical and diagrammatic representation of electronic configuration.
(a) K⁺ F^–, from 19K and 9F, (b) Ca²⁺ O^2- from ₂₀Ca and ₈O.
(Use your brainpower)
Answer:
(a) K⁺F^– from ₁₉K and ₉F
Potassium fluoride:
1. Potassium has atomic number 19 and electronic configuration 2, 8, 8, 1.
2. Potassium atom has 1 electron in its outermost shell.

3. It loses one electron from its outermost shell, i.e. N shell. Then its M shell becomes the outermost shell with a stable octet. The nucleus of potassium atom has 19 protons but the number of electrons in the atom has become 18. So there is a net unit positive charge giving a potassium cation (K⁺).

4. On the other hand, fluorine has electronic configuration 2, 7. Fluorine has 7 electrons in the outermost shell and requires one electron to complete its octet.

5. Thus, the electron lost by potassium is taken up by chlorine.

6. When fluorine atom gains one electron, octet of fluorine is completed, its K and L shells have together 10 electrons and the nucleus has 9 protons. This leads to the formation of an ion (F^–).

7. Thus, a fluorine atom accepts one electron from a potassium atom and consequently a fluoride ion with one unit negative charge and a potassium ion with one unit positive charge are formed.

8. Potassium and fluoride ions, being oppositely charged, attract each other due to electrostatic force of attraction. An ionic bond is formed and this results in the formation of potassium fluoride (KF) molecule.

(b) Ca²⁺O^2- from ₂₀Ca and ₈O.

Distinguish between the following:

Question 1.
Physical change and Chemical change:
Answer:

Physical change	Chemical change
1. In this change, the composition of the substance does not change. No new substance is formed.	1. In this change, the composition of the compounds change and new compounds are formed.
2. In this case, physical properties such as state, colour, density, etc. are changed.	2. In this case, physical and chemical properties are entirely changed.
3. This change is temporary.	3. This change is permanent.
4. In this case, the original substance can be recovered by simple means or by merely reversing the process.	4. In this case, the original substance cannot be recovered by easy means or by reversing the process.

Question 2.
Ionic bond and Covalent bond:
Answer:

Ionic bond	Covalent bond
1.  Ionic bond is formed due to the transfer of electrons from one atom to another.	1. Covalent bond is formed due to the sharing of electrons between two or more atoms.
2. Atoms of metals and nonmetals combine to form ionic bonds.	2. Atoms of nonmetals combine to form covalent bonds.
3. Molecules of the compounds formed due to ionic bond split up into ions in aqueous solution.	3. Molecules of the compounds formed due to covalent bond so not split up into ions in a solution.

Give scientific reasons:

Question 1.
Ionic compounds are formed due to the combination of metallic and nonmetallic atoms.
Answer:
Metallic atoms have a tendency to lose electrons from their outermost orbits to establish the octet state in their penultimate orbits. Conversely, nonmetallic atoms gain electrons to establish the octet state of their outermost orbits.

When a metallic atom and a nonmetallic atom come close together, the metallic atom loses electrons and gets c converted into positively charged ion, while the nonmetallic’ atom gets converted into negatively charged ions so formed, develop an ionic bond and this results in the formation of an ionic compound. Hence, ionic compounds are formed due to the combination of metallic and nonmetallic atoms.

Activity-based questions:

Activity 1:

Take the lemon juice in a clean glass. Take two drops of the lemon juice in a spoon and taste. Add a pinch of baking soda in the glass of lemon juice. Did you notice bubbling around the particles of soda? Did you hear a sound on taking your ear near the glass? Now again taste it. Did it taste as sour as it was in the beginning? (Above activity is to be done using clean apparatus and edible material. Then only it is possible to test the ’taste’, otherwise keep in mind that the testing of ‘taste’ cannot be done.)

Activity 2:
Take some freshly prepared lime water (solution of calcium hydroxide) in a test tube. Keep on blowing in it with a blow tube. What is seen after some time? Did the colourless lime water turn milky? After some more time you will find that a white insoluble solid settles at the bottom of the test tube. This is a precipitate of calcium carbonate. The turning lime water milky means that the blown gas mixed in it was carbon dioxide.

Write a chemical equation for the above word equation.
Chemical equation:
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O