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Page No 68:

Question A:

Solve the following crossword puzzle:
Figure

Across
3. Heating causes ...................... of iron ring
5. No new substance is formed in this change and it can be reversed

Down
1. When something is transformed into other things
2. Digestion of food is a ................... change
4. It is necessary for some changes to take place

Answer:



Page No 69:

Question B.1:

Bursting of cracker is
(a) reversible change
(b) irreversible change
(c) both of these
(d) none of these

Answer:

(b) irreversible change
A busted cracker cannot be converted back to its original state.

Page No 69:

Question B.2:

A change may manifest itself by change in
(a) shape
(b) size
(c) colour
(d) all of these

Answer:

(d) all of these
The change occurred can be identified by change in shape, size and colour of anything.

Page No 69:

Question B.3:

Rusting of iron nail is
(a) reversible change
(b) irreversible change
(c) periodic change
(d) non-periodic change

Answer:

(b) irreversible change
Rusted iron cannot be converted back to its original state.

Page No 69:

Question B.4:

Formation of curd from milk is the example of
(a) slow
(b) chemical change
(c) periodic change
(d) physical change

Answer:

(b) chemical change
Formation of curd from milk is an example of chemical change.

Page No 69:

Question C:

Fill in the blanks to say whether the following changes are 'reversible' or irreversible':
1. A change which cannot be reversed is called ............................... .
2. Setting of cement is an example of a ............................... change.
3. Absorption of water by a sponge is an example of a ............................... change.
4. Cooking of food represents ............................... change.
5. Digestion of food is a ............................... change.

Answer:

1. A change which cannot be reversed is called irreversible change.
2. Setting of cement is an example of a irreversible change.
3. Absorption of water by a sponge is an example of a reversible change.
4. Cooking of food represents irreversible change.
5. Digestion of food is a chemical change.

Page No 69:

Question D:

Match the items is Column A with the items in Column B

Column A Column B
1. Condensing steam into water (a) Contact
2. Dyeing a cloth (b) Change in size
3. Evaporating water to get salt (c) Heating
4. Inflated balloon (d) Periodic change
5. Burning a matchstick (e) Reversible change
  (f) Change in colour

Answer:

Column A Column B
1. Condensing steam into water (e) Reversible change
2. Dyeing a cloth (f) Change in colour
3. Evaporating water to get salt (c) Heating
4. Inflated balloon (b) Change in size
5. Burning a matchstick (a) Contact

Page No 69:

Question E:

Write True (T) or False (F) against the following statements in the given brackets:
1. Digestion of food is a Q irreversible change. ( )
2. Rusting of iron is a reversible change. ( )
3. Melting of wax is a reversible change. ( )
4. Curding of milk is an irreversible change. ( )
5. Melting of ice cream is a reversible change. ( )

Answer:

1. True (T)

2. False (F)
Rusting of iron is an irreversible change, as rusted iron cannot be converted back to its original state.

3. True (T)

4. True (T)

5. True (T)



Page No 70:

Question 1:

Do all changes involve exchange of energy? Give examples supporting your answer.

Answer:

Yes, all changes involve exchange of energy. Energy is either absorbed or released during changes. Energy is always present and it only takes different forms without which, no change can happen.
Example:

  • When ice is melted, solid state changes to liquid. Here, energy is supplied in the form of heat. This heat energy is absorbed by the ice cubes and they start melting. Hence, ice cubes (solid state) changes to water (liquid state) by exchange of energy.
  • Breaking a wooden log with axe by a person. Here, the energy is exerted by the person through axe on the wooden log. Thus, energy is transformed from the person to axe and then to the wooden log. Hence, exchange of energy causes the breaking of wooden log. 

Page No 70:

Question 2:

De all changes involves interaction? By interaction do we mean only physical interaction? Find out examples of changes that affect each other non-physically.

Answer:

Yes all changes involve interactions.
No, we cannot mean only physical interaction here. It may also be non-physical interactions. Let us give some examples having non-physical interactions.

  • Conversion of water to vapour by evaporation: Here, water molecules attain kinetic energy due to the surrounding heat, thus they interact with each other and evaporate to vapour.
  • Chemical reaction between iron and sulphur on heating: By heating, the atoms present in the mixture of iron and sulphur attains energy, interact with each other and form iron sulphide.

Page No 70:

Question 3:

Why does your mother put two glass tumblers that get fixed tightly to each other in hot water?

Answer:

This is because we cannot separate two glass tumblers, fixed tightly to each other, easily without breaking. The separation is rather done by placing them in hot water. By doing so, the bottom glass tumbler expands a little due to increase in temperature compared to the top one. Thus, the top glass tumbler gets separated easily.

Page No 70:

Question 4:

Why is the change of season a periodic change?

Answer:

Periodic changes are changes that replicate themselves after specific intervals of time. As the change of seasons repeats after a specific period of time, it is considered as a periodic change.



Page No 71:

Question A.1:

What is a change? Explain with the help of a suitable example.

Answer:

Change is nothing but modification, where one thing is altered to another thing. For example, water on heating changes to vapour.

Page No 71:

Question A.2:

What types of changes are classified as reversible?

Answer:

Physical and periodic changes are classified as reversible.

Page No 71:

Question A.3:

What type of change produces an entirely new product?

Answer:

An irreversible change produces an entirely new product.

Page No 71:

Question A.4:

List two conditions which help in bringing about a change.

Answer:

The two conditions that help in bringing about a change are:

  • Interaction among substances
  • Heating of substances

Page No 71:

Question A.5:

What type of change is involved in burning of a fuel?

Answer:

Burning of a fuel is an irreversible change. The fuel burnt is utilised as energy. It is a permanent change and cannot be reversed.

Page No 71:

Question A.6:

Identify two changes that can not be reversed while preparing dishes at home.

Answer:

The two changes that cannot be reversed while preparing dishes at home are:

  • Cooking of vegetables
  • Preparing dosa

Page No 71:

Question B.1:

Give four examples of reversible changes occurring around you.

Answer:

Four examples of reversible changes occurring around us:

  • Dissolving salt in water
  • Inflating balloon
  • Melting ice
  • Folding paper

Page No 71:

Question B.2:

What is a reversible change? Can all the changes be reversed?

Answer:

A change in which no new materials are formed and that can be reversed is called a reversible change. No, we cannot reverse all the changes. Only a few changes are reversible.

Page No 71:

Question B.3:

What is an irreversible change? Give two examples.

Answer:

A change that cannot be reversed is called an irreversible change. It is permanent in nature and forms new materials. Examples:
Cooking of rice: Rice once cooked cannot be reversed to original raw rice.
Burning of candleCandle once burned cannot be reversed to its original state.

Page No 71:

Question B.4:

How is a metal rim fixed on a wooden wheel?

Answer:

On heating, metals undergo expansion and contract to their original size on cooling. Therefore, a metal rim that is fixed on the wooden wheel is a little smaller in size. On heating the metal rim, it expands and then it is fixed on to the wooden wheel. After cooling, metal rim contracts and gets fixed firmly to the wooden wheel. This is how a metal rim is fixed on a wooden wheel.

Page No 71:

Question B.5:

Can we get milk from curd again? Which type of changes is it?

Answer:

No, we cannot get milk from curd again.
Formation of curd from milk is an irreversible change.

Page No 71:

Question C.1:

List three characteristics of reversible changes.

Answer:

The three characteristics of reversible changes are:

  • No new materials are produced.
  • No permanent change takes place.
  • The change can be reversed.

Page No 71:

Question C.2:

How can you say that burning of a candle is an irreversible change?

Answer:

Burning of a candle is an irreversible change that proceeds only in one direction. It is because original candle cannot be recovered from the burnt candle.

Page No 71:

Question C.3:

Explain with an example that interaction is essential of change.

Answer:

For any change to happen, interaction is essential.
For example, if we want to light a match stick, interaction between the matchstick and the side of the matchbox is necessary.

Page No 71:

Question C.4:

What is the difference between the following changes:
(i) Rolling of chapati from dough and
(ii) Baking of chapati.

Answer:

Rolling of chapati from dough Baking of chapati
This is a reversible process. This is an irreversible process.
We can convert rolled chapati to dough any number of times. We cannot convert baked chapati to dough again.
No new material is formed during rolling of chapati from dough. New material is formed on baking of chapati.
The change occurs in mutual direction. The change occurs in single direction.

Page No 71:

Question C.5:

Why do changes occur around us? Explain it with examples.

Answer:

Change is nothing but a modification where one thing is altered to another thing. Change occurs in each and every thing present in this universe. Every change has a cause. For example, we notice the change from day to night. This change is caused by the Earth's rotation. We also notice changes in weather. These changes are caused by the heat of the Sun.

Page No 71:

Question C.6:

In what way heat helps in bringing a change?

Answer:

Heat is one of the necessary conditions required for changes to happen. Heating of substances can cause changes. For example, mixture of iron and sulphur does not react with each other. Once we heat the mixture of iron and sulphur, they no more remain as mixture but react with each other to form a new compound called iron sulphide.



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