NCERT Exemplar Solutions Class 6 Science Changes around us

Multiple Choice Questions (MCQ’s)

Question 1:  Pick the change that can be reversed from the following.
(a) Cutting of trees.
(b) Melting of ghee.
(c) Burning of candle.
(d) Blooming of flower.

Solution1:  The right thing for which selection is made is (b) Melting of ghee. 

A reversible change from the following is, melting of ghee. When we heat ghee, it turns into liquid and then after cooling it returns back to its original form.

Question 2:  Which of the following changes cannot be reversed?
(a) Hardening of cement.
(b) Freezing of ice cream.
(c) Opening a door.
(d) Melting of chocolate.

Solution2:  The right thing for which selection is made is (a) Hardening of cement.

The change that cannot be reversed is hardening of cement because it is an irreversible change.

Question 3:  An iron ring is heated. Which of the following statement about it is incorrect?
(a) The ring expands.
(b) The ring almost comes to the same size on cooling.
(c) The change in this case is reversed.
(d) The ring changes its shape and the change cannot be reversed.

Solution 3:  The right thing for which selection is made is (d) the ring changes its shape and the change cannot be reversed. 

The ring changes its shape and the change cannot be reversed is the incorrect statement. The change in this case can be reversed because when an iron ring is heated the ring expands and on cooling, the ring almost comes to the same size.

Question 4:  While lighting a candle, Paheli observed the following changes:

1. Wax was melting.
2. Candle was burning.
3. Size of the candle was reducing.
4. Melted wax was getting solidified.

Of the above, the changes that can be reversed are:

a)      (1) and (2)

b)      (2) and (3)

c)       (3) and (4)

d)      (1) and (4) 

Solution 4:  The right thing for which selection is made is (d) (1) and (4).

Melting of wax and solidification of melted wax both can be seen in case of lighting a candle.

When we lighting a candle, max will be melted and then melted wax would getting solidified. So, this changes are reversible change.

Question 5:  Salt can be separated from its solution (salt dissolved in water) because:

a)      Mixing of salt in water is a change that can be reversed by heating and melting of salt.

b)      Mixing of salt in water is a change that cannot be reversed.

c)       Mixing of salt in water is a permanent change.

d)      Mixing of salt in water is a change that can be reversed by evaporation.

Solution 5:  The right thing for which selection is made is (d) mixing of salt in water is a change that can be reversed by evaporation.

Salt dissolved in water can be separated from its solution because it is a change that can be reversed by evaporation.

Evaporation is a process that separates dissolved salt from water by means of heat transfer via vaporization or boiling.

Question 6:  Rolling of chapatti and baking of chapatti are the changes that

a)      can be reversed

b)      cannot be reversed

c)       can be reversed and cannot be reversed, respectively  

d)      cannot be reversed and can be reversed, respectively

Solution 6:  The right thing for which selection is made is (c) can be reversed and cannot be reversed, respectively.

Rolling of chapatti can be reversed or reversible change.

Baking of chapatti cannot be reversed or irreversible change.

Question 7:  Iron rim is made slightly smaller than the wooden wheel. The rim is usually heated before fixing into the wooden wheel because on heating the iron rim

a)      expands and fits onto the wooden wheel

b)      contracts and fits onto the wooden wheel

c)       no change in the size takes place

d)      expands first, then on cooling contracts and fits onto the wooden wheel

Solution 7:  The right thing for which selection is made is (d) expands first, then on cooling contracts and fits onto the wooden wheel.

The rim is usually heated before fixing into the wooden wheel because on heating the iron rim, it expands first, then after cooling it contracts and fits onto the wooden wheel.

Very Short Answer Type Questions

Question 8:  Look at the figures given below, which show three situations 

(a) burning candle (b) an extinguished candle (c) melting wax.

Which of these shows a reversible change and why?

Solution 8:   Melting of wax shows a reversible change because after cooling it returns back to its original form i.e. after cooling, it becomes solid wax again.

Whereas, when candle is burning, the wax in it produces carbon dioxide gas water that all go into air and we cannot combine all these products to get back the original candle. So, burning candle and an extinguished candle shows an irreversible change.

Question 9:  A piece of iron is heated till it becomes red hot. It then becomes soft and is beaten to a desired shape. What kinds of changes are observed in this process; reversible or irreversible?

Solution 9:   The changes are reversed in this process because iron is heated till it becomes red hot and when it becomes soft then is beaten to a desired shape.  But after it is cooled, it returns back to its original form i.e. it becomes hard again. So, this is a reversible change.

Question 10:   Paheli had bought a new bottle of pickle from the market. She tried to open the metal cap to taste it but could not do so. She then took a bowl of hot water and immersed the upper end of the bottle in it for five minutes. She could easily open the bottle now. Can you give the reason for this?

Solution 10:   When metal cap is in contact with hot water, it increases in size. So, she could easily open the bottle because of expansion of metal cap.

Short Answer Type Questions

Question 11:  Can we reverse the following changes? If yes, suggest the name of the method.

a)      Water into water vapor

b)      Water vapor into water

c)       Ice into water

d)      Curd into milk

Solution 11:

a)      Yes, water vapor can be converted into water by using condensation method.

b)      Yes, water converted into its vapor form by using evaporation method.

c)       Yes, water converted into ice by freezing it.

d)      No, it’s not possible. It is an irreversible change.

Question 12:   Which of the following changes cannot be reversed?

a)      Blowing of a balloon.

b)      Folding a paper to make a toy aero plane.

c)       Rolling a ball of dough to make roti.

d)      Baking cake in an oven.

e)      Drying a wet cloth.

f)       Making biogas from cow dung.

g)      Burning of a candle.

Solution 12:   The Changes that cannot be reversed are reversible changes. From in the above given changes, the reversible changes are as follows: 

(d) Baking cake in an oven.
(f) Making biogas from cow dung.
(g) Burning of a candle.

Question 13:   Boojho’s sister broke a white dove, a symbol of peace, made of Plaster of Paris (POP). Boojho tried to reconstruct the toy by making a powder of the broken pieces and then making a paste by mixing water. Will he be successful in his effort? Justify your answer.

Solution 13:   Making of toy from Plaster of Paris (POP) is a change that cannot be reversed because it is not possible to convert the broken pieces of POP into its original form i.e. POP powder. On adding water to Plaster of Paris, it will immediately sets to a hard mass. So, the setting of plaster of Paris on mixing water is an irreversible chemical change and Boojho will not be successful to reconstruct the toy.

Question 14:  Tearing of paper is said to be a change that cannot be reversed. What about paper recycling?

Solution 14:   Tearing of paper is a change that cannot be reversed. By paper recycling, we can get the paper but the color and texture of the paper change i.e. the same original paper not return back.

Long Answer Type Questions

Question 15:  Give one example in each case.

1. Change which occurs on heating but can be reversed.
2. Change which occurs on heating but cannot be reversed.
3. Change which occurs on cooling but can be reversed.
4. Change which occurs on mixing two substances but can be reversed.
5. Change which occurs on mixing two substances but cannot be reversed. 

Solution 15:  The example of each case is as follows:

1. Heating of an iron rod
2. Baking cake in an oven
3. Formation of ice from water
4. Formation of salt solution
5. Mixing of cement and water

Question 16:  A potter working on his wheel shaped a lump of clay into a pot. He then baked the pot in an oven. Do these two acts lead to the same kind of changes or different? Give your opinion and justify your answer.

Solution 16:   These two acts lead to the two different kinds of changes. When potter working on his wheel shaped a lump of clay into a pot that time clay was wet and he can convert back to wet clay into the original clay. So, this act is a reversible change.

 Whereas, when he baked the clay pot in an oven, and now he cannot convert back to baked clay pot into original clay. So, this act is an irreversible change.

Question 17:  Conversion of ice into water and water into ice is an example of change which can be reversed. Give four more examples where you can say that the changes can be reversed.

Solution 17:  Changes which can be reversed are called reversible changes. Some examples of reversible changes are as follows:

1. Blowing of a balloon.
2. Folding a paper to make a toy aero plane.
3. Rolling a ball of dough to make roti.
4. Melting of wax.

Question 18:  Change of a bud into a flower is a change which cannot be reversed. Give four more such example.

Solution 18:   Changes which cannot be reversed are called irreversible changes. Some examples of irreversible changes are as follows:

1. Hardening of cement.
2. Making biogas from cow dung.
3. Burning of a candle.
4. Milk into curd.

Question 19:  Paheli mixed flour and water and (a) made a dough, (b) rolled the dough to make a chapatti, (c) baked the chapatti on a pan, (d) dried the chapatti and ground it in a grinder to make powder. Identify the changes (a) to (d) as the changes that can be reversed or that cannot be reversed.

Solution 19:
(a) Made a dough of flour and water, it is an irreversible change.
(b) Rolled the dough to make a chapatti, it is a reversible change.
(c) Baked the chapatti on a pan, it is an irreversible change.
(d) Dried the chapatti and ground it in a grinder to make powder, it is an irreversible change.

Question 20:  It was Paheli’s birthday, her brother Simba was helping her to decorate the house for the birthday party and their parents were also busy making other arrangements. Following were the activities going on at Paheli’s home:
(a) Simba blew balloons and put them on the wall.
(b) Some of the balloons got burst.
(c) Paheli cut colorful strips of paper and put them on the wall with the help of tape.
(d) She also made some flowers by origami (paper folding) to decorate the house.
(e) Her father made dough balls.
(f) Mother rolled the dough balls to make puries.
(g) Mother heated oil in a pan.
(h) Father fried the puries in hot oil.

Identify the activities at Paheli’s home, as those that can be reversed and those, which cannot be reversed.

Solution 20:  Changes that can be reversed are as follows:
(a) Simba blew balloons and put them on the wall.
(d) Making of some flowers by origami (paper folding) to decorate the house.
(e) Her father made dough balls.
(f) Mother rolled the dough balls to make puries.
(g) Mother heated oil in a pan.

Changes that cannot be reversed are as follows:
(b) Some of the balloons got burst.
(c) Paheli cut colorful strips of paper and put them on the wall with the help of tape.
(h) Father fried the puries in hot oil.