NCERT Exemplar Problems Class 6 Science – Changes Around US

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
Solution:
(b) Melting of ghee is a reversible change because on melting, ghee turns into liquid form and 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
Solution:
(a) Hardening of cement is an irreversible change. Changes that cannot be reversed are called irreversible changes.

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:
(d) The correct statement is that when an iron ring is heated the ring expands and the change can be reversed.

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:
(d) Melting of wax and solidification of melted wax, both can be seen in case of lighting a candle.

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:
(d) Mixing of salt in water is a change that can be reversed by evaporation.
The common salt dissolved in water can be separated by the process of evaporation.

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:
(c) Rolling of chapatti can be reversed and baking of chapatti cannot be reversed.

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:
(d) On heating the iron rim, it expands first, then on cooling it contracts and fits onto the wooden wheel.
Note: Expansion occurs on heating whereas contraction occurs on cooling.

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:
Melting of wax in (c), which on cooling, changes back to solid wax but burning candle (a) and an extinguished candle (b) show chemical irreversible change.
Note: The burning of wax produces carbon dioxide gas water vapour and soot, which all go into air. We cannot combine all the products of burning of wax to get back the original candle. So burning of candle is 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 kind of changes are observed in this process; reversible or irreversible?
Solution:
The softening of iron on heating to red hot stage is a reversible change. And when the hot iron is cooled, it becomes hard again.

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:
She can easily open the bottle because of expansion of metal cap (increase in size) due to heating.
Note: When a metal object is heated, it increases in size and when the hot object is cooled, it decreases in size and comes back to the original size.
Short Answer Type Questions

Question 11:
Can we reverse the following changes? If yes, suggest the name of the method.
(a) Water into water vapour
(b) Water vapour into water
(c) Ice into water
(d) Curd into milk
Solution:
(a) Yes, by condensation water vapour can be converted into water.
(b) Yes, by evaporation water converted into its vapour form.
(c) Yes, by freezing water converted into ice.
(d) Not possible.

Question 12:
Which of the following changes cannot be reversed?
(a) Blowing of a balloon
(b) Folding a paper to make a toy aeroplane
(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:
Changes that cannot be reversed 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:
Boojho will not be successful because making of toy from Plaster of Paris (POP) is a change that cannot be reversed. Plaster of Paris, immediately sets to a hard mass on adding water to it. So, the setting of plaster of Paris on mixing water is an irreversible chemical change.

Question 14:
Tearing of paper is said to be a change that cannot be reversed. What about paper recycling?
Solution:
We can get the paper on paper recycling but it is not the same original paper that we get. The colour and texture of the paper change.

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:
The example of each case is as follows:

1. Heating of an iron rod
2. Baking of chapatti
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:
These two acts are of different kinds. Making a lump of clay into a pot is a reversible change. This is because the wet clay pot can be converted back into the original clay.
The baking of clay pot in an oven is an irreversible change. This is because the baked clay pot cannot be changed back into the original clay.

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:
Changes that can be reversed are called reversible changes. Some examples are as follows:

1. Melting of wax
2. Folding of a paper
3. Knitting of a sweater
4. Inflating of a tyre

Question 18:
Change of a bud into a flower is a change which cannot be reversed. Give four more such example.
Solution:
Changes that cannot be reversed are called irreversible changes. Some examples are as follows :

1. Milk into curd
2. Burning of wood
3. Ripening of fruits
4. Digestion of food

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:
(a) Made a dough this change cannot be reversed.
(b) Rolled the dough to make a chapatti. This change can be reversed.
(c) Baked the chapatti on a pan, this change cannot be reversed.
(d) Dried the chapatti and ground it in a grinder to make powder, this change cannot be reversed.

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 colourful 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:
Reversed changes 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 colourful strips of paper and put them on the wall with the help of tape.
(h) Father fried the puries in hot oil.