Changes Around Us
Class 6th Science NCERT Exemplar Solution
- Pick the change that can be reversed from the followingA. Cutting of trees B. Melting of…
- Which of the following change cannot be reversed?A. Hardening of cement B. Freezing of ice…
- An iron ring is heated. Which of the following statement about it is incorrect?A. The ring…
- While lighting a candle, Paheli observed the following changes (i) Wax was melting. (ii)…
- Salt can be separated from its solution (salt dissolved in water), becauseA. mixing of…
- Rolling of chapatti and baking of chapatti are the changes thatA. can be reversed. B.…
- Iron rim is made slightly smaller than the wooden wheel. The rim is usually heated before…
- Look at Fig. 6.1 which shows three situations (a) a burning candle (b) an extinguished…
- A piece of iron is heated till it becomes red-hot. It then becomes soft and is beaten to…
- Paheli had bought a new bottle of from the market. She tried to open the metal cap to…
- Can we reverse the following changes? If yes, suggest the name of the method. (a) Water…
- Which of the following changes cannot be reversed? (a) Blowing of a balloon (b) Folding a…
- Boojho’s sister broke a white dove, a symbol of peace, made of plaster of paris (POP).…
- Tearing of paper is said to change that cannot be reversed. What about paper recycling?…
- Give one example in each case (a) Change which occurs on heating but can be reversed. (b)…
- A potter working on his wheel-shaped a lump of clay into a pot. He then baked the pot in…
- Conversion of ice into water and water into ice is an example of change which can be…
- Change of a bud into a flower is a change which cannot be reversed. Give four more such…
- Paheli mixed flour and water and (i) made a dough, (ii) rolled the dough to make a…
- It was Paheli ’s birthday, her brother Simba was helping her to decorate the house for the…
Multiple Choice Questions
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
Answer:
Melting of ghee is a physical change. In this, only the physical state of ghee changes from solid to liquid state and this change can be reversed, upon cooling.
All the other options include physical as well as chemical change, and hence cannot be reversed back.
● When we cut trees, they are physically separated into two parts, and also the chemical bonds are broken, changing their chemical properties.
● The burning of a candle decreases it’s length as a physical change and involves combustion as a chemical change.
● Blooming of a flower changes its physical state from bud to an open flower and has the role of some chemical substances, which can not be changed back to original form.
Question 2.
Which of the following change cannot be reversed?
A. Hardening of cement
B. Freezing of ice cream
C. Opening a door
D. Melting of chocolate
Answer:
When powdered cement is mixed with water, it hardens. This is because of formation of new structures, called crystals, which after a fixed time get attached to each other, providing strength and hardening the cement. Therefore, as hardening of cement includes a change in its physical state and it’s chemical properties, it cannot be reversed back.
● A frozen ice- cream melts when kept in a warm room, a door can be closed after it is opened and a melted chocolate can be frozen back when kept in a refrigerator.
● As all these changes can be undone, or the items used can be brought back to their original state, they are said to be reversed.
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.
Answer:
Consider the example of a wooden wheel in which an iron rim ( ring ) has to be fit. The iron ring is made a bit smaller than the wheel. The ring, when heated, expands and gets bigger in size to fit on the wooden wheel. When the ring cools down, it contracts, that means it’s size reduces back to its original size and therefore it gets tightly fixed over the wheel. As there is change only in the physical property of shape, the change can be reversed.
Question 4.
While lighting a candle, Paheli observed the following changes
(i) Wax was melting.
(ii) Candle was burning.
(iii) Size of the candle was reducing.
(iv) Melted wax was getting solidified.
Of the above, the changes that can be reversed are
A. (i) and (ii)
B. (ii) and (iii)
C. (iii) and (iv)
D. (i) and (iv)
Answer:
Melting of wax is a physical change where the physical state of wax changes from solid to liquid. The melted wax that solidifies, can be changed to a liquid state on heating. These two are physical changes and thus, can be reversed.
● Burning of candle is a chemical change which involves breaking of chemical bonds in a process called combustion.
● The size of the burning candle reduces due to this change and cannot be reversed immediately.
● These two changes involve changes in chemical properties and thus cannot be reversed.
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
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.
Answer:
When salt and water solution is heated, the water particles get evaporated and we get only salt left in the container. By this method, we can separate salt from its solution. Thus, the mixing of salt in water can be reversed by 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.
Answer:
When a chapatti is rolled from a dough, only it’s shape changes from spherical to a flat round one. There is no change in its chemical properties and hence, the change can be reversed.
When a chapatti is baked, heat brings about chemical changes in its properties. Therefore, baking of chapatti is a change which 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.
Answer:
The iron rim is generally made slightly smaller than the wooden wheel. When heated, the metal expands, and increases in size, being able to fit on the wheel. As the metal cools down, it comes back to its original state and regains it’s size, fitting tightly onto the wooden wheel.
This is an example of physical, reversible change.
Very Short Answer Questions
Question 1.Look at Fig. 6.1 which shows three situations (a) a burning candle (b) an extinguished candle (c) melting the wax.
Which of these shows a reversible change and why?
Answer:
(c) Melting wax shows a reversible change. This is because it is a physical process, where only the physical state of wax changes from solid to liquid, and can be reversed back on cooling.
● A burning candle(a) and (b) an extinguished candle show irreversible changes. This is because these involve a change in chemical properties of the candle and wax. When a candle is burned, the wax around the wick melts and is converted to fumes. This change is chemical in nature and therefore cannot be reversed.
● Similarly, an extinguished candle can be lightened up again, but that will be the start of a new combustion process, not the reversal of fumes that were produced during burning to flame again. Thus, this change also cannot be reversed.
Question 2.
A piece of iron is heated till it becomes red-hot. It then becomes soft and is beaten to the desired shape. What kind of changes are observed in this process-reversible or irreversible?
Answer:
The changes during this process are reversible in nature.
A piece of iron when heated, becomes red -hot and soft. There is change only in its physical state, from solid and hard, to flexible and soft. Then, it is beaten to desired shape, in which again, there is change only in its physical form or shape. This piece of iron can be re -heated and molded back to its original shape.
Thus, the changes are reversible.
Question 3.
Paheli had bought a new bottle of from the market. She tried to open the metal cap to taste it but could not do so. She then took a bowl 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?
Answer:
Paheli could do so because of the reversible change occurring in the metal cap, on the application of warm water. When Paheli first tried, the metal cap was fit tightly on the bottle. On immersion in hot water, the metal expanded and the cap became loose, making it easy for her to open the cap.
This expansion of metal on heating is a reversible change, and the metal cap will come back to its original size on cooling.
Short Answer Questions
Question 1.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 the water.
(d) Curd into milk.
Answer:
(a) Yes, the change of water into water vapour can be reversed. The process used here is condensation, which is the cooling of vapours to give liquid state water.
(b) Yes, the change of water vapour into water can be reversed. This can be done by evaporation. Water vapour, once changed to water, can be heated again to give vapours or gaseous state of water.
(c) Yes, Melting of ice changes its state from solid form to liquid form. On cooling, or refrigeration, this water can be changed back to the ice. Therefore, it is a reversible change.
(d) No, Changing of curd into milk is a chemical change which involves the use of bacteria. The milk once made into curd, cannot be brought back in milk or liquid form.
Question 2.
Which of the following changes cannot be reversed?
(a) Blowing of a balloon
(b) Folding a paper to make a toy airplane
(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
Answer:
(a) Can be reversed
(b) Can be reversed
(c) Can be reversed
(d) Cannot be reversed
(e) Can be reversed
(f) Cannot be reversed
(g) Cannot be reversed
Therefore, from the given options, the changes which cannot be reversed are (d), (f) and (g).
Question 3.
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.
Answer:
No, he will not be successful in his effort. When a toy is made from plaster of Paris, the POP hardens because of formation of new chemical structures, that is a chemical change, which cannot be reversed. The properties of the original pop change and hence it cannot be brought back to its original form. Therefore, Boojho will not be able to fix the broken pieces back by using water and powder.
Question 4.
Tearing of paper is said to change that cannot be reversed.
What about paper recycling?
Answer:
In paper recycling, the old, waste paper is used to make a new type of paper. In this, the original form, shape, size, and texture of paper are replaced by new shape, size and texture. Therefore, tearing of paper remains a change which cannot be reversed, as the actual form of torn paper used in recycling is not got back.
Long Answer Questions
Question 1.Give one example in each case
(a) Change which occurs on heating but can be reversed.
(b) Change which occurs on heating but cannot be reversed.
(c) Change which occurs by mixing two substances, but can be reversed.
(d) Change which occurs by mixing two substances, but cannot be reversed.
Answer:
(a) Expansion of metal rim on heating, which contracts back on cooling down.
(b) Baking of a cake, which requires heating, but the contents cannot be got back in their original form.
(c) Mixing of salt in water. The substances can be separated by evaporation of water.
(d) Mixing of sugar in milk. These substances cannot be separated out, once mixed together.
Question 2.
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.
Answer:
No, these two actions lead to two different types of changes.
1. The shaping of clay into a pot is a physical change, only in the shape of the clay. This change can be reversed.
2. Baking of pot in the oven brings about a change in the chemical properties of the clay, and therefore this change cannot be reversed.
Hence, the first action is reversible, whereas the second action is a non – reversible change.
Question 3.
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 four more examples where you can say that the changes can be reversed.
Answer:
Four examples of changes which can be reversed are:
1. Blowing of a balloon, which comes back to its original form when the air escapes out.
2. Rolling of the ball from a dough, which is a change only in the shape of the dough
3. Making a paper boat, by folding the paper. This can be reversed by opening up the folded paper.
4. Melting of ice cream in a hot room, which can be reversed by keeping it in the refrigerator.
Question 4.
Change of a bud into a flower is a change which cannot be reversed. Give four more such example.
Answer:
Four examples of changes which cannot be reversed are:
1. Bursting of a blown balloon. The balloon loses its original shape, which cannot be brought back.
2. Baking of a cake, where the flour used cannot be got back in original powder form, once it is heated.
3. Cutting of a paper to make an aeroplane, boat, etc.
4. Burning of a candle, which includes both physical and chemical changes, which cannot be reversed.
Question 5.
Paheli mixed flour and water and (i) made a dough, (ii) rolled the dough to make a chapatti, (iii) baked the chapatti on a pan, (iv) dried the chapatti and ground it in a grinder to make powder. Identify the changes (i) to (iv) as the changes that can be reversed or that cannot be reversed.
Answer:
The changes that can be reversed are
(I) Making a dough
(II) Rolling the dough to make a chapatti
In both these steps, there is change only in the physical state or form of the dough, which can be reversed.
The changes that cannot be reversed are:
(iii) baking the chapatti
(iv) drying and grinding the chapatti to make powder
As these steps involve a change in the chemical properties of dough, they cannot be reversed.
Question 6.
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 :
(i) Simba blew balloons and put them on the wall.
(ii) Some of the balloons got burst.
(iii) Paheli cut colourful strips of paper and put them on the wall with the help of tape.
(iv) She also made some flowers with origami (paper folding) to decorate the house.
(v) Her father made dough balls.
(vi) Mother rolled the dough balls.
(vii) Mother heated oil in a pan.
(viii) Father fried the puries in hot oil.
Identify the activitie4s at Paheli’s home as those that can be reversed and those which cannot be reversed.
Answer:
The activities which can be reversed are :
(i) Simba blew balloons and put them on the wall.
(iv) She also made some flowers with origami ( paper folding) to decorate the house
(v) Her father made dough balls
(vi) Mother rolled the dough balls
(vii) Mother heated oil in a pan
The activities which cannot be reversed are :
(ii) Some of the balloons got burst
(iii) Paheli cut colourful strips of paper and put them on the wall with the help of tape.
(viii) Father fried prices in the oil pan.