NCERT Solutions for Class 6th: Ch 6 Changes Around us Science
Page No: 51
Exercises
1. To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?
Answer
2. You accidentally dropped your favourite toy and broke it. This is a change you did not want. Can this change be reversed?
Answer
No, this change can't be reversed.
3. Some changes are listed in the following table. For each change, write in the blank column, whether the change can be reversed or not.
| S. No. | Change | Can be reversed (Yes/No) |
| 1. | The sawing of a piece of wood | |
| 2. | The melting of ice candy | |
| 3. | Dissolving sugar in water | |
| 4. | The cooking of food | |
| 5. | The ripening of a mango | |
| 6. | Souring of milk |
Answer
| S. No. | Change | Can be reversed (Yes/No) |
| 1. | The sawing of a piece of wood | No |
| 2. | The melting of ice candy | Yes |
| 3. | Dissolving sugar in water | Yes |
| 4. | The cooking of food | No |
| 5. | The ripening of a mango | No |
| 6. | Souring of milk | No |
4. A drawing sheet changes when you draw a picture on it. Can you reverse this change?
Answer
We can reverse this change if the picture is made by pencil on drawing sheet.
We can't reverse this change if the picture is made by pen, oil colour or sketch pen.
5. Give examples to explain the difference between changes that can or cannot be reversed.
Answer
(i) Paper can be folded to make different shapes. This is reversible change as shapes of paper can be unfolded back into paper sheet. But when paper is burnt and turned into ash, it can't be reversed.
(ii) If we fill balloon with air, the shape and size of the balloon changes. This change can be reversed but if balloon burst while inflating then this change can't be reversed.
(iii) The shape of rubber band can be changed by stretching which can be reversed
6. A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilised. Can the change in POP be reversed?
Answer
No, the change in POP can't be reversed as it became hard on drying and new product is formed which cannot be changes back in its original form. It is type of chemical change.
7. A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?
Answer
No, the changes which have occurred in the cement couldn't be reversed. When sunlight falls on the wet cement, chemical changes occur which is a irreversible process.