Changes Around Us

1. Yes.The length of the dress can again be increased by unfolding it. Hence, this change can be reversed.
2. No. This change cannot be reversed.
   

3. S. No.	Change	Can be reversed (Yes/No)
   1.	Sawing of a piece of wood	No
   2.	Melting of ice candy	Yes
   3.	Dissolving sugar in water	Yes
   4.	Cooking food	No
   5.	Ripening of a mango	No
   6.	Souring of milk	No

4. If we draw a picture on a drawing sheet with a pencil, we can get back the original drawing sheet by erasing the drawing with an eraser. In this case, the change can be reversed.
   However,if we draw with a pen, then the original drawing sheet cannot be obtained back because we cannot erase ink. Hence, in this case, the change cannot be reversed.
5. To explain the differences between changes that can or cannot be reversed, some examples are given below:
   (i) If we inflate a balloon, the size and shape of the balloon undergoes a change. However, the original size and shape of the balloon can be obtained back by allowing the air to escape from the balloon. This means that the change that occurs by inflating a balloon can be reversed. But, if the balloon bursts after being inflated, then its original size and shape cannot be obtained back. Thus, in this case, the change cannot be reversed.
   (ii) If we fold a piece of paper, then the shape and size of the paper undergoes a change. In this case, the original shape and size of the original paper can be obtained back. Thus, this change can be reversed. However, if we cut the piece of paper, the change in the shape and size of the paper cannot be reversed.
   (iii) After we roll out a chapattifrom a ball of dough, the chapattican be converted back into a ball. Thus, the change occurring here can be reversed. But, if we cook the chapattion a tawa, then it cannot be converted back into a ball of dough. Thus, the change is irreversible.
   
6. When water is added to plaster of Paris (POP), it changes to another substance and on drying it hardens. Once the POP has hardened, its shape cannot be changed. Therefore, the change in POP cannot be reversed.
7. In this case, the changes cannot be reversed. This is because the cement that hardens up after getting wet cannot be obtained back.