Changes Around Us

NCERT Solution

Answer the following questions:

To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?

Answer: Yes

You accidentally dropped your favourite toy and broke it. This is a change you did not want. Can this change be reversed?

Answer: No

A drawing sheet changes when you draw a picture on it. Can you reverse this change?

Answer: If the drawing is made by pencil, then the change can be reversed. If the drawing is made by using water colour or oil colour, then the change cannot be reversed.

Some changes are listed in the following table. For each change, write in the blank column, whether the change can be reversed or not.

Change Can Be Reversed (yes/no)
The sawing of a piece of wood No
The melting of ice candy Yes
Dissolving sugar in water Yes
The cooking of food No
The ripening of mango No
Souring of milk No

Answer the following questions:

Give examples to explain the difference between changes that can or cannot be reversed.

Answer: Melting of ice is a reversible change. Water can be turned into ice by keeping it in the freezer. When ice cubes are kept at room temperature, they turn into water after some time. On the other hand, burning of a piece of paper is an irreversible change. When a piece of paper is burnt, it turns into ash. The ash cannot be turned back into paper. Hence, it is an irreversible change.

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: This is an example of chemical change which cannot be reversed by using simple physical means. However, this change can be reversed using some chemical means.

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: The cement in bag becomes a huge lump of solid cement. This is an example of chemical change which cannot be reversed.

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