Chapter-6 Changes Around Us

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

Explanation:

Yes, the change of shortening the length of the dress by folding it can be reversed by unfolding it. Once the waterlogged area is crossed, the dress can be unfolded to its original length. This change is a physical change and can be easily reversed without any permanent alteration to the dress.

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

Explanation:

If the toy is completely shattered or irreparably damaged, then the change cannot be reversed. But, if the toy is only partially broken or damaged, it might be possible to repair it and reverse the change.

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.


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


Explanation:

No.

Change

Can Be Reversed Yes/No

1

The sawing of a piece of wood

No, sawing of wood is a physical change but it is irreversible.

2

The melting of ice candy

Yes, Melting is a physical change, and the ice candy can be refrozen to form the original solid candy.

3

Dissolving sugar in water

Yes, Dissolving is a physical change, and sugar can be recovered by evaporating the water.

4

The cooking of food

No, Cooking is a chemical change that alters the chemical composition of food, and it cannot be reversed

5

The ripening of a mango

No, Ripening is a chemical change. Once a mango is ripe, it cannot be unripe.

6

Souring of milk

No, Souring is a chemical change.


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

Explanation:

The change that occurs when a picture is drawn on a drawing sheet is a physical change.

If it is drawn using a pencil then the change can be reversed by erasing the drawing which will restore the drawing sheet to its original state.

Any other materials like paint or ink is used then the change is most likely irreversible.


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

Explanation:

Here are a few examples to explain the difference between changes that can or cannot be reversed:

  1. Melting of Ice vs Burning of Paper: The melting of ice is a physical change that can be reversed by freezing the water again, while the burning of paper is a chemical change that cannot be reversed. Once the paper is burned, it cannot be restored to its original state.

  2. Mixing of Salt and Water vs Cooking of an Egg: The mixing of salt and water is a physical change that can be reversed by evaporating the water, leaving behind the salt. Cooking an egg, on the other hand, is a chemical change that cannot be reversed. Once the egg is cooked, it cannot be restored to its raw state.

Freezing of Water vs Rusting of Iron: The freezing of water is a physical change that can be reversed by melting the ice, while rusting of iron is a chemical change that cannot be reversed. Once the iron is rusted, it cannot be restored to its original state.

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 immobilized. Can the change in POP be reversed?

Explanation:

When a thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone and left to dry, it undergoes a chemical change to form a hard and rigid structure. This change is irreversible and cannot be reversed. Once the POP has hardened, it cannot be returned to its original state by applying any force or action.


 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?

Explanation:

When cement gets wet, a chemical change occurs, and it hardens as it dries. If the cement has hardened it will not revert to its original state by drying it in the sun. As it is an irreversible process.