### Theory:

Reversible change:
The changes that can be reversed to form its original shape and size is termed as reversible change.
Example:
Freezing of water
Melting of ice
Stretching  of a rubber band
Blowing a balloon
Folding a paper
Blowing a balloon

1. A balloon is blown in such a way that it does not burst. There is a change in the shape and size of the balloon on blowing air into it. Now, when the air is made to escape out of the balloon, it turns to its original shape and size.

Making paper cup

2. A toy ship is made by folding a sheet of paper. When we unfold the paper again, it comes to its original shape.

Making shapes with dough

3. Likewise, a dough is made into a ball and rolled it out into a roti. If its shape is not good, it can be changed back into a ball of dough again.

In all the above examples, the substances are changed into their original shape and size.
Irreversible change
The changes that cannot be reversed to form its original shape and size is termed as irreversible change.
Example:
Burning of a candle
Rusting of iron
Burning of paper
This type of change is permanent and cannot be undone by any means. A new substance is formed in this type of change.

Bursting a balloon

1. Bursting a balloon: If a balloon is blown to its full size and pricked with the pointed tip of a pencil or needle, it cannot regain its original form.

2. Cutting a paper: On a sheet of paper, the shape of an aeroplane is drawn and cut along its outline.

3. Baking a roti: When the dough is baked into a roti, it cannot be changed into a dough again. These changes cannot be reversed.

Baking a roti

4. Pot making: A potter makes a pot using a lump of clay. The shaped pot can be rolled again into a lump which is a reversible process. But when the same pot is baked in an oven, it cannot be shaped into a lump of clay again. So, this process becomes irreversible.
Pot making
Expansion and contraction
When a substance is heated, it becomes larger in size. The process where an object expands on heating is known as expansion. In general, all the metals expands on heating.

When a substance is cooled, it shrinks in size. The process where an object contracts on cooling is known as contraction.