3. Reversible and irreversible changes

  

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.

  

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

  

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.

This type of change is permanent and cannot be undone by any means. A new substance is formed in this type of change.
 

  

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.

 

  

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.

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.