1.When is there physical change and chemical change of matter?

Chemical Change:

Change in a substance involving a rearrangement of the atoms in the molecules to produce a different substance or substances.

These are very useful because they help us to produce new substances from existing substances, which then have entirely different characteristics. We produce medicines in this manner

Physical Change:

Change not involving chemical change in a body or substance; e.g., freezing, expansion, magnetization. All these are actually changes of state only. If you want to change the state of something to use it now and then later to bring it back to its original state you use physical changes.

Burning a candle the wax turns to a liquid (physical)

The burning wick (chemical)

They are useful because they happen all the time around us.