Question

Give reasons:-

1)Melting of wax is reversible, but burning of candle is irreversible.

2)Irreversible changes should be allowed to take place only when neccessary.

3)Lighting of machstick is an irreversible change.

Answer 1

1) Melting of wax is physical change and burning of candle is a chemical change.In burning the constituent of wax is used up to produce CO₂,H₂O and heat.
2) In irreversible change we are always end up with new product and starting is lost.So it should take place on requirement only.
3) In lighting of matchstick the chemical on matchstick is used up to give light. As the matchstick stop giving light then even we cannot get the starting material (that was pasted on matchstick) back.Hence it is irreversible change.