What is the main difference between physical changes and chemical changes

1. A physical change is reversible, a chemical change is not. For example, the freezing of water would be a physical change because it can be reversed, whereas the burning of wood is a chemical change - you can't 'unburn' it

2. A physical change is a change in which no new substance is formed; a chemical change results in the formation of one or more new substances. Again, consider the previous examples: Freezing water into ice just results in water molecules which are 'stuck' together - it's still H2O. Whereas burning wood results in ash, carbon dioxide, etc, all new substances which weren't there when you started.

Physical changes are something which you can change for example, water into ice and ice into water. Chemical changes are something you can't change for example, burning paper into ashes, you cannot make the ashes into paper again.