Chemical changes happen when chemical reactions occur. They involve the formation of new chemical elements or compounds.
Physical changes do not lead to new chemical substances forming. In a physical change, a substance simply changes physical state, eg from a solid to a liquid.
Liquid water becoming steam (when water boils) is a physical reaction:
Liquid water decomposing into hydrogen and oxygen, eg when an electric current is passed through water, is a chemical reaction:
For a chemical reaction to happen, particles must collide with each other. However, not all collisions between particles lead to a chemical reaction.
This may be because of the reactivity of the substances involved. For example, gold does not react with oxygen (because gold is so unreactive) even though oxygen particles in the air collide with gold in jewellery.
Iron is a more reactive metal than gold so iron reacts with oxygen. However, the reaction of iron with oxygen in the air (rusting) is slow. This shows that not all collisions between oxygen and iron particles lead to a reaction taking place. The reason for this is that collisions may not happen with enough energy for the reaction to take place.