When a substance reacts with oxygen, its chemical composition changes. This is because oxygen atoms bond with atoms of the other substance, forming new compounds. For example, when iron is exposed to oxygen, it rusts. This is because the iron atoms react with oxygen atoms to form iron oxide, which is a new compound.

Physical changes, on the other hand, do not change the chemical composition of a substance. For example, when water freezes, it changes from a liquid to a solid, but the chemical composition of the water does not change.