Boiling water is a physical change, as it rearranges molecules but does not affect the internal structures. Boiling water forces the water molecules away from each other as the liquid changes to vapor. In a chemical change, new chemical substances are created or formed.
Physical changes affect the state of an item, and a chemical change happens at a molecular level. Chemical changes involve atomic bonds being created or broken during chemical reactions. When iron rusts because it is exposed to the oxygen, that is a chemical change. In this instance, the molecules change their structure and become iron oxide.