The science that embraces the properties, composition, and structure of matter, the changes in structure and composition that matter undergoes, and the accompanying energy changes. It is important to distinguish chemical change, implicit in this definition, and changes in physical form. An example of the latter is the conversion of liquid water to solid or gas by cooling or heating; the water substance is unchanged. In chemical change, such as the rusting of iron, the metal is consumed as it reacts with air in the presence of water to form the new substance, iron oxide.