Although physical and chemical changes can both result in a change of state, such as from liquid to gas or solid, the two processes are completely different. In a physical change, the matter simply undergoes a change in state, such as by freezing or being dissolved in water.
However, the actual molecules in the substance remain chemically identical despite this change. This is the opposite of a chemical change, where the molecules undergo a chemical reaction that forms new chemical substances. This process is not reversible, as the actual chemical structure of the various atoms has been changed, unlike in a physical change, which can always be reversed. In chemical changes, the atoms in the substance always break apart or join together to form new compounds.
For instance, water can also be boiled to create steam or frozen to create ice, but each of these substances still have the same chemical makeup, H20. However, when gasoline is burned, the result is two new substances, carbon dioxide and water vapor. Similarly, when iron is exposed to oxygen, the two react to form iron oxide, or rust. Although the layers of iron underneath the rust are still pure, the chemical change at the surface is permanent, as the iron oxide cannot be changed back into pure iron.