Air is not a pure substance but a mixture of different gases. It is composed of approximately 78 percent nitrogen and 21 percent oxygen, with small amounts of about a dozen other gases making up the remainder.
Chemists define a pure substance as one made up of only one type of atom or molecule. Pure water is a pure substance because only water molecules are present. Water is a compound. It is made up of hydrogen and oxygen ions that have bonded together to form a new substance that is chemically different from either of its constituent elements. A mixture, such as air, is made up of elements that are physically blended but remain chemically distinct.