Pure substance:A substance that hasa fixedchemical composition throughout.A puresubstance does not have to be ofa single chemicalelement or compound, however.A mixture ofvarious chemical elements or compounds alsoqualifies asa pure substance as long as the mixtureis homogeneous. For example, air is considered tobea pure substance.•A mixture of two or more phases ofa pure substanceis stilla pure substance as long as the chemicalcomposition of all phases is the same.•A mixture of liquid air and gaseous air is nota puresubstance since the mixture is no longer chemicallyhomogeneous. This is due to different components inair condensing at different temperatures ataspecified pressure