Whereas a mixture has variable composition, a pure substance always has the same composition. Pure substances are either elements or compounds. For example, pure water is a compound containing individual H2O molecules. However, as we find it in nature, liquid water always contains other substances in addition to pure water-it is a mixture. This is obvious from the different tastes, smells, ans colors or water samples obtained from various sources (such as oceans, lakes, rivers, and the earth's interior), we always end up with the same pure substance-water, which is made up of only H2O molecules.
Pure water always has the same physical and chemical properties and always consists of molecules containing hydrogen and oxygen in exactly the same proportions, regardless of the original source of water. The proporties of a pure substances make it possible to identify that substance conclusively.