In some cases, a separation may fully divide the mixture into its pure constituents. Separations are carried out based on differences in chemical properties such as size, shape, mass, or chemical affinity between the constituents of a mixture, and are often classified according to the particular differences they use to achieve separation. In the case that no single difference can be used to accomplish a desired separation, multiple processes will often be performed in combination to achieve the desired end.

Barring a few exceptions, almost every element or compound is naturally found in an impure state. Often these impure raw materials must be separated into their purified components after they can be put to productive use, making separation processes essential for the modern industrial economy. In some cases these separations require total purification, as in the electrolysis refining of bauxite ore for aluminum metal, but a good example of an incomplete separation process is oil refining. Crude oil occurs naturally as a mixture of various hydrocarbons and impurities. The refining process splits this mixture into other, more valuable mixtures such as natural gas, gasoline and chemical feedstocks, none of which are pure substances, but each of which must be separated from the raw crude. In both these cases a series of separations is necessary to obtain the desired end products. In the case of oil refining, crude is subjected to a long series of individual distillation steps, each of which produces a different product or intermediate.