FRACTIONATION, in chemistry, consists in separating into fractions materials which differ in rates of crystallization, boiling points or other characteristics, rendering possible a sharp separation. Gases may be separated by fractional combustion, as in burning oxygen out of the air to secure the nitrogen (see NITROGEN, FIXATION OF). Fractional crystallization requires con ditions favourable for the crystallization of one component in a reasonably pure state. Further purification is obtained by re dissolving and re-crystallization. Various conditions, temperature and a choice of solvents are important, as well as a knowledge of the characteristics of the solids to be separated. Fractional dis tillation is dependent upon differences in boiling points, the best example being the concentration of alcohol (see ALCOHOL IN INDUSTRY). Here the lower boiling point of the alcohol allows its removal in a high state of purity from the water with which it is admixed. Fractionating columns are of various designs, the object in all cases being to concentrate to a high purity with low heat consumption. Complex mixtures of liquids are of ten sep arated by several distillations, in the course of which one after another is removed, according to predetermined conditions. Frac tionation may also be applied to precipitation, where one material after another may be thrown out of a mixed solution, depending upon the reagents used.