Distillation is the process by which a liquid is boiled and its vapor condensed. It is used, like the processes of crystallization and sublimation, for purposes of purification. If impure water, for instance, is placed in a boiler to which a condensing apparatus (an apparatus for cooling the steam) is attached, the vapor or steam given off when the water is boiled, is condensed. It then becomes pure or distilled water, all the non-volatile impurities having been left in the boiler. The water has a lower boiling point than the impurities, hence it boils first, and is thus enabled to leave the impurities behind.