Distillation

Distillation is the separation of a volatile liquid from a non-volatile substance by the application of heat. If more than one volatile liquid be present, the distillation may also have for its object the partial or complete separation of the different products, as in fractional distillation. In cases where a solid is strongly heated so that it is destroyed and decomposed, with the evolution of gaseous products, the process is known as destructive distillation. The essential parts for a distillation are - (1) the still, the vessel in which the substance is heated; (2) the condenser, in which the volatile products are condensed to the liquid state; (3) the receiver, where the liquid is collected. For a simple distillation this is all that is required. In the chemical laboratory the still usually takes the form of a flask; the neck, into which a thermometer may be placed, being provided with a side-tube, which is connected to the Liebig's, or other condenser(q.v.); the receiver being either a flask or a bottle placed below the end of the condenser. In the arts, various forms of stills, generally of copper, and specially arranged condensers are employed, according to the nature of the special distillation for which they are required. When it is necessary to separate the products, the liquid is heated in the usual way, and the temperature slowly rises. The portions which boil off between fixed ranges of temperature are separately collected, and the portions re-distilled and again collected as before. By this means it is found, after a few distillations, that the greater bulk of the liquid can be obtained in a few portions, each boiling at a nearly constant temperature and nearly pure; but to obtain very pure substances by this means - fractional distillation - is a laborious and sometimes practically impossible process. Frequently different condensers, cooled to different temperatures, are employed, the least volatile liquid being first condensed, the more volatile portions passing on to following and colder condensers. Special forms of apparatus, in which the less volatile portions condense above the still and run back into it, are also largely employed both in the laboratory and the manufactory. In certain cases, as in the purification of mercury, distillation has to be effected under reduced pressure - i.e. in a more or less complete vacuum - and at other times, owing to the action of air upon some of the products, all the apparatus has to be kept filled with an inert gas, usually nitrogen or carbon-dioxide. Destructive distillation is employed little in pure chemistry, but largely in industrial process, as e.g. in the production of coal-gas.