Desiccation

Desiccation, the process or processes by which a substance, which may be either solid, liquid, or gaseous, is freed from accompanying moisture. For solids, heat alone is the most common mode of accomplishing this, steam or air ovens being generally employed. The action may also be accelerated by the passage of a current of hot dry air over the solid. In cases where heating is inadmissible, the drying may be effected by placing the substance in an exhausted vessel containing also a quantity of some desiccator. Liquids are generally allowed to stand over, or shaken with, some substance which combines readily with water, but does not act upon the liquid in question. The chief of such substances - desiccants or desiccators - in ordinary use are metallic sodium, phosphorus pentoxide (P₂O₅), sulphuric acid, calcium chloride, and quicklime. Gases are usually dried by passing through tubes containing one or other of the above desiccants. The commonest forms of such drying-tubes are U-tubes containing pieces of pumice saturated with sulphuric acid, or loosely filled with calcium chloride.