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(or drying agent), a liquid or solid that takes up moisture, used for drying gases, liquids and, more rarely, solids. Desiccants are divided into three major groups: (1) water-adsorbing desiccants, (2) desiccants that form hydrates with water, and (3) desiccants that form other chemical compounds with water. The first group includes aluminum oxide, silica gel, and other adsorbents; the second group comprises certain anhydrous salts and fused hydroxides; and the third group includes metallic sodium and calcium, calcium oxide, and phosphorus pentoxide. The state of aggregation of the substance to be dried, the moisture content of the substance, and the drying conditions determine the selection of a particular desiccant. It is absolutely essential that the desiccant does not react with the substance to be dried.