Sodium \So"di*um\, n. [NL., fr.E. soda.] (Chem.) A common metallic element of the alkali group, in nature always occuring combined, as in common salt, in albite, etc. It is isolated as a soft, waxy, white, unstable metal, so readily oxidized that it combines violently with water, and to be preserved must be kept under petroleum or some similar liquid. Sodium is used combined in many salts, in the free state as a reducer, and as a means of obtaining other metals (as magnesium and aluminium) is an important commercial product. Symbol Na (Natrium). Atomic weight 23. Specific gravity 0.97. Sodium amalgam, an alloy of sodium and mercury, usually produced as a gray metallic crystalline substance, which is used as a reducing agent, and otherwise. Sodium bicarbonate, a white crystalline substance, HNaCO3, with a slight alkaline taste resembling that of sodium carbonate. It is found in many mineral springs and also produced artificially,. It is used in cookery, in baking powders, and as a source of carbonic acid gas (carbon dioxide) for soda water. Called also cooking soda, saleratus, and technically, acid sodium carbonate, primary sodium carbonate, sodium dicarbonate, etc. Sodium carbonate, a white crystalline substance, Na2CO3.10H2O, having a cooling alkaline taste, found in the ashes of many plants, and produced artifically in large quantities from common salt. It is used in making soap, glass, paper, etc., and as alkaline agent in many chemical industries. Called also sal soda, washing soda, or soda. Cf. Sodium bicarbonate, above and Trona. Sodium chloride, common, or table, salt, NaCl. Sodium hydroxide, a white opaque brittle solid, NaOH, having a fibrous structure, produced by the action of quicklime, or of calcium hydrate (milk of lime), on sodium carbonate. It is a strong alkali, and is used in the manufacture of soap, in making wood pulp for paper, etc. Called also sodium hydrate, and caustic soda. By extension, a solution of sodium hydroxide.