term applied to various compounds of sodium, and particularly to sodium carbonate, Na2CO3, and to sodium bicarbonate, NaHCO3. Sodium carbonate—sp.gr. 2.53, m.p. 851° C (1563.8° F)—is a white powder with strong alkaline properties and occurs in nature dissolved in the waters of inland lakes called soda lakes. The compound occurs also in some salt beds. Several hydrated forms of sodium carbonate are manufactured, chief among which are the decahydrate, Na2CO3 • 10H2O, called washing soda or sal soda, and the monohydrate, Na2CO3 H2O, called crystal carbonate.

Sodium carbonate was originally prepared from the ashes of seaweed and was called soda ash, but it was not used on a large scale until the French chemist Nicolas Leblanc (1742–1806) devised a method, called the Leblanc process, for the production of the compound from ordinary table salt, sodium chloride. The Leblanc process was superseded by the less expensive Solvay process, invented by the Belgian chemist Ernest Solvay, in an attempt to utilize the ammonia obtained as a by-product in the coke industry. In the Solvay process sodium chloride is treated with ammonia gas and then with carbon dioxide, resulting in the formation of sodium bicarbonate, NaHCO3, and ammonium chloride. The sodium bicarbonate precipitate is filtered from the solution of ammonium chloride and is dried and heated to form sodium carbonate. Approximately 4 million tons of sodium carbonate are produced in the U.S. annually by the Solvay process, and 3 million tons are recovered from natural deposits. The compound is used in the manufacture of glass and ceramics, in the pulping of wood to make paper, and in the manufacture of soap. It is also used in petroleum refining, as a water softener, as a cleaner and degreaser in washing compounds, as well as in the manufacture of other sodium-containing compounds, such as sodium hydroxide.

Sodium bicarbonate, or baking soda, is a white powder with sp.gr. 2.16. It decomposes when heated in air above 55° C (131° F), losing carbon dioxide and water and forming sodium carbonate. It is an important constituent of baking powder and is also employed as a source of carbon dioxide in fire extinguishers. The compound is used medicinally to neutralize excessive acid in the stomach and industrially to moderate the alkalinity of sodium carbonate. It occurs naturally in many mineral springs and is manufactured by treating sodium carbonate with water and carbon dioxide or as an intermediary in the Solvay process. For other compounds of sodium, which are designated as soda with a qualifying term, see ALKALIES,; SODIUM,.