carbonate [key], chemical compound containing the carbonate radical or ion, CO3−2. Most familiar carbonates are salts that are formed by reacting an inorganic base (e.g., a metal hydroxide) with carbonic acid. Normal carbonates are formed when equivalent amounts of acid and base react; bicarbonates, also called acid carbonates or hydrogen carbonates, are formed when the acid is present in excess. Sodium carbonate, Na2CO3, sodium bicarbonate, NaHCO3, and potassium carbonate, K2CO3, are widely used. Smelling salts is ammonium carbonate. Calcium carbonate is found in shells of animals and in Iceland spar, limestone, and marble; it is used in the production of lime (calcium oxide). Barium carbonate occurs as the mineral witherite. Magnesium carbonate occurs as magnesite and in dolomite (with calcium carbonate). Iron carbonate is a ferrous compound that occurs in nature as siderite. White lead used as a pigment in paints is basic lead carbonate. Only ammonium, potassium, and sodium carbonates are readily soluble in water. Alkali metal carbonates are stable when heated, but other carbonates decompose, releasing carbon dioxide. Carbonates also give off carbon dioxide when treated with dilute acids, e.g., hydrochloric acid.
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