OXIDE

any of a large and important class of chemical compounds in which oxygen is combined with another element. Nearly all the elements form oxides, which vary in properties according to their composition. Metal oxides are crystalline solids that contain a metal cation and the oxide anion, O2-. They typically react with water to form bases or with acids to form salts. Calcium oxide (CaO), for example, reacts with water to form calcium hydroxide , a strong base, and with hydrochloric acid to form calcium chloride (CaCl2), a salt. Nonmetal oxides are volatile compounds in which the oxygen atoms are linked covalently to the nonmetal atom. They react with water to form acids or with bases to form salts. Thus, sulfur trioxide (SO3) reacts with water to form sulfuric acid (H2SO4), a strong acid, and with sodium hydroxide to form sodium sulfate (Na2SO4), a salt. Amphoteric oxides contain oxygen along with cations such as aluminum, tin, or zinc; they may combine with either acids or bases to form salts. Aluminum oxide (Al2O3), for example, reacts with hydrochloric acid to form aluminum chloride (AlCl3) and with sodium hydroxide to form sodium aluminate (NaAlO2). Certain organic compounds react with oxygen or other oxidizing agents to produce substances called oxides. Thus, amines, phosphines, and sulfides form amine oxides, phosphine oxides, and sulfoxides, respectively, in which the oxygen atom is covalently bonded to the nitrogen, phosphorus, or sulfur atom. The so-called olefin oxides are cyclic ethers.