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(also called bisulfites), acid salts of sulfurous acid, H2S03—for example, KHS03. Hydrosulfites are produced by the reaction K2C03 + 2S02 + H20 = 2KHSO 3+ C02. As opposed to the majority of neutral salts of H2S03, or sulfites, all hydrosulfites are readily soluble in water. In solutions hydrosulfites are gradually oxidized by atmospheric oxygen to salts of sulfuric acid. When sodium or potassium hydrosulfite is heated, pyrosulfites, which are often called metabisulfites, are formed: 2KHS03 = K2S2O5 + H2O. Sodium hydrosulfite, NaHSO3, is used in photography and for bleaching various materials; calcium hydrosulfite, Ca(HS03)2, is used to obtain cellulose from wood.