acid anhydride

acid anhydride (ănhī`drīd, –drəd), chemical compound that reacts with water to form an acid (see acids and bases). Anhydrides of inorganic acids are usually oxides of nonmetallic elements. Carbon dioxide, CO₂, is the anhydride of carbonic acid, H₂CO₃ . Nitrogen pentoxide, N₂O₅, is the anhydride of nitric acid, HNO₃ . Phosphorus pentoxide, P₂O₅, is the anhydride of phosphoric acid, H₃PO₄ . Sulfur dioxide, SO₂, is the anhydride of sulfurous acid, H₂SO₃ . Sulfur trioxide, SO₃, is the anhydride of sulfuric acid, H₂SO₄ . Anhydrides of organic acids, like the acids themselves, contain the carbonyl group, CO. Organic anhydrides include acetic anhydride or ethanoic anhydride, (CH₃C=O)₂O, and benzoic anhydride, (C₆H₅C=O)₂O.