acid anhydride (ănhĪˈdrĪd, –drəd) [key], 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&eq;O)2O, and benzoic anhydride, (C6H5C&eq;O)2O.