Anhydride, an HY dryd, is a chemical substance that forms acids or bases when combined with water. For example, water and sulfur trioxide, an acid anhydride, make sulfuric acid. Water and calcium oxide, a basic anhydride, make basic calcium hydroxide. Anhydrides can be formed by dehydrating acids and bases, or by synthesis directly from other substances. Anhydrides are used in the manufacture of artificial fibers, plastics, photographic films, and medicines. See also Acid; Base.