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Anhydride is a term used in chemistry to describe a compound that has lost water. This loss can occur in a few ways. For organic compounds, an anhydride is often formed by the removal of a water molecule from two molecules of a carboxylic acid, or from a single molecule containing two carboxylic acid groups. For example, acetic anhydride is formed from acetic acid. Inorganic anhydrides are often metal oxides that react with water to form bases, or nonmetal oxides that react with water to form acids. For instance, calcium oxide reacts with water to form calcium hydroxide, making calcium oxide an inorganic anhydride. Sulfur trioxide reacts with water to form sulfuric acid, so sulfur trioxide is also an inorganic anhydride. The term "anhydride" can also refer to an acid or base that has lost water and can regain it. The presence of the anhydride functional group, which consists of two acyl groups bonded to the same oxygen atom, is characteristic of organic anhydrides. These compounds are often reactive and are used as acylating agents in various chemical syntheses. They are generally less reactive than acid halides but more reactive than esters.