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Anhydride, an oxide which combines with water to form an acid. Anhydrides may therefore be regarded as acids deprived of water, the latter being essential for the exhibition of those properties which are characteristic of .acids. Thus, a solution of an anhydride in ether, or some other non-aqueous solvent, is not capable of reddening litmus paper [Ex. Sulphuric anhydride, which combines with water to form sulphuric acid].