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Compare with acid .Nonmetallic oxides or organic compounds that react with water to form acids . For example, SO 2 , CO 2 , P 2 O 5 , and SO 3 are the acid anhydrides of sulfurous, carbonic, phosphoric, and sulfuric acids, respectively. Acetic anhydride (CH 3 CO) 2 O) reacts with water to form acetic acid.