Acid anhyrides are nonmetal oxides. When acid anhydrides react with water, the solutions are acidic.

For example,

SO₃ %2B H₂O ---> H₂SO₄

Exhaust from coal burning has produced SO_x products that have contributed to acid rainfall .

Metal oxides such as CaO are basic. The oxide will accept a proton (Bronsted-Lowry). Oxide is a stronger base than hydroxide. In water, metal oxides become metal hydroxides. In Bronsted-Lowry terms, the OH^- is the conjugate acid of the base O^2-.

Ca^2%2B %2B O^2- %2B H₂O --> Ca^2%2B %2B OH^- %2B OH^-

Starting on the left of the third row of the periodic table, and working to the right, Na₂O and MgO react with water to form strong bases. Aluminum oxide (Al₂O₃) forms Al(OH)₃^.3H₂O in water. Aluminum hydroxide is amphoteric, reacting both with NaOH and HCl. SiO₂ does not readily form hydoxides with water. However, SiF₄ hydrolyzes in water to produce H₄SiO_4, silicic acid and HF. Tetraphosphorus decoxide reacts with water to produce H₃PO₄ (phosphoric acid). SO₃ reacts with water to produce the strong acid H₂SO_4. Perchloric acid is one of the strongest acids.

Acid anhydride can also refer to a reaction between two organic acids. Two molecules of acetic acid will react to form acetic anhydride and water. The acid anhydride linkage usually forms a bond that is energy rich. Acid anhydride linkages occur between the phosphate moieties of ATP (biochemical energy storage molecule).