Oxyacid

oxyacid, any oxygen-containing acid. Most covalent nonmetallic oxides react with water to form acidic oxides; that is, they react with water to form oxyacids that yield hydronium ions (H₃O⁺) in solution. There are some exceptions, such as carbon monoxide, CO, nitrous oxide, N₂O, and nitric oxide, NO.

The strength of an oxyacid is defined by the extent to which it dissociates in water (i.e., its ability to form H⁺ ions). In general, the relative strength of oxyacids can be predicted on the basis of the electronegativity and oxidation number of the central nonmetal atom. The acid strength increases as the electronegativity of the central atom increases. For example, because the electronegativity of chlorine (Cl) is greater than that of sulfur (S), which is in turn greater than that of phosphorus (P), it can be predicted that perchloric acid, HClO₄, is a stronger acid than sulfuric acid, H₂SO₄, which should be a stronger acid than phosphoric acid, H₃PO₄. For a given nonmetal central atom, the acid strength increases as the oxidation number of the central atom increases. For example, nitric acid, HNO₃, in which the nitrogen (N) atom has an oxidation number of +5, is a stronger acid than nitrous acid, HNO₂, where the nitrogen oxidation state is +3. In the same manner, sulfuric acid, H₂SO₄, with sulfur in its +6 oxidation state, is a stronger acid than sulfurous acid, H₂SO₃, where a +4 oxidation number of sulfur exists.

The salt of an oxyacid is a compound formed when the acid reacts with a base: acid + base → salt + water. This type of reaction is called neutralization, because the solution is made neutral.