Why do many nonmetal oxides form acidic solutions in water?

Nonmetal oxides form acidic solutions in water because they are covalent compounds that typically react with water to produce oxyacids. These resulting oxyacids dissociate in aqueous solution to yield hydronium ions ($H_3{O}^{+}$), which characterize the acidic nature of the solution.12

The behavior of these oxides generally falls into two categories:

• Acid Anhydrides: Oxides where the nonmetal is in a common oxidation state (such as sulfur trioxide or dinitrogen pentoxide) react directly with water to form oxyacids without changing the oxidation number of the nonmetal.1
• Disproportionation: Oxides where the nonmetal is not in a common oxidation state (such as nitrogen dioxide) react with water through a process where the nonmetal is simultaneously oxidized and reduced, ultimately producing an acid.1

While most nonmetal oxides are acidic, there are exceptions, such as carbon monoxide, nitrous oxide, and nitric oxide, which do not form acidic solutions in water.2