Why do most covalent oxides of nonmetals produce acidic solutions when mixed with water?

Most covalent oxides of nonmetals produce acidic solutions when mixed with water because they act as acid anhydrides or undergo chemical reactions that result in the formation of oxyacids.12

When these nonmetal oxides react with water, they form compounds that dissociate to yield hydronium ions ($H_3{O}^{+}$) in aqueous solution, which characterizes the solution as acidic. This behavior occurs in two primary ways:12

• Acid Anhydride Formation: Many nonmetal oxides, such as sulfur trioxide ($S{O}_3$) and dinitrogen pentoxide ($N_2{O}_5$), react directly with water to form oxyacids without changing the oxidation number of the nonmetal.1
• Disproportionation: In cases where the nonmetal is not in a common oxidation state, such as nitrogen dioxide ($N{O}_2$), the oxide may undergo a disproportionation reaction. In this process, the nonmetal is simultaneously oxidized and reduced, ultimately producing an oxyacid.1

While most nonmetal oxides exhibit this acidic character, there are notable exceptions, such as carbon monoxide ($CO$), nitrous oxide ($N_{2}O$), and nitric oxide ($NO$), which do not form acidic solutions in water.2