1. Acidic oxides: oxides that can react with alkali to form salts and water. Most nonmetallic oxides are acidic oxides.

For example: SO2, CO2, SO3, N2O5, P2O5, etc.

Most acidic oxides can react with water to generate corresponding acids, which are corresponding to the valence of nonmetallic elements.

For example, CO2+H2O=H2CO3, and the valence of carbon before and after the reaction is +4;

SO2+H2O=H2SO3

SO3+H2O=H2SO4

A few acidic oxides can't react with water to generate corresponding acids, for example, SiO2 can't react with water.

Basic oxides: oxides that can react with acids to form salts and water, and most metal oxides are basic oxides.

For example: Fe2O3, CuO, Na2O, CaO, etc.