Generally nonmetal oxides tend to react with water to form acidic solutions. For example CO2 is an acidic oxide. It dissolves in rainwater to form carbonic acid, H2CO3. This rainwater solution contains an equilibrium mixture of carbonic acid, H2CO3, and the ions, HCO31- , CO32-, H3O1+ and OH1-. This rainwater mixture has a pH of roughly 5.6 and is naturally acidic.

Carbon dioxide reacts with water according to the following equation.

CO2(g) + H2O(l) ---->H2CO3(aq)

Carbonic acid is a weak acid with two ionization steps. The ionization constants are dramatically different . The first ionization step is shown here with an ionization constant -----Ka1 = 4.45 x 10-7.

H2CO3(aq) + H2O(l) ----> HCO3-1(aq) + H3O1+(aq)

The second ionization step is shown here with an ionization constant Ka2 = 4.7 x 10-11

HCO3-1(aq) + H2O(l) ----> CO3-2(aq) + H3O1+(aq)