How does CO2 dissolve in water? Here is a sketchy outline of the process. As with the O2, the CO2 must cross the surface of the liquid.
It's a little easier for the CO2 to do so than for oxygen, because the oxygen ends of the molecule have a partial negative charge are better able to hydrogen-bond to the water as a result. The CO2 rather slowly acquires a shell of water molecules. A fraction of these hydrated carbon dioxide molecules react with the water to produce carbonic acid (H2CO3):
CO2(aq) + H2O H2CO3(aq)
The equilibrium constant for this reaction is about 1.6×10-3 around room temperature, which means that most of the dissolved carbon dioxide is present as hydrated CO2. Only about 16% reacts with water to form carbonic acid. The reaction is rather slow. It involves bending a stable, linear CO2 molecule (with a water parked oxygen-down over the carbon) into a Y-shaped O=C(OH)2 molecule.
The carbonic acid is a weak acid, and it can dissociate to form bicarbonate ion (HCO3-) and (in basic solution) carbonate ion: (CO32-):
H2CO3(aq) H+(aq) + HCO3-(aq)
HCO3-(aq) H+(aq) + CO32-(aq)