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:
CO2(g) CO2(aq)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)
Because the carbonic acid forms slowly, adding a base to carbonated water causes the pH to jump up (as the small amount of carbonic acid is quickly neutralized) and then, slowly down again (as new carbonic acid forms from hydrated carbon dioxide). For a vivid demonstration of this point, see D. M. Kern's article "The Hydration of Carbon Dioxide", J. Chem. Ed., 37, 14 (1960).
Stephen Lower discusses these equilibria in Carbonate equilibria in natural waters (you'll need the Adobe Acrobat reader to view the document).
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