How do nonpolar molecules like CO₂ and O₂ dissolve in water?

How does O₂ dissolve in water? The oxygen has to cross the air/water interface- often a slow process. Just how slow depends on whether the water is still or running, how much surface is in contact with the air, what's dissolved in the water, and whether any films (like soap, or broken bacteria or algae cell walls) are floating on the surface. Once the oxygen crosses the surface, it is caged by water molecules.

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: (CO₃^2-):

H₂CO₃(aq) H⁺(aq) + HCO₃^-(aq)
HCO₃^-(aq) H⁺(aq) + CO₃^2-(aq)

Notice that hydrogen ions are produced. That means that any water with dissolved CO₂ will be slightly acidic. In fact, distilled water in equilibrium with air has a pH of around 5.7 and not 7.0, due to these reactions (air contains between 300 and 400 ppm of CO₂). Check it out for yourself. Measure the pH of freshly distilled water. Let the same water sit for a day or two, and measure the pH again. You can show how slowly the process occurs by timing your measurements.

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).

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2001 by Fred Senese
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Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101-hidden/solutions/faq/dissolving-gases.shtml