Dissolution of Gases in Liquids

Gases dissolve in liquids, but usually only to a small extent.

When a gas dissolves in a liquid, the ability of the gas molecules to move freely throughout the volume of the solvent is greatly restricted. If this latter volume is small, as is often the case, the gas is effectively being compressed. Both of these effects amount to a decrease in the entropy of the gas that is not usually compensated by the entropy increase due to mixing of the two kinds of molecules. Such processes greatly restrict the solubility of gases in liquids.

liquid solvent, solute `=>`	gas
energy to disperse solute	nil
energy to introduce into solvent	medium to large
increase in entropy	negative
miscibility	usually very limited

Solubility of gases in water

One important consequence of the entropy decrease when a gas dissolves in a liquid is that the solubility of a gas decreases at higher temperatures; this is in contrast to most other situations, where a rise in temperature usually leads to increased solubility. Bringing a liquid to its boiling point will completely remove a gaseous solute.Some typical gas solubilities, expressed in the number of moles of gas at 1 atm pressure that will dissolve in a liter of water at 25° C, are given below:

solute	formula	solubility, mol L–1 atm–1
ammonia	NH3	57
carbon dioxide	CO2	0.0308
methane	CH4	0.00129
nitrogen	N2	0.000661
oxygen	O2	0.00126
sulfur dioxide	SO2	1.25

As we indicated above, the only gases that are readily soluble in water are those whose polar character allows them to interact strongly with it.