Have you ever wondered why oil and water don't mix while sugar and water
do? The answer to
that question, and many others, comes from an analysis of the factors
affecting solubility--the
propensity of a solute to form a solution with a given solvent.
A thermodynamic
cycle constructed for the formation of a solution shows that a solution
will form when the strength of
the interaction (from dipole-dipole, van der
Waals, hydrogen
bonding, etc.) between
solvent and solute is greater than the energies of the separated solute and
solvent. The strength of
that solute-solvent interaction is largely determined by the structures of
the solvent and the solute.
Like dissolves like--only solutes with similar properties or structural
features in common with the
solvent will be dissolved.
The solubility of solids in aqueous solution, generally
increases with the
temperature of the solution. There are a few exceptions to this rule, such as
Na2SO4 and all gasses. That temperature effect is
governed by the
entropy of solution. Solids become less ordered when dissolved in
water giving a positive
ΔSsoln. Because ΔG = ΔH - TΔS, a positive entropy
term will lead to an
increase in solubility with increasing temperature. Gasses become more
ordered when dissolved in
water (due to the large negative change in volume for the gas) so they have
negative entropies of
solvation Therefore, gasses have a decreasing solubility with increasing
temperature.
However, it is possible to increase the solubility of a gas in a given
solvent by increasing the pressure
of the gas above the solvent. That effect reduces the size of the negative
entropy of solvation for
gasses by increasing the order of the gas in the gas phase. The equation
that describes this effect is
called Henry's law.
