The partial pressure a vapor in equilibrium with the liquid is psat. Making this substitution for p in the above equation and integrating from a reference pressure psat,ref at temperature T where gg = gg,ref yields:


psat,ref is the saturation pressure in the absence of any external agent (such as an inert gas) to cause the total pressure to differ from the saturation pressure..

Pressure in excess of the saturation pressure alters the free energy of this phase by:

When integrated from psat,ref to some higher pressure p, neglecting the pressure effect on the molar volume of the condensed phase, the result is:

Although the total pressure p enters directly into the free energy of the condensed phase, it only indirectly affects gg via its effect on psat. Since the vapor and the condensed phase are always in equilibrium (gg = gc), equating the right-hand sides of Eqs (5.16a) and (5.16b) yields:

sat _ exp psat,ref RT

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