Excess molar quantities are properties of mixtures which characterize the nonideal behaviour of real mixtures. They are the difference between the partial molar property of a component in a mixture and that of the pure component. The most frequently used excess molar quantities are the excess molar volume, excess molar enthalpies and heat capacities, excess chemical potential

Excess chemical potential

The excess chemical potential is defined as the difference between the chemical potential of a given species and that of an ideal gas under the same conditions ....

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Definition

By definition, excess properties of a mixture are related to those of the pure substance by:


Here denotes the pure substance, the excess molar property, and corresponds to the specific property under consideration. From the definition of partial molar properties,


substitution yields: