Excess molar quantity

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 real mixture and that of the component in an ideal mixture. The most frequently used excess molar quantities are the excess molar volume, excess molar enthalpies and heat capacities, excess chemical potential. For volumes, internal energies and enthalpies the excess quantities are identical to the mixing quantities. They can be expressed as functions of derivatives of the activity coefficients.