Deviations from ideality can be described by the use of Margules functions or activity coefficients.  A single Margules parameter may be sufficient to describe the properties of the solution if the deviations from ideality  are modest; such solutions are termed regular.
In contrast to ideal solutions, where volumes are strictly additive and mixing is always complete, the volume of a non-ideal solution is not, in general, the simple sum of the volumes of the component pure liquids and solubility is not guaranteed over the whole composition range. By measurement of densities, thermodynamic activity of components can be determined.