The equations used for the activity coefficients of the components in a liquid as a function of the composition of the mixture. Developed in 1929 by Dutch chemist Johannes van Laar (1860–1938) for liquid solutions based on the van der Waals’ equation of state, it was based on observations that strongly non-ideal solutions are associated with large heats of mixing. The van Laar equation gives good agreement for many binary mixtures. The equations can be extended to mixtures of more than two components, but the equation becomes unwieldy, and is therefore rarely used beyond binary mixtures.