Chemically similar compounds (e.g., ethanol and water or benzene and toluene) generally form mixtures that are close to ideal, as evidenced by activity coefficients that are near unity and by small excess Gibbs energies of mixing. On the other hand, chemically dissimilar species (e.g., benzene and water or toluene and ethanol) form strongly nonideal mixtures. Show, by considering the binary mixtures that can be formed from ethanol, water, benzene, and toluene, that the UNIFAC model predicts such behavior.