Raoult's Law

Deviations from Raoult's law are as informative as the law itself. It holds strictly only when the forces between unlike molecules match those between like molecules, a condition approached by mixtures of closely related substances such as benzene and toluene. Where unlike interactions are weaker than like interactions, molecules escape the liquid more readily than the law predicts and the mixture shows a positive deviation, which may produce a minimum-boiling azeotrope; stronger unlike interactions give negative deviation and a maximum-boiling azeotrope. The law also supplies the basis of the colligative properties, since lowering the vapour pressure of a solvent by adding a non-volatile solute raises the boiling point and depresses the freezing point in proportion to the mole fraction of solute added. Fractional distillation is designed around the resulting vapour-liquid equilibria.