Raoult's Law

Raoult's law states that the partial vapor pressure of each volatile component in an ideal liquid mixture is equal to the vapor pressure of that component in its pure form multiplied by its mole fraction in the mixture. French chemist Francois-Marie Raoult formulated the law based on experimental work published between 1886 and 1888, establishing one of the foundational quantitative relationships of solution chemistry. The law implies that adding a non-volatile solute to a pure solvent lowers the solvent's vapor pressure in proportion to the solute's mole fraction, an effect that explains related colligative properties of solutions, including boiling point elevation and freezing point depression. Real mixtures generally deviate from Raoult's law to some degree depending on whether the interactions between unlike molecules are weaker or stronger than the interactions between like molecules, so the law holds most precisely for mixtures of chemically similar substances or in the limit of a very dilute solution, and it remains a standard starting point in physical chemistry for describing vapor-liquid equilibrium in the design of industrial distillation processes.