Relates the partial vapor pressure of a component in an ideal solution to the mole fraction of that component in the liquid phase and the vapor pressure of the pure component: p_i = x_i · p_i*.

Principle

In an ideal solution, intermolecular interactions between unlike molecules equal those between like molecules, so each volatile component contributes to total vapor pressure proportionally to its liquid-phase mole fraction.

Demonstration

A mixture of two chemically similar, nonpolar liquids (approximate ideality) shows a total vapor pressure equal to the sum of each pure-component vapor pressure weighted by its mole fraction; measured vapor composition matches predictions within experimental uncertainty.

Misapplication

Applying Raoult's law to strongly non-ideal mixtures (large polarity differences, hydrogen bonding, ionic solutes) or using it at high concentrations without activity corrections produces inaccurate vapor pressure and phase behavior predictions.

Consequence

When valid, Raoult's law lets one compute vapor pressures, vapor compositions, and boiling-point changes from liquid composition and pure-component vapor pressures, and serves as a baseline for identifying deviations (positive/negative).

Reversal

Deviations from Raoult's law (positive or negative) occur when unlike interactions differ from like interactions; strong deviations can produce azeotropes or nonideal phase diagrams that invert expected boiling order.

Boundary

Valid for ideal or nearly ideal liquid solutions, typically for chemically similar species at moderate concentrations and temperatures. Excludes electrolyte solutions, strong associating mixtures, and situations requiring activity coefficients to account for non-ideality.

Semantic Tension

Can be confused with Henry's law (which links dissolved gas concentration to partial pressure) or Dalton's law (partial pressures in a gas mixture); Raoult's law specifically addresses the contribution of a liquid solution component to vapor pressure under ideality assumptions.

Synthesis

Raoult's law is the ideal-solution rule that each component's partial vapor pressure equals its pure-component vapor pressure scaled by its liquid mole fraction; it is the reference model for vapor–liquid equilibria and a diagnostic for non-ideal behavior.