Raoult's Law Definition in Chemistry

Raoult's Law can be used to calculate vapor pressure.
Raoult's Law can be used to calculate vapor pressure. Steve McAlister, Getty Images

Raoult's law is a chemical law that relates the the vapor pressure of a solution is dependent on the mole fraction of a solute added to solution.
Raoult's Law is expressed by the formula:
Psolution = ΧsolventP0solvent
Psolution is the vapor pressure of the solution
Χsolvent is mole fraction of the solvent
P0solvent is the vapor pressure of the pure solvent
If more than one solute is added to the solution, each individual solvent's component is added to the total pressure.

Raoult's law is akin to the ideal gas law, except for solution. The ideal gas law assumes ideal behavior in which the intermolecular forces between dissimilar molecules equals forces between similar molecules. Raoult's law assumes the physical properties of the components of a chemical solution are identical.

Deviations From Raoult's Law

If there are adhesive or cohesive forces between two liquids, there will be deviations from Raoult's law.

There is negative deviation when the vapor pressure is lower than expected from the law. This occurs when forces between particles are stronger than those between particles in pure liquids. This behavior is observed in a mixture of chloroform and acetone. Here, hydrogen bonds cause the deviation. Another example of negative deviation is in a solution of hydrochloric acid and water.

Positive deviation occurs when the cohesion between similar molecules exceeds adhesion between unlike molecules. The result is higher than expected vapor pressure. Both components of the mixture escape solution more readily than if the components were pure. This behavior is observed in mixtures of benzene and methanol and mixtures of chloroform and ethanol.


  • Raoult, F. M. (1886). "Loi générale des tensions de vapeur des dissolvants" (General law of vapor pressures of solvents), Comptes rendus, 104 : 1430-1433.
  • Rock, Peter A. (1969). Chemical Thermodynamics. MacMillan. p.261 ISBN 1891389327.