Raoult's Law Definition in Chemistry

Determining vapor pressure in relation to solutes in solutions

Distilling flasks filled with water in a laboratory
Disillaton is an application of Raoult's Law.

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Raoult's law is a chemical law that states that the vapor pressure of a solution is dependent on the mole fraction of a solute added to the 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 as it relates to the properties of a 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.

When the vapor pressure is lower than expected from the law, the result is a negative deviation. This occurs when forces between particles are stronger than those between particles in pure liquids. For example, this behavior can be 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.
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Helmenstine, Anne Marie, Ph.D. "Raoult's Law Definition in Chemistry." ThoughtCo, Feb. 16, 2021, thoughtco.com/definition-of-raoults-law-605591. Helmenstine, Anne Marie, Ph.D. (2021, February 16). Raoult's Law Definition in Chemistry. Retrieved from https://www.thoughtco.com/definition-of-raoults-law-605591 Helmenstine, Anne Marie, Ph.D. "Raoult's Law Definition in Chemistry." ThoughtCo. https://www.thoughtco.com/definition-of-raoults-law-605591 (accessed June 5, 2023).