Raoult's law

Raoult's law concerns the vapour pressure of the solvent above a solution. This is always lower than the vapour pressure of the pure solvent, and the form of the law first proposed by François-Marie Raoult in 1887 was that the fractional lowering of the vapour pressure of the solvent is proportional to the amount fraction of solute. A more general statement, which allows for the dissociation of the solute, is that the vapour pressure is proportional to the amount fraction of solvent.

Both of these statements hold only for dilute solutions, and a solution which obeys these simple forms of Raoult's law is termed an ideal solution. Limited non-ideal behaviour can be accounted for by taking fugacities instead of vapour pressures, and this approach is used to define the activity of the solvent in a solution.