Its mathematical expression is:
PA=p xA where pa is the vapor partial pressure of the solvent in the solution; P is the vapor pressure of pure solvent; XA is the substance fraction of the solvent.
This law was put forward by French physicist F.M. Raoul on the basis of 1887 experiment, and it is one of the basic laws of dilute solution. For different solutions, although the concentration range of the law is different, any solution can strictly follow the above formula under the condition of xA→ 1. Raoul's law was first summed up when studying the dilute solution of non-volatile non-electrolyte, and later it was found that it was also correct for other solvents in dilute solution. In any solution satisfying xA→ 1, the force on solvent molecules is almost the same as that on molecules in pure solvents. Therefore, in solution, if the molecules of a component are subjected to the same action as the pure state, the vapor pressure of the component obeys Raoul's law.
Raoul's law is the basis of solution thermodynamics research, which has a guiding role in the study of phase equilibrium and solution thermodynamic function.