Maxwell equal-area construction

Summary

The Maxwell equal-area construction corrects the isotherms of an equation of state such as the van der Waals equation below the critical temperature. It replaces the non-monotone loop of an isotherm by a horizontal segment. The segment lies at the pressure where the two areas cut off by it are equal. The segment describes the coexistence of liquid and vapour, and its pressure is the vapour pressure. The equal-area rule expresses the equality of the chemical potentials of the two coexisting phases.