The lever rule finds phase fractions from a horizontal tie line on a phase diagram. For solid and liquid concentrations and bulk concentration , conservation gives . Thus is the liquid fraction, provided . For pure solid, .
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The Lever Rule is a principle used in materials science and thermodynamics to determine the relative amounts of different phases in a two-phase system at equilibrium. It is particularly useful in the context of phase diagrams, such as binary alloy phase diagrams, where two phases coexist at a specific temperature and composition. The basic idea of the Lever Rule is based on the balance of masses between the two phases. When two phases are present, their compositions can be determined from the phase diagram.