Roozeboom's Rules

two rules relating the equilibrium compositions of a two-component solid solution and the coexisting liquid to the melting point of the solid phase. The rules were formulated by H. Roozeboom in 1899.

The first rule holds that at equilibrium the solid solution is relatively richer than the liquid phase in that component whose addition increases the temperature at which crystallization begins. If on a phase diagram’s curve depicting the dependence of the temperature at the end of the melting stage (or at the beginning of crystallization) or the system’s composition there are no extrema, then this law reduces to a simpler formulation. Here, the solid solution at equilibrium, in relation to the liquid, is richer in that component whose melting point in the pure state is higher.

The second rule holds that at equilibrium, the extrema of the melting point curve are points at which the solid and liquid phase have identical compositions. These rules are analogous to Konovalov’s rules, which describe two-component liquid systems in equilibrium with vapor. Roozeboom’s rule have great practical significance since they make it possible to determine the rate of crystallization of solid solutions.