Equations similar to generalized van der Waals differential equations but written in the metric of the Gibbs potential open with respect to the solvent are analyzed for two-phase solution - solid equilibria under isothermal-isobaric conditions. It is shown that for systems with one disparate component (solvent), in particular, water-salt systems, there exist analogues of the Gibbs-Konovalov laws for liquid-vapor equilibria and the Gibbs-Roozeboom rules for melt-solid phase equilibria. The results obtained substantiate the hypothesis of topological isomorphism of melting-point diagrams of n-component systems in the temperature-composition (mole fraction) coordinates and isothermal solubility diagrams of (n + 1)-component systems in the water activity-composition (Yeneke index) coordinates.