A closed system containing at least two phases is called a heterogeneous system. Reactions (changes) between or among phases are driven by energy manifested in temperature of chemical potentials. When there is no net change in a closed system among the phases, the system is said to have reached an equilibrium condition. The application of the equilibrium principles to describe the changes between solids and their solutions is the focus in this section. For these salts, the concept of solubility product, Ksp, is used to described the formation of solutions and solids.