Solubility equilibrium is any type of chemical equilibrium relationship between solid and dissolved states of a compound at saturation.Ksp stands for "solubility product" or "solubility equilibrium". It is the equilibrium constant for the reaction in which a solid salt dissolves to give its constituent ions in solution. Solubility equilibria involve application of chemical principles and constants to predict solubility of substances under specific conditions (because solubility is sensitive to the conditions, while the constants are less so).