Ionic compounds that are not very soluble in water but that do dissociate to some extent are called sparingly soluble. In solution, these substances exhibit equilibrium between the dissolved and undissolved solute. Let's consider the sparingly soluble compound calcium fluoride (CaF2). We show its equilibrium in solution as
The equilibrium constant Ksp, called a solubility product constant, governs this reaction. The numerical value of Ksp equals the product of the equilibrium concentrations of all the ions, raised to the power of their coefficients in the balanced equation.