Solubility the concentration of a saturated solution of a solute in a particular solvent at a particular temperature.
A special case of equilibrium involves any situation where excess solute is in equilibrium with its aqueous solution. This type of equilibrium can be established by dissolving excess salt in a solvent.The excess solute precipitates and the solution remain saturated. The dissolved ions will be at equilibrium with the precipitated crystals of the solute.
Consider the chemical equation and equilibrium law equation for a weak electrolyte such as copper (I) chloride. Notice that since copper(I) chloride dissolves very little in water, almost any amount dropped into water will result in the formation of a saturated solution, and there will be a heterogeneous equilibrium between the solid and dissolved Cu+ (aq) and Cl- (aq) ions.
For any solute that forms ions in solution, the solubility equilibrium constant is the product of the concentrations of the ions in solution raised to the power equal to the coefficient of each in the balanced equation, and for that reason is often called the solubility product constant of the substance, symbolized as Ksp. The equilibrium law equation for the copper(I) chloride equilibrium is written
Ksp = [Cu+(aq)][Cl-(aq)]
Ksp values are given only for ionic compounds with low solubility. Highly soluble ionic compounds do not form precipitates and their solutions are not saturated. Solubilities of highly soluble substances are listed in mol/L or g/100 mL values rather than as Ksp values.