The concentration of ions are very small, we can set their activities equal to their molar concentration. , is the constant for the equilibrium established between a solid solute and its ions in a saturated solution: solubility products vary with temperature. Calculations involving k sp are generally more subject to error than are those involving other equilibrium constants, but the results are suitable for many purposes. According to le ch telier s principle, an equilibrium mixture responds to a forced increase in the concentration of one of its reactants by shifting in the direction in which that reactant is consumed. In the lead(ii) iodide solubility equilibrium, addition of the common ion, i reaction to be favored, leading to a new equilibrium. The addition of the common ion shirts the equilibrium of a slightly soluble ionic compound toward the undissolved compound, causing more to precipitate. Thus, the solubility of the compound is reduced.
