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The common ion effect is a phenomenon that occurs when a solution containing a weak electrolyte and a strong electrolyte, which share a common ion, experience a decrease in the solubility of the weak electrolyte due to the presence of the common ion from the strong electrolyte.

Solubility equilibrium is the state in which the rate of dissolution of an ionic solid is equal to the rate of precipitation. For an ionic compound AB, this equilibrium can be represented by the following equation: \[ AB_{(s)} ⇌ A^{+}_{(aq)} + B^{-}_{(aq)} \] Where AB is the ionic solid, and A^+ and B^- are the dissolved ions in the solution. The solubility product constant, K_sp, describes the equilibrium position of this reaction and can be represented as: \( K_{sp} = [A^{+}] [B^{-}] \)

When a common ion is added to the solution, the equilibrium shifts according to Le Châtelier's principle to counteract the change in the system. If we consider the above equilibrium and add another soluble ionic compound containing either A^+ or B^-, the concentration of that common ion will increase in the solution. According to Le Châtelier's principle, the equilibrium will shift to oppose the change, in this case towards the left, in order to reduce the concentration of the common ion. This shift causes an increase in the formation of AB and a decrease in the solubility of the ionic compound. For example, if we have a saturated solution of lead(II) chloride (PbCl₂) and we add a sodium chloride (NaCl) solution to it, the chloride ion (Cl^-) is the common ion between these two compounds. The equilibrium will shift to the left as follows: \[ PbCl_{2(s)} ⇌ Pb^{2+}_{(aq)} + 2Cl^{-}_{(aq)} \] The increased concentration of Cl^- ions from the addition of NaCl causes the excess Pb^2+ ions to combine with Cl^- ions and form precipitated PbCl₂, thus decreasing the solubility of PbCl₂ in the solution. In summary, the common ion effect significantly decreases the solubility of ionic compounds in water by shifting the solubility equilibrium towards precipitation due to the presence of a common ion from another electrolyte.