Final answer:
Le Châtelier's principle explains the common ion effect as a shift in equilibrium that occurs when a solution already containing a common ion is introduced to a weak acid or base equilibrium, resulting in decreased solubility or ionization.
Step-by-step explanation:
Le Châtelier’s principle explains the common ion effect through a change in equilibrium dynamics. When a strong electrolyte containing a common ion is added to a weak acid or weak base that is at equilibrium, the system will shift towards the reactant side to restore balance. This results in a decrease in the solubility of an ionic species or in the ionization of the weak acid or base.
For example, considering the dissolution of silver iodide (AgI), the equilibrium:
AgI(s) ⇒ Ag+ (aq) + I¯(aq)
If a solution containing additional silver (I) or iodide ions is introduced, the system will respond by shifting the equilibrium left, in favor of the reactant or undissolved solid, thereby decreasing the concentrations of dissolved Ag+ and I¯ in accordance with Le Châtelier's principle.