The Common Ion Effect states that the addition of an ion already present in the equilibrium mixture will shift the equilibrium away from that ion.

• $AgCl(s)\rightleftharpoons A{g}^{+}(aq)+C{l}^{-}(aq)$
• $HCl$ provides $H^{+}$ and $C{l}^{-}$ ions.
• $NaCl$ provides $N{a}^{+}$ and $C{l}^{-}$ ions.
• In both cases, the concentration of $C{l}^{-}$ (a common ion) increases. According to Le Chatelier's principle, the equilibrium shifts to the left, causing more $AgCl$ to precipitate.
• Consequently, the solubility of $AgCl$ decreases in both solutions.