Explain the solubility of a salt in the presence of a common ion.

(N/A) The solubility of a sparingly soluble salt decreases in the presence of a common ion. This is known as the common ion effect.
According to Le Chatelier's principle,when the concentration of one of the ions (either the cation or the anion) of a sparingly soluble salt is increased by adding a strong electrolyte containing a common ion,the equilibrium shifts to the left to consume the excess ions,resulting in the precipitation of the salt and a decrease in its solubility.
Example $1$: In a saturated solution of $AgCl$,if $NaCl$ is added,the $Cl^{-}$ ion concentration increases. If $AgNO_{3}$ is added,the $Ag^{+}$ ion concentration increases. In both cases,the equilibrium $AgCl(s) \rightleftharpoons Ag^{+}(aq) + Cl^{-}(aq)$ shifts to the left,causing more $AgCl$ to precipitate and decreasing its solubility.
Example $2$: When $HCl$ gas is passed through a saturated solution of $NaCl$,the concentration of $Cl^{-}$ ions increases. This causes the equilibrium $NaCl(s) \rightleftharpoons Na^{+}(aq) + Cl^{-}(aq)$ to shift to the left,leading to the precipitation of solid $NaCl$ and a decrease in its solubility.