Write about the common ion effect.

(N/A) The common ion effect is a phenomenon based on $Le \text{ } Chatelier's$ principle.
It is defined as the shift in equilibrium upon adding a substance that provides more of an ionic species already present in the dissociation equilibrium.
Example: Consider the dissociation equilibrium of acetic acid:
$CH_3COOH_{(aq)} \rightleftharpoons H^{+}_{(aq)} + CH_3COO^{-}_{(aq)}$
$\therefore K_a = \frac{[H^{+}][CH_3COO^{-}]}{[CH_3COOH]}$
If we add sodium acetate $(CH_3COONa)$ to this solution,the concentration of acetate ions $[CH_3COO^{-}]$ increases.
According to $Le \text{ } Chatelier's$ principle,the equilibrium shifts to the left to consume the excess acetate ions,thereby decreasing the concentration of $H^{+}$ ions and suppressing the dissociation of acetic acid.