In precipitation of saturated solution of a common salt, HCl gas is added and not HCl acid because if we add HCl acid to a saturated solution of common salt, the equilibrium of equation (1) will be shifted to left side by increasing concentration of Cl– (eq). As a result, NaCl will recrystallize in the solution.
NaCl ➝ Na+ + Cl– …(1)
HCl ➝ H+ + Cl– …(2)