Discuse common ion effect on the solubility of an ionic salt.

Explanation

(1) Purification of common salt :
In a saturated solution of common salt, the following equilibrium exists
`NaClhArrNa_(aq)^(+)+Cl_(aq)^(-)`
`K_(sp)=[Na^(+)][Cl^(-)]` = a constant at constant temperature. If HCl gas is passed into the saturated solution, it ionises producing plenty of `Cl^(-)` ions `HCl toH^(+)+Cl^(-),Q_(sp)=[Na^(+)][Cl^(-)]` increases. According to Le Chatelier's principle, the reverse reaction gets favoured until `Q_(sp)` again becomes equal to `K_(sp)`. More NaCl(s) precipitates. Thus, pure NaCl crystals are obtained.
Impurities present in common salt remain in the solution.
(ii) Salting out of soap:
Soap consists of sodium salts of fatty acids like `C_(17)H_(35)COONa`. A saturated solution of `C_(17)H_(35)COONa` formed during saponification contains plenty of dissolved `C_(17)H_(35)COONa`.
It is recovered as follows : `C_(17)H_(35)COONa(s)hArrC_(17)H_(35)COO_(aq)+Na_(aq)^(+)`
In a saturated solution, `K_(sp)=[C_(17)H_(35)COO^(-)][Na^(+)]` = a constant at constant temperature.
If plenty of common salt is added to the above saturated solution, `[Na^(+)]` increases because NaCl ionises as `NaCl toNa^(+)+Cl^(-)`
`Q_(sp)=[C_(17)H_(35)COO^(-)][Na^(+)]` increases. Now the backward reaction gets favoured until `Q_(sp)` becomes equal to `K_(sp).C_(17)H_(35)COONa(s)` precipitates.