The common ion effect

Saturated sodium chloride solution contains solid NaCl in equilibrium with the aqueous ions:NaCl(s) Na+(aq) + Cl–

(aq)

We know this sodium chloride solution is saturated because there are crystals of solid NaCl at the bottom of the bottle.

Some of the saturated solution is poured into a test tube.

Since this liquid is clear we know that all the ions are in solution.

Concentrated hydrochloric acid contains a high concentration of Cl–(aq).

Some of the colourless conc HCl solution is put in a dropper, ready to add to the saturated salt solution.

A single drop of HCl falls into the saturated solution and immediately forms crystals of NaCl.

It’s more impressive if we give it a good squirt…

Saturated NaCl contains about 6 mol L–1 of NaCl.

[Na+] = 6 mol L–1; [Cl–] = 6 mol L–1

[Na+][Cl–] = Ks = 36

Concentrated HCl is about 11 mol L–1, which significantly increases the concentration of chloride ions.

Since the ionic product, [Na+][Cl–], exceeds the Ks, a precipitate of NaCl(s) forms.The solubility of a compound will decrease in the presence of another compound with a common ion.

You may have separated soap (sodium stearate) from the mixture of glycerol and unreacted sodium hydroxide by adding saturated salt.