the common ion effect. saturated sodium chloride solution contains solid nacl in equilibrium with...

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.