1. The common ion effect

2. 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.

3. 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.

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

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

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

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

8. Saturated NaCl contains about 6 mol L –1 of NaCl. [Na+] = 6 mol L –1 ; [Cl–] = 6 mol L –1 [Na + ][Cl – ] = K s = 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 K s, a precipitate of NaCl(s) forms. The solubility of a compound will decrease in the presence of another compound with a common ion.

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