Common-ion effect  The common-ion effect refers to the decrease in solubility of an ionic precipitate by the addition to the solution of a soluble compound with an ion in common with the precipitate. This behaviour is a consequence of Le Chatelier's principle for the equilibrium reaction of the ionic association/dissociation. The effect is commonly seen as an effect on the solubility of salts and other weak electrolytes. Adding an additional amount of one of the ions of the salt generally leads to increased precipitation of the salt, which reduces the concentration of both ions of the salt until the solubility equilibrium is reached. The effect is based on the fact that both the original salt and the other added chemical have one ion in common with each other. Hydrogen sulphide (H2S) is a weak electrolyte. It is weakly ionized in its aqueous solution. There exists an equilibrium between unionized molecules and the ions in an aqueous medium as follows:  By applying the law of mass action, we have  To the above solution of H2S , if we add hydrochloric acid, then it ionizes completely as   This makes H+ a common ion and creates a common ion effect. Due to the increase in concentration of H+ ions, the equilibrium of dissociation of H2S shifts to the left and keeps the value of Ka constant. Thus the ionization of H2S is decreased. The concentration of unionized H2S is increased. As a result, the concentration of sulphide ions is decreased. Barium iodate, Ba(IO3)2, has a solubility product Ksp = [Ba2+][IO3-]2 = 1.57 x 10-9. Its solubility in pure water is 7.32 x 10-4 M. However in a solution that is 0.0200 M in barium nitrate, Ba(NO3)2, the increase in the common ion barium leads to a decrease in iodate ion concentration. The solubility is therefore reduced to 1.40 x 10-4 M, about five times smaller. A practical example used very widely in areas drawing drinking water from chalk or limestone aquifers is the addition of sodium carbonate to the raw water to reduce the hardness of the water. In the water treatment process, highly soluble sodium carbonate salt is added to precipitate out sparingly soluble calcium carbonate. The very pure and finely divided precipitate of calcium carbonate that is generated is a valuable by-product used in the manufacture of toothpaste.