Solubility Effects
The solubility of a sparingly soluble salt is reduced in a solution that contains an ion in common with that salt. For instance, the solubility of silver chloride in water is reduced if a solution of sodium chloride is added to a suspension of silver chloride in water.
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.
The salting out process used in the manufacture of soaps benefits from the common ion effect. Soaps are sodium salts of fatty acids. Addition of sodium chloride reduces the solubility of the soap salts. The soaps precipitate due to a combination of common ion effect and increased ionic strength.
Sea, brackish and other waters that contain appreciable amount of Na+ interfere with the normal behavior of soap because of common ion effect. In the presence of excess Sodium ions the solubility of soap salts is reduced, making the soap less effective.
Read more about this topic: Common-ion Effect
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