common-ion effect, decrease in solubility of an ionic salt, i.e., one that dissociates in solution into its ions, caused by the presence in solution of another solute that contains one of the same ions as the salt. The common-ion effect is an example of chemical equilibrium. For example, silver chloride, AgCl, is a slightly soluble salt that in solution dissociates into the ions Ag+ and Cl−, the equilibrium state being represented by the equation
AgClsolid ⥨ Ag++Cl−. According to Le Châtelier's principle, when a stress is placed on a system in equilibrium, the system responds by tending to reduce that stress. In the system taken as an example, if another solute containing one of those ions is added, e.g., sodium chloride, NaCl, which supplies Cl− ions, the solubility equilibrium of the solution will be shifted to remove more Cl− from the solution, so that at the new equilibrium point there will be fewer Ag+ ions in solution and more AgCl precipitated out as a solid.
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