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
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