The difference in the solubilities of strong and weak electrolytes is what is harnessed to purify or separate compounds by shifting the equilibrium of a specific equation in a system of multiple equilibria.
An ionic compound that dissociates completely into its component ions in solution is called a strong electrolyte. In aqueous solutions, strong electrolytes exist only as ions. KI, the salt used to cause AgI to precipitate out of solution, is an example of a strong electrolyte. The equilibrium equation for KI in solution is usually written with a one-way arrow to indicate that its equilibrium constant is so large that the reaction goes to completion:
In comparison, a weak electrolyte is an ionic compound that only partially dissociates into its component ions in solution. The equilibrium constant of a weak electrolyte is small, and the chemical equation is written with a double arrow to indicate that the reaction goes both ways. AgI is an example of a weak electrolyte:
The difference in the solubilities of strong and weak electrolytes is what is harnessed to purify or separate compounds by shifting the equilibrium of a specific equation in a system of multiple equilibria. Consider the common ion effect, demonstrated with solutions of KI and AgI. Both K+ and Ag+ have the common ion I–. It is because KI is a strong electrolyte that KI does not form a precipitate in the presence of dissolved AgI. The equilibrium for the KI dissolution is shifted so far to the right that a reduction in [I–] has no real effect on the solubility of KI. The only way for solid KI to exist in an aqueous solution is if the amount of solid KI is increased. Both species of a strong electrolyte must be present in large concentrations for a solid precipitate to form.