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Ionic compounds tend to dissolve in polar solvents such as water.
The solubility is due to the solvation (or in the case of water, hydration), of ions by the polar water molecules.
A solution is saturated when no more solute will dissolve in it.
If the solute is an ionic compound, then these ions in the saturated solution are in dynamic equilibrium with the excess solid. In the case of silver chloride for example, the following equilibrium is set up:
By applying the equilibrium law:
Ksp = [Ag+][Cl-]eq
Ksp is called the solubility product of silver chloride.
The AgCl(s) term is constant and is incorporated in to Ksp.
The solubility of silver chloride can be expressed in terms of the concentration (c) of dissolved AgCl in water.
This concentration equals the concentrations of both ions in solution:
[Ag+] = [Cl-] = c
So, the solubility of silver chloride can be related to its solubility product by:
The units of Ksp in the case of AgCl are (concentration)2 or mol2dm-6.
Let us now consider the equilibrium established when silver chloride dissolves in water to form a saturated solution:
If we now add more chloride ions (by adding sodium chloride solution for example) the equilibrium shifts to the left.
This effect is known as the common ion effect. In this case, chloride is the common ion. It is common to both silver chloride and the sodium chloride.
Since the addition of the common ion results in shifting the equilibrium to the left, silver chloride is precipitated. The addition of the common ion has reduced its solubility. The same effect is produced if silver nitrate solution is added. In this case the common ion is silver.
It follows that the solubility of an electrolyte in an aqueous solution containing a common ion is less than the solubility in water.