Dear student,

Silver chloride is so insoluble in water (.0.002 g/L) that a saturated solution contains only about 1.3 x 10^-5 moles of AgCl per liter of water.

	H2O
AgCl(s)		Ag+(aq)	+	Cl-(aq)

Strict adherence to the rules for writing equilibrium constant expressions for this reaction gives the following result.

(Water isn't included in the equilibrium constant expression because it is neither consumed nor produced in this reaction, even though it is a vital component of the system.)

The [Ag⁺] and [Cl^-] terms represent the concentrations of the Ag⁺ and Cl^- ions in moles per liter when this solution is at equilibrium. The third term[AgCl]is more ambiguous. It doesn't represent the concentration of AgCl dissolved in water because we assume that AgCl dissociates into Ag⁺ ions and Cl^- ions when it dissolves in water. It can't represent the amount of solid AgCl in the system because the equilibrium is not affected by the amount of excess solid added to the system. The [AgCl] term has to be translated quite literally as the number of moles of AgCl in a liter of solid AgCl.