25.1.10—Common ion effect and solubility

Adding an ion already present in a dissolution equilibrium shifts the equilibrium toward the solid, reducing the solubility. Ksp remains constant at fixed temperature; the ion concentrations change.

Set up the common-ion concentration before adding the small solubility contribution, then substitute into Ksp. The approximation is valid only when the added ion dominates.

AgCl is less soluble in NaCl solution because added Cl⁻ shifts AgCl(s) ⇌ Ag⁺ + Cl⁻ left. Calculate [Ag⁺] from Ksp/[Cl⁻] when [Cl⁻] is known.

The common ion does not change Ksp itself and does not always make precipitation instantaneous.