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The common-ion effect

A salt dissolves less in water that already contains one of its ions. That is just Le Chatelier applied to Ksp: the shared ion pushes the dissolution equilibrium back toward the solid.

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A shared ion pushes back.

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The common-ion effect is Le Chatelier's principle applied to a dissolution equilibrium. Adding an ion that is already part of the equilibrium is a stress that shifts it toward the solid (back toward the reactant side).

As a result, the salt is less soluble in a solution that already contains one of its ions than in pure water. For AgCl in a solution already containing Cl⁻, the extra Cl⁻ pushes the equilibrium back, dropping the solubility.

Ksp itself is unchanged (it is a concentration stress, not temperature); the shift simply lowers how much solid can dissolve.

UNIT 7 TOPIC 7.12 • COMMON-ION EFFECT COMMON ION LAB BEFORE ADDING NaCl AgCl(s) ⇌ Ag⁺ + Cl⁻ Ag⁺ Cl⁻ Ag⁺ Cl⁻ Cl⁻ Pure water: dissolved Ag⁺ and Cl⁻ are equal, and some solid remains. AFTER ADDING NaCl NaCl adds Na⁺ and Cl⁻ Na⁺ Cl⁻ Na⁺ Ag⁺ Cl⁻ Cl⁻ Na⁺ Cl⁻ More Cl⁻ shifts equilibrium left. Dissolved Ag⁺ decreases, and more solid AgCl forms. PARTICLE ACCOUNTING FIXED The after-panel includes Na⁺ spectators, so total positive and negative charge stays balanced. QUANTITATIVE EFFECT Ksp = [Ag⁺][Cl⁻] If [Cl⁻] = 0.10 M: [Ag⁺] = Ksp / 0.10 much lower TAKEAWAY Adding a common ion lowers molar solubility by shifting the dissolution equilibrium left. Spectator ions must still preserve charge balance. AP Chemistry · Unit 7 · Equilibrium
Fig. 7.12.1 The common-ion effect: adding an ion the salt already contains stresses the dissolution equilibrium, shifting it back toward the solid. The salt is less soluble in a solution already containing one of its ions than in pure water.
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Applying the common-ion effect.

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Treat the shared ion as a Le Chatelier stress.

  1. Identify the common ion. An ion the added solution shares with the dissolution equilibrium.
  2. Apply the stress. The common ion adds product-side stress, shifting the equilibrium toward the solid.
  3. Conclude reduced solubility. The salt dissolves less than it would in pure water.
  4. Keep Ksp fixed. Ksp is unchanged; only the amount dissolved decreases.
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The pieces you'll meet.

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Le Chatelier meets solubility.

common ion
Common ion
An ion shared with the dissolution equilibrium.
shift
Shift toward solid
The shared ion pushes toward the reactant (solid).
less soluble
Lower solubility
The salt dissolves less than in pure water.
Ksp fixed
Ksp unchanged
A concentration stress does not change Ksp.
Le Chatelier
Le Chatelier
The common-ion effect is this principle applied.
pure water
Versus pure water
Solubility is highest in pure water, lower with a common ion.
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Worked example: AgCl with added chloride.

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Setup. AgCl(s) ⇌ Ag⁺ + Cl⁻. You dissolve AgCl in a solution that already contains Cl⁻ (from, say, NaCl).

Stress. The extra Cl⁻ is a common ion — a product-side stress on the equilibrium.

Shift. By Le Chatelier, the equilibrium shifts left, toward the solid, so less AgCl dissolves.

Result. AgCl is less soluble in the chloride solution than in pure water, even though Ksp is unchanged.

§5

Mistakes that cost real points.

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Pitfall · 01

"Adding a common ion increases the salt's solubility."

The opposite is true: a common ion shifts the dissolution equilibrium toward the solid, decreasing solubility. A salt is less soluble in a solution that already contains one of its ions than in pure water.

Fix. Read the common-ion effect as decreasing solubility; the shared ion pushes the equilibrium back toward the solid.

Pitfall · 02

"The common-ion effect changes Ksp."

The common-ion effect is a concentration stress, so it shifts the equilibrium position but does not change Ksp. Ksp is fixed at a given temperature; only the amount that dissolves changes.

Fix. Keep Ksp constant; the common-ion effect lowers solubility without altering Ksp.

Pitfall · 03

"A salt is equally soluble in pure water and in a solution of a common ion."

A salt is more soluble in pure water than in a solution already containing one of its ions. The common ion suppresses dissolution, so the solubilities differ.

Fix. Expect lower solubility in a common-ion solution than in pure water.

§6

Skill Check.

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