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Common-Ion Effect

The common-ion effect applies Le Chatelier to solubility: adding an ion that is already part of a dissolution equilibrium stresses it, shifting the equilibrium toward the solid. The result is that a salt is less soluble in a solution already containing one of its ions than in pure water.

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
The common-ion effect: adding an ion the salt already contains stresses the dissolution equilibrium, shifting it back toward the solid. The salt becomes less soluble in a solution that already contains one of its ions than in pure water.
Common Ion Lab · Open the sandbox →

The trap mismodels the common-ion effect — for example, expecting the added ion to increase solubility, or misapplying the shift. Adding a common ion pushes the equilibrium back toward the undissolved solid, lowering solubility.

The work

3 ways in · any order
Lesson
Common-Ion Effect ›

Adding a common ion shifts a dissolution equilibrium toward the solid, lowering solubility. The lesson applies Le Chatelier to Ksp, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 7.12 misconception: the common-ion effect mismodeled — the added common ion's effect on solubility read backward.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first