Intermediate Topic 481 of 1102 Kinetics & Equilibrium
Common Ion Effect
Adding an ion common to an equilibrium reduces solubility or ionization.
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In-depth explanation of Common Ion Effect
The common ion effect describes how the presence of an ion that is already part of an equilibrium shifts that equilibrium away from dissolution or ionization. Adding a common ion increases the concentration of one product, so Q exceeds the equilibrium constant and the reaction shifts toward the undissolved or unionized form. As a result the solubility of a sparingly soluble salt decreases in a solution that already contains one of its ions. The effect is important in buffer solutions, where a weak acid or base is combined with its own salt, and in analytical separations based on precipitation. It is a direct application of Le Chatelier's principle to ionic equilibria.
Going deeper: Common Ion Effect
Key ideas: Method: put the common ion concentration in the initial row of the ICE table, then solve with the approximation that x is small. For AgCl in 0.10 M NaCl, s = Ksp/0.10 = 1.8 x 10^-9 M. In weak acid problems, added salt lowers ionization and raises pH, which is the foundation for buffers.
Common mistakes and connections: Students think that the common ion dissolves the solid by reacting or that the solubility is unchanged because Ksp is a constant. Ksp stays fixed but solubility drops. Another error is to use total ion concentration without noting the source of each ion. At high ionic strength or with complexation, as in concentrated chloride, solubility of AgCl can rise again. Link to buffers and Le Chatelier.
Visualization of Common Ion Effect
Adding chloride from NaCl pushes AgCl solubility from 13 uM in pure water down to 0.018 uM in 10 mM NaCl, as Le Chatelier predicts. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Silver chloride is less soluble in sodium chloride solution than in pure water, because the common chloride ion suppresses dissolution.
Frequently asked questions about Common Ion Effect
What is Common Ion Effect?
Adding an ion common to an equilibrium reduces solubility or ionization.
Is Common Ion Effect a beginner, intermediate or advanced topic?
Common Ion Effect is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Common Ion Effect?
Study these earlier topics in Kinetics & Equilibrium first: Ostwald Process, Equilibrium and Free Energy, Solubility Product.
What comes after Common Ion Effect?
The next topics in Kinetics & Equilibrium are Precipitation Prediction and Selective Precipitation.
Can I practise Common Ion Effect with quizzes?
Yes. The interactive lesson includes a self-check question on Common Ion Effect, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Common Ion Effect fits in Kinetics & Equilibrium
Common Ion Effect is topic 62 of 91 in Kinetics & Equilibrium.