October 4, 2025

What Is The Common Ion Effect

Q: How does the Common Ion Effect relate to Le Châtelier's Principle?

It's a direct consequence: adding a common ion increases the concentration of a product in a solubility equilibrium, causing the equilibrium to shift towards the reactants (undissolved solid) to relieve the stress, thus reducing solubility.

Q: Does the Common Ion Effect apply to highly soluble salts?

While technically present, its effect is negligible for highly soluble salts because their solubility is already very high, and a slight decrease does not significantly change their overall dissolved amount.

Q: In what real-world scenarios is this effect utilized?

It's used in water treatment to precipitate undesirable ions, in manufacturing to purify chemicals through controlled crystallization, and in analytical chemistry for selective precipitation of ions.

Q: What happens to solubility if an ion *not* common to the sparingly soluble salt is added?

Adding a non-common ion usually has a negligible effect on solubility, or in some cases, can slightly increase it due to ionic strength effects, but it will not cause the significant decrease seen with a common ion.

Discover how the presence of a common ion affects the solubility of sparingly soluble ionic compounds in a solution, explained concisely.

Have More Questions →

Defining the Common Ion Effect

The Common Ion Effect describes the decrease in the solubility of an ionic compound when a salt containing a 'common ion' is added to the solution. This effect is most noticeable for sparingly soluble salts, meaning those that dissolve only to a small extent in water. When a soluble salt introduces an ion already present from the sparingly soluble salt, the equilibrium shifts to reduce the solubility of the latter.

How the Common Ion Effect Works (Le Châtelier's Principle)

This phenomenon is a direct application of Le Châtelier's Principle. For a sparingly soluble ionic compound, dissolving in water is an equilibrium process between the undissolved solid and its dissociated ions in solution. Adding an ion that is already part of this equilibrium (a 'common ion') increases the concentration of that product. According to Le Châtelier's Principle, the system will counteract this change by shifting the equilibrium back towards the undissolved solid, thereby decreasing the solubility of the sparingly soluble compound.

A Practical Example: Silver Chloride Solubility

Consider the sparingly soluble salt silver chloride (AgCl). In water, it establishes an equilibrium: AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq). If we add sodium chloride (NaCl) to this solution, NaCl dissociates completely into Na⁺(aq) and Cl⁻(aq), increasing the concentration of Cl⁻ ions. Since Cl⁻ is a common ion, the equilibrium for AgCl shifts to the left, causing more AgCl to precipitate out of the solution, thus reducing its solubility.

Importance and Applications in Chemistry

The Common Ion Effect is crucial in quantitative analysis, particularly in gravimetric analysis and controlling precipitation reactions. It allows chemists to precisely control the precipitation of ionic compounds, ensuring maximum yield of a desired solid product or preventing unwanted precipitation. It also plays a role in understanding and controlling the formation of mineral deposits and the behavior of certain buffer solutions in biological and industrial processes.

Frequently Asked Questions

How does the Common Ion Effect relate to Le Châtelier's Principle?

Does the Common Ion Effect apply to highly soluble salts?

In what real-world scenarios is this effect utilized?

What happens to solubility if an ion *not* common to the sparingly soluble salt is added?