Explain the common ion effect and how it shifts solubility.

Explain the common ion effect and how it shifts solubility.

• A. Adding a salt containing a common ion shifts the dissolution toward more dissolution, increasing solubility.
• B. The common ion effect occurs with acids only.
• C. The common ion effect has no impact on solubility.
• D. Adding a salt that contains a common ion shifts the dissolution equilibrium toward the solid and decreases solubility.

Answer: (D)

The common ion effect is an application of Le Chatelier’s principle to a dissolution equilibrium. When you dissolve a sparingly soluble salt, it establishes an equilibrium between solid and ions in solution: the solid can dissolve to produce ions, and those ions can recombine to form more solid. If you introduce a salt that supplies one of those ions—the common ion—the concentration of that ion in solution increases. To compensate, the system shifts the equilibrium toward the side with fewer ions, which is the solid form. As a result, less of the salt dissolves, so the observed solubility decreases. For example, adding a salt that supplies Cl− to a solution containing AgCl shifts the balance toward AgCl(s), reducing the amount of AgCl that can dissolve at that temperature, even though the solubility product is still the same. The other statements either claim the effect increases solubility, applies only to acids, or has no impact, which contradicts how the equilibrium responds to a higher concentration of a common ion.

The common ion effect is an application of Le Chatelier’s principle to a dissolution equilibrium. When you dissolve a sparingly soluble salt, it establishes an equilibrium between solid and ions in solution: the solid can dissolve to produce ions, and those ions can recombine to form more solid. If you introduce a salt that supplies one of those ions—the common ion—the concentration of that ion in solution increases. To compensate, the system shifts the equilibrium toward the side with fewer ions, which is the solid form. As a result, less of the salt dissolves, so the observed solubility decreases. For example, adding a salt that supplies Cl− to a solution containing AgCl shifts the balance toward AgCl(s), reducing the amount of AgCl that can dissolve at that temperature, even though the solubility product is still the same. The other statements either claim the effect increases solubility, applies only to acids, or has no impact, which contradicts how the equilibrium responds to a higher concentration of a common ion.