The common ion effect reduces the solubility of a sparingly soluble salt in the presence of a solution containing a common ion. Which statement is true?

• A. It increases the dissolution of the salt
• B. It has no effect on dissolution
• C. It decreases solubility by Le Chatelier's principle
• D. It depends on the solvent's color

Answer: (D)

The common ion effect is about Le Chatelier's principle applied to a dissolution equilibrium. For a sparingly soluble salt AB(s) that dissolves to A+(aq) and B−(aq), the solubility product Ksp = [A+][B−]. If the solution already contains a common ion, such as an extra A+, the increased concentration pushes the product [A+][B−] above Ksp. The system responds by shifting the equilibrium toward the solid, reducing the amount of salt that can dissolve. In other words, solubility decreases. The color of the solvent has no effect on this equilibrium, so that factor doesn’t influence solubility in this context.