Which statement about solubility in a solution with a common ion is true?

Which statement about solubility in a solution with a common ion is true?

• A. Increases
• B. Decreases
• C. Remains the same
• D. Depends on temperature

Answer: (B)

The common ion effect explains why solubility drops when the solution already contains one of the ions from the dissolving solid. For a sparingly soluble salt that dissolves into A+ and B−, the solubility is governed by Ksp = [A+][B−]. If the solution already has extra A+ (a common ion), adding more solid would push the product [A+][B−] above Ksp, so the dissolution shifts to the left and fewer ions dissolve. The system reaches a new equilibrium with less dissolved substance, i.e., lower solubility. Temperature can change overall solubility, but the defining outcome of the common ion effect is the decrease due to the added ion, with the exact amount depending on temperature via Ksp(T).

The common ion effect explains why solubility drops when the solution already contains one of the ions from the dissolving solid. For a sparingly soluble salt that dissolves into A+ and B−, the solubility is governed by Ksp = [A+][B−]. If the solution already has extra A+ (a common ion), adding more solid would push the product [A+][B−] above Ksp, so the dissolution shifts to the left and fewer ions dissolve. The system reaches a new equilibrium with less dissolved substance, i.e., lower solubility. Temperature can change overall solubility, but the defining outcome of the common ion effect is the decrease due to the added ion, with the exact amount depending on temperature via Ksp(T).