If a saturated solution of MgCO3 is placed in contact with a large supply of Mg2+ ions, what happens to its solubility?

• A. Increases
• B. Decreases
• C. Unchanged
• D. Doubles

Answer: (B)

The amount of a sparingly soluble salt that dissolves is controlled by its solubility product, Ksp = [Mg2+][CO3 2−]. In a saturated MgCO3 solution, the concentrations are tied so their product equals Ksp. If a large amount of Mg2+ is added, the [Mg2+] increases, and to keep the product at Ksp, the system must decrease [CO3 2−] by precipitating more MgCO3. With fewer carbonate ions in solution, less MgCO3 can dissolve, so the solubility drops. In other words, the common ion effect causes the solubility to decrease.