Which statement correctly describes the effect of adding a salt containing a common ion on the dissolution of a sparingly soluble salt?

Which statement correctly describes the effect of adding a salt containing a common ion on the dissolution of a sparingly soluble salt?

• A. It increases solubility.
• B. It decreases solubility, because the common ion shifts the dissolution equilibrium toward the solid.
• C. It has no effect on solubility.
• D. It converts the salt to a more soluble form.

Answer: (B)

The effect being tested is how a common ion influences the dissolution of a sparingly soluble salt. When a salt that barely dissolves in water dissolves, it establishes an equilibrium between the solid and its ions in solution, with a constant solubility product (Ksp) at a given temperature: the product of the concentrations of the ions equals Ksp. If you add a salt that contains one of those ions, you raise its concentration in solution. Because Ksp must stay fixed, the system responds by shifting the dissolution equilibrium in a way that lowers the concentrations of the dissolved ions—precipitating more of the solid. In other words, more of the solid forms to counter the increase in the common ion, so less of the sparingly soluble salt dissolves. That means the overall solubility decreases. For example, adding a salt that supplies the same anion as the sparingly soluble salt increases that anion’s concentration, pushing the equilibrium toward the solid and reducing solubility. The other described outcomes—solubility increasing, no effect, or converting to a more soluble form—would violate the constant-Ksp relationship and Le Châtelier’s principle.

The effect being tested is how a common ion influences the dissolution of a sparingly soluble salt. When a salt that barely dissolves in water dissolves, it establishes an equilibrium between the solid and its ions in solution, with a constant solubility product (Ksp) at a given temperature: the product of the concentrations of the ions equals Ksp.

If you add a salt that contains one of those ions, you raise its concentration in solution. Because Ksp must stay fixed, the system responds by shifting the dissolution equilibrium in a way that lowers the concentrations of the dissolved ions—precipitating more of the solid. In other words, more of the solid forms to counter the increase in the common ion, so less of the sparingly soluble salt dissolves. That means the overall solubility decreases.

For example, adding a salt that supplies the same anion as the sparingly soluble salt increases that anion’s concentration, pushing the equilibrium toward the solid and reducing solubility. The other described outcomes—solubility increasing, no effect, or converting to a more soluble form—would violate the constant-Ksp relationship and Le Châtelier’s principle.