Which statement correctly distinguishes Ksp from the common ion effect?

• A. Ksp refers to complete dissolution; common ion effect increases dissolution
• B. Ksp is the solubility product of sparingly soluble salt; common ion effect suppresses dissolution due to a common ion
• C. Ksp depends on temperature only; common ion effect depends on pressure
• D. Ksp applies to gases; common ion effect applies to solids

Answer: (B)

Ksp describes the dissolution equilibrium of a sparingly soluble solid in water. For a salt that dissociates into two ions, the solubility product constant Ksp is the product of the ion concentrations at equilibrium, [A+][B−], at a given temperature. This constant characterizes how much solid can coexist with the dissolved ions, not how fully the solid dissolves.

The common ion effect is about what happens when a solution already contains one of the ions produced by dissolution. Adding a common ion increases that ion’s concentration, which, by Le Chatelier’s principle, shifts the equilibrium toward the solid and reduces the amount that can dissolve. So the common ion effect suppresses dissolution, whereas Ksp defines the equilibrium limit for dissolution.

For example, dissolving AgCl involves Ag+ and Cl− in solution. If you add more Cl−, the product [Ag+][Cl−] must remain equal to Ksp, so [Ag+] decreases and less AgCl dissolves. This illustrates how Ksp governs the dissolution equilibrium and how the common ion effect lowers solubility by increasing a common ion’s concentration.

The other statements miscast Ksp or the common ion effect: Ksp isn’t about complete dissolution, and the common ion effect isn’t about increasing dissolution; Ksp’s dependence on temperature is more nuanced than “depends only on temperature,” and the common ion effect is governed by ion concentrations rather than pressure or gas behavior.