What does the solubility product constant (Ksp) indicate?

• A. It measures how quickly a sparingly soluble salt dissolves.
• B. It measures how much a sparingly soluble salt dissolves at equilibrium.
• C. It measures the color change of dissolution.
• D. It equals the sum of ion concentrations.

Answer: (B)

Ksp is the equilibrium constant for the dissolution of a sparingly soluble salt in water. At a given temperature, the concentrations of the dissolved ions in solution adjust so that their product (each raised to its stoichiometric power) equals a fixed value. This tells you how much of the salt can dissolve before the solution becomes saturated. A larger Ksp means more ions can be present in solution, so more of the salt dissolves; a smaller Ksp means only a small amount dissolves. It does not measure how fast dissolution occurs (that’s about kinetics), nor does it describe a color change or be simply the sum of ion concentrations. For a salt that dissolves as M2+ and X− with the formula M X, Ksp equals [M2+][X−]^2, illustrating that the constant is a product of ion concentrations, not a sum.