Which statement is true about the solubility product constant Ksp and precipitation?

• A. Ksp is a fixed value for any salt at all temperatures.
• B. Ksp equals [cation][anion] for sparingly soluble salts; precipitation occurs when Q > Ksp; dissolution when Q < Ksp.
• C. Ksp applies to gases dissolved in water.
• D. Ksp depends only on the color of the precipitate.

Answer: (B)

The main idea tested is how the solubility product constant (Ksp) governs when a solid will precipitate or dissolve based on the ionic product of the dissolved ions. For a sparingly soluble salt that dissociates into A+ and B-, the dissolution equilibrium is AB(s) ⇌ A+(aq) + B-(aq), and Ksp = [A+][B-] at a given temperature. The ionic product, Q, is the current [A+][B-] in solution. If Q exceeds Ksp, the solution is supersaturated and the system shifts toward forming solid, causing precipitation until Q returns to Ksp. If Q is less than Ksp, the solution can dissolve more solid until Q reaches Ksp. This shows why Ksp is a temperature-dependent constant (it can change with temperature) and why the concept applies specifically to sparingly soluble salts, with the exact expression potentially involving powers if stoichiometry differs.

The statement that Ksp is fixed for all temperatures is not correct because Ksp changes with temperature. The idea that Ksp applies to gases dissolved in water is incorrect; gas dissolution follows different principles (Henry’s law). Finally, Ksp does not depend on the color of the precipitate; color is unrelated to the thermodynamics described by Ksp.