Which statement best describes the relation of ion product to Ksp when a sparingly soluble salt is in solution?

Which statement best describes the relation of ion product to Ksp when a sparingly soluble salt is in solution?

• A. Ion product equals Ksp when precipitation has just begun.
• B. Ion product is greater than Ksp always.
• C. Ion product equals Ksp indicates the solution is saturated and at equilibrium.
• D. Ion product is independent of Ksp.

Answer: (D)

Ion product is the actual product of the concentrations of the dissolved ions in solution, while Ksp is the value that product takes when the system is at equilibrium for a sparingly soluble salt. As the salt dissolves, the ion concentrations rise, so the ion product changes. When the ion product reaches the value of Ksp, the solution is saturated and the rates of dissolution and precipitation are equal, i.e., the system is at equilibrium. If dissolution continues and the ion product exceeds Ksp, precipitation will occur to pull the value back down to Ksp. If the ion product is still below Ksp, more solid can dissolve. Therefore, the ion product is not independent of Ksp; it is the current value that is compared to Ksp to determine whether the solution will dissolve more or precipitate.

Ion product is the actual product of the concentrations of the dissolved ions in solution, while Ksp is the value that product takes when the system is at equilibrium for a sparingly soluble salt. As the salt dissolves, the ion concentrations rise, so the ion product changes. When the ion product reaches the value of Ksp, the solution is saturated and the rates of dissolution and precipitation are equal, i.e., the system is at equilibrium. If dissolution continues and the ion product exceeds Ksp, precipitation will occur to pull the value back down to Ksp. If the ion product is still below Ksp, more solid can dissolve. Therefore, the ion product is not independent of Ksp; it is the current value that is compared to Ksp to determine whether the solution will dissolve more or precipitate.