The ion product Q is used to predict whether a precipitate will form.

• A. It determines the rate of the reaction.
• B. It measures the reaction's energy change.
• C. It is always equal to Ksp.
• D. It predicts precipitation events.

Answer: (D)

The ion product tells you whether the current ion concentrations will lead to precipitation. It is the product of the concentrations of the ions involved in the dissolution equilibrium (raised to their stoichiometric powers). Compare this to the solubility product constant, Ksp, which is the value at equilibrium. If the ion product exceeds Ksp, the solution is supersaturated with respect to the solid and a precipitate forms as the system shifts toward reducing the ion concentrations. If the ion product is below Ksp, the ions remain dissolved and no precipitate appears. If it equals Ksp, the solution is at equilibrium with the solid present. So the ion product is used to predict precipitation events, rather than to measure reaction rate or energy change, and it is not always equal to Ksp except at equilibrium.