The ion product Q is defined as the product of the ions in solution at any given moment.

• A. The sum of the ion concentrations in solution.
• B. The ratio of the product to the reactant concentrations.
• C. The product of the ion concentrations in solution at any given moment.
• D. The equilibrium constant for a precipitation reaction.

Answer: (C)

At heart, the ion product measures the immediate product of the dissolved ion concentrations for a sparingly soluble salt. For a solid that dissolves into two ions, the dissolution equilibrium can be written as AB(s) ⇌ A+(aq) + B-(aq). The ion product Q uses the current concentrations: Q = [A+][B-]. This is the same kind of idea as the equilibrium constant, but it’s evaluated with whatever concentrations are actually present at that moment, not just the equilibrium ones.

This makes Q a useful diagnostic: if Q is larger than the solubility product Ksp, the solution is supersaturated and precipitation tends to occur; if Q is smaller than Ksp, more solid can dissolve; if Q equals Ksp, the solution is saturated and in equilibrium. The option describing the product of the ion concentrations matches this idea exactly.

It isn’t a sum of concentrations, which would not track dissolution/precipitation behavior, and it isn’t a ratio of products to reactants. That ratio concept relates to the general reaction quotient for reactions, but for dissolution/precipitation the relevant expression is the product of the ion concentrations. It also isn’t the equilibrium constant itself (Ksp); Ksp is the special case of Q when the system is at equilibrium.