The physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates.

• A. Saturated solution
• B. Unsaturated solution
• C. Solution equilibrium
• D. Supersaturated solution

Answer: (C)

Dynamic equilibrium between dissolution and crystallization is reached when a solid solute is in contact with a saturated solution. At this point, molecules or ions leave the solid and join the solution just as fast as those in solution revert to solid, so the overall amount of dissolved solute stays constant. The system is saturated at that temperature, meaning the solubility limit has been reached and any small disturbance won’t change the concentration until temperature or pressure changes. This balancing act—the ongoing, equal rates of both processes—defines solution equilibrium. If the solution were unsaturated, more solute could dissolve, and if it were supersaturated, excess solute would crystallize to re-establish equilibrium.