For the phase equilibrium NaCl(s) ⇌ NaCl(aq), the NaCl(aq) must be a solution that is:

• A. Concentrated
• B. Saturated
• C. Supersaturated
• D. Unsaturated

Answer: (B)

At this temperature, the solid and dissolved form of a salt are in balance only when the water holds the maximum amount of salt it can dissolve—that is, the solution is saturated with respect to NaCl. In the phase equilibrium NaCl(s) ⇌ NaCl(aq), the rates of dissolution and crystallization are equal, so the amount of NaCl in solution stays constant while solid and dissolved salt coexist. If the solution were unsaturated, more salt would dissolve and no solid would be needed to maintain equilibrium. If it were supersaturated, it would be unstable and tend to crystallize to return to the saturated state. The term concentrated isn’t precise enough to define the equilibrium condition here, whereas saturated directly describes the balance at the solubility limit.