In a saturated solution at a given temperature, the undissolved solutes are in equilibrium with

Analyze the question

The question asks what the undissolved solute is in equilibrium with in a saturated solution at a fixed temperature.

Explain the concept

A saturated solution holds the maximum amount of solute it can dissolve at a given temperature. Any extra solute stays undissolved at the bottom. This is a dynamic equilibrium:

• Solute particles leave the solid and enter the solution (dissolving).
• Dissolved solute particles return to the solid (crystallizing).
• At equilibrium, the rate of dissolving equals the rate of crystallizing, so the amount of undissolved solid stays constant.

Solid solute ⇌ dissolved solute (ions or molecules)

For example: NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq)

Evaluate the options

• A. The solvent: The solvent is the medium. The equilibrium is not between the solid and the solvent itself, so this is wrong.
• B. Dissolved solute particles: The solid exchanges particles with the dissolved ones. This is the equilibrium described above, so it is correct.
• C. The saturated solution: This is a loose answer, because the solution contains the solvent and the dissolved solute. The precise equilibrium is with the dissolved solute particles in it. B is the more accurate choice.
• D. Insoluble solute particles: A solute that is in equilibrium is soluble to some extent, and "insoluble particles" does not describe the dissolved/undissolved exchange. This is wrong.

Answer

The correct option is B.

Tip

When you see "saturated solution" together with "equilibrium", think of dissolved ⇌ undissolved with equal rates in both directions. The equilibrium is always between the solid and the dissolved particles of the same substance.