A change in the temperature of a saturated solution disturbs the equilibrium between the

Analyze the question It asks which equilibrium in a saturated solution is disturbed when the temperature changes.

Concept A saturated solution is one in which the dissolved solute is in dynamic equilibrium with the undissolved solute:

solute (undissolved) ⇌ solute (dissolved)

At equilibrium, the rate of dissolving equals the rate of crystallisation. Solubility depends on temperature, so changing the temperature changes one of these rates more than the other, and the equilibrium shifts.

Solution

• Raising the temperature (for most solids, where dissolving is endothermic) increases solubility. More undissolved solute dissolves, and the equilibrium shifts toward the dissolved side.
• Lowering the temperature decreases solubility. Solute crystallises out, and the equilibrium shifts toward the undissolved side.

In both cases, the balance that changes is between the dissolved solute and the undissolved solute.

Checking the other options:

• A and B: the solvent is not one of the two species in the equilibrium. It is just the medium.
• D: the dissolved solute is part of the solution, so this is not a separate equilibrium.

Answer: C. Dissolved solute and the undissolved solute

Tip: For "saturated solution equilibrium" questions, remember that the equilibrium is always between the solid solute and the solute in solution. The solvent is never one of the two sides.