The saturated CuSO4(aq) shown above [VISUAL] is left uncovered on a lab bench at a constant temperat — Equilibrium Chemistry Question

STEPS:

1. Identify the initial chemical state of the system:
The question states that the copper(II) sulfate solution is saturated and is maintained at a constant temperature.

2. Understand solubility equilibrium in a saturated solution:
A saturated solution exists in a state of dynamic equilibrium between the dissolved ions and the undissolved solid solute present in the beaker. This chemical equilibrium is represented by the equation:
$$\text{CuSO}_4(s) \rightleftharpoons \text{Cu}^{2+}(aq) + \text{SO}_4^{2-}(aq)$$
At a given constant temperature, the maximum concentration of dissolved ions that the solution can hold is a fixed value governed by the solubility of the substance (or its solubility product, $K_{sp}$).

3. Analyze the physical effect of evaporation:
As the beaker remains uncovered on the lab bench, water molecules continuously evaporate, which decreases the volume of the solvent.

4. Predict the equilibrium shift (Le Châtelier's Principle):
* If the concentration of dissolved ions were to increase due to the loss of water volume, the ion product would exceed the solubility limit ($Q_{sp} > K_{sp}$).
* To relieve this stress and maintain equilibrium at a constant temperature, the system shifts to the left.
* Consequently, dissolved $\text{Cu}^{2+}$ and $\text{SO}_4^{2-}$ ions combine and precipitate out of the solution as solid $\text{CuSO}_4(s)$ crystals at the bottom of the beaker.

5. Determine the net effect on ion concentration over time:
Because the excess dissolved solute precipitates in direct proportion to the volume of water lost, the ratio of dissolved moles of sulfate to the remaining volume of water remains perfectly unchanged. Thus, the concentration of sulfate ions, $[\text{SO}_4^{2-}]$, remains constant. This confirms Option D as the correct answer.

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WHY_OTHERS_WRONG:

• Option A is incorrect: During evaporation, only volatile water molecules ($\text{H}_2\text{O}$) have enough thermal energy to escape into the gas phase. The dissolved copper and sulfate ions are highly non-volatile and cannot leave the beaker with the evaporating water.
• Option B is incorrect: While it is true that copper sulfate precipitates, this precipitation is a feedback mechanism that maintains the saturation equilibrium. It does not cause the concentration of dissolved ions to decrease; it simply keeps the concentration constant as the solvent volume shrinks.
• Option C is incorrect: Even though the phase change of water from liquid to gas is endothermic, the problem specifies that the system is kept at a constant temperature. Because the temperature does not change, there is no temperature-induced shift in solubility, and the concentration of sulfate ions does not increase or fluctuate.