Question

Identify from following salts so that the solubility of salt in water decreases with increase in temperature.

• A. \( \text{NaBr} \)
• B. \( \text{NaCl} \)
• C. \( \text{NaNO}_3 \)
• D. \( \text{Na}_2\text{SO}_4 \)

Hint:

$\text{Na}_2\text{SO}_4$ is an exception where solubility decreases after the transition temperature.

Answer 1

The Correct Option is D

Step 1: Concept
Most salts show increased solubility with temperature (endothermic dissolution). Some show exothermic dissolution or phase changes.

Step 2: Meaning
Sodium sulfate exhibits a unique solubility curve due to the transition between hydrated and anhydrous forms.

Step 3: Analysis
Above 32.4 °C, $\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}$ (Glauber's salt) loses water to become anhydrous $\text{Na}_2\text{SO}_4$, whose solubility decreases as temperature increases.

Step 4: Conclusion
Unlike $\text{NaCl}$ or $\text{KNO}_3$, sodium sulfate's solubility decreases in its anhydrous phase at higher temperatures. Final Answer: (D)