Which from following ionic solids exhibits decrease in its solubility in water with increase of temperature?

Among the given options, sodium sulfate \(\left(\mathrm{Na}_2 \mathrm{SO}_4\right)\) exhibits a decrease in solubility with an increase in temperature. This behavior is unusual because, for most ionic solids, solubility tends to increase with temperature. However, for \(\mathrm{Na}_2 \mathrm{SO}_4\), its dissolution is exothermic, meaning it releases heat when it dissolves in water. According to Le Chatelier's principle, if the dissolution process is exothermic, increasing the temperature will shift the equilibrium to favor the undissolved solid, thereby reducing solubility at higher temperatures.
Solubility trends for other salts:
- (1) NaCl (Sodium chloride): NaCl 's solubility increases with temperature.
- (2) \(\mathrm{KNO}_3\) (Potassium nitrate): \(\mathrm{KNO}_3\) 's solubility increases significantly with temperature.
- (3) \(\mathrm{NaNO}_3\) (Sodium nitrate): \(\mathrm{NaNO}_3\) 's solubility also increases with temperature.