On dissolving sugar in water at room temperature it feels cool to the touch. Under which of the following cases, dissolution of sugar will be most rapid?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Powdered sugar in Hot Water
Sugar feeling cool when it dissolves shows that the dissolution is endothermic (ΔsolH > 0). Two factors decide how fast a solid dissolves:
Temperature — at a higher temperature the molecules move faster, so dissolution is faster. Because the dissolution is endothermic, a higher temperature (Le Chatelier’s principle) also raises the solubility.
Surface area — powdered sugar offers a much larger surface in contact with water than crystals do.
Both factors favour powdered sugar in hot water.
Why the other options are incorrect
Option (1) Sugar Crystals in Cold Water — Incorrect. Small surface area and low temperature, so this is the slowest case.
Option (2) Sugar Crystals in Hot Water — Incorrect. The temperature helps, but crystals expose far less surface than powder.
Option (3) Powdered Sugar in Cold Water — Incorrect. The surface area is large, but cold water slows an endothermic dissolution.
NCERT (2026-27 edition):
Effect of Temperature – “The solubility of a solid in a liquid is significantly affected by temperature changes. Consider the equilibrium represented by equation 1.10. This, being dynamic equilibrium, must follow Le Chateliers Principle. In general, if in a nearly saturated solution, the dissolution process is endothermic (ΔsolH > 0), the solubility should increase with rise in temperature and if it is exothermic (ΔsolH < 0) the solubility should decrease. These trends are also observed experimentally.”