Question

On dissolving sugar in water at room temperature, the solution feels cool to touch. Under which of the following cases dissolution of sugar will be most rapid?
(A)Sugar crystals in cold water
(B)Sugar crystals in hot water
(C)Powdered sugar in cold water
(D)Powdered sugar in hot water

Answer 1

Hint: Endothermic reactions absorb heat from the surrounding. Molecules with greater surface area absorb better than molecules having smaller surface area. Hot solutions provide greater energy to the molecules as compared to the colder mediums.

Complete step by step answer:
Given in question is that, when sugar is dissolved in water, the solution feels cool to touch. This meant that, when sugar is dissolved in water, it absorbed the heat of the provided water to get solvated. However, when the reaction going on is absorbing heat from the medium it means that the reaction is endothermic. Therefore, the dissolution of sugar in water is endothermic.
Since sugar is absorbing heat from the water, it means to get dissolved effectively it will be needing much heat, hence, the provided or supplied water should be hot to provide enough energy to the sugar molecules to get dissolved. Heat provides energy to the sugar molecules, through which they move faster and collide with each other to dissolve quickly.
Also, when molecules have large surface area, it will be able to absorb more heat, through the exposed surface.Powdered sugar has increased surface area as compared to normal cubes of sugar. Therefore, hot solution and increased surface area will be favourable for the dissolution of sugar.

Therefore, the correct answer is (D) option.

Note: According to Le Chatelier's principle, at equilibrium , the rate of endothermic reaction will be increased when temperature is increased. However, for exothermic reactions, Increase in temperature decreases the rate of reaction.