Question

During hot sunny days, people sprinkle water on the roof because ___________________.
A) They have more water to sprinkle
B) Water is the cheapest thing available
C) It has a high latent heat of vaporization
D) None of the above

Answer 1

Hint: Water molecules are held together by hydrogen bonding. To evaporate the water sufficient energy is required to break the hydrogen bond between the water molecules. As the water heats the hydrogen bond makes it difficult to separate the water molecules from the liquid surface. Thus water absorbs energy from the surrounding and changes its state from liquid to vapour.

Complete answer:
The latent heat of vaporization is an amount of heat consumed by the matter when matter changes its state from liquid to gas at a constant temperature. The latent heat of vaporization is an amount of heat combusted by the matter.
Water has a high boiling point equal to $\text{ 10}{{\text{0}}^{\text{0}}}\text{C }$. Water contains the long network of hydrogen bonding between the molecules. Thus high energy input is required to transform one gram of liquid water into water vapour. The amount of heat required to break the hydrogen bonding is the latent heat of vaporization. Water has a latent heat of vaporization value equal to $\text{ 40}\text{.65 kJ /mol }$ . Thus water requires a definite amount of energy for the change in the phase of water.
As the water heats up the heat energy is required to break the hydrogen bonding to separate the water molecules. When hydrogen reaches the $\text{ 10}{{\text{0}}^{\text{0}}}\text{C }$ heat can break the hydrogen bonding between the molecules.
Since water has a high latent heat of vaporization water absorbs the heat from the surrounding. The absorption of heat cools down the surrounding and heats the water. This converts the liquid water into water vapours.
Thus during hot sunny days, people sprinkle water on the roof because water has a high latent heat of vaporization.

Hence, (C) is the correct option.

Note: The cooling effect of water is also observed in biological activities. Evaporation of sweat or water from the body removes the heat from the skin and cools the body. Like water, acetone has a greater cooling effect than water. It is a volatile liquid and absorbs heat from surrounding.