Question

Which of the following statements are correct pertaining to evaporation processes ?
(A) The gross escape of water molecules from the liquid state to the gaseous state constitutes evaporation.
(B) The latent heat of vaporization at about 685 cal/g of evaporated water must be provided by the water body.
(C) The rate of evaporation increases always with the wind speed.
(D) Deep water bodies have more heat storage than shallow ones.
Choose the correct answer from the options given below :

• A. (A), and (B) only.
• B. (B), (C) and (D) only.
• C. (B) and (D).
• D. (D) only.

Hint:

- Latent heat of vaporization of water (\(\lambda\)) at \(0^{\circ}C\) is about 597 cal/g, and at \(100^{\circ}C\) it is 539 cal/g. At a standard room temperature of \(20^{\circ}C\), it is approximately 585 cal/g.
- Deep reservoirs show a "thermal lag" effect, delaying seasonal evaporation peaks.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Evaporation is the physical process by which water is converted from its liquid state to a gaseous state (water vapor) and transferred into the atmosphere.
This process is governed by thermal energy, vapor pressure gradients, wind velocity, and the heat storage capacity of water bodies.

Step 2: Detailed Explanation:
Let us analyze each statement to determine its validity:
- Statement (A): Evaporation is defined as the net escape of water molecules from the liquid to the gaseous state.
Since water molecules are constantly escaping (evaporating) and returning (condensing) simultaneously, the gross escape alone does not constitute net evaporation, making this statement technically incorrect.
- Statement (B): The latent heat of vaporization of water at normal ambient temperatures (around \(20^{\circ}C\)) is approximately 585 calories per gram (cal/g), not 685 cal/g.
Thus, this statement is incorrect.
- Statement (C): While wind increases the rate of evaporation by removing the saturated boundary layer above the water surface, this relationship does not hold "always."
Beyond a certain critical wind velocity, further increases in wind speed have negligible effect on the evaporation rate if the air is already saturated or if other factors like solar radiation are limiting, making this statement incorrect.
- Statement (D): Deep water bodies have a much larger volume and higher thermal mass, allowing them to store a significant amount of heat energy during the summer and slowly release it during the winter.
This high heat storage capacity delays peak evaporation rates compared to shallow water bodies, making this statement correct.
Therefore, only statement (D) is correct.

Step 3: Final Answer:
The correct option is (D), representing (D) only.