Question

The rate of evaporation from a water surface increases if:
A. difference of vapour pressure between water and air is increased
B. velocity of wind is decreased
C. concentration of soluble solids in water is decreased
D. altitude is high
Choose the correct answer from the options given below:

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

Hint:

To remember evaporation drivers:
- Higher Temperature, Higher Wind Speed, Higher Altitude $\rightarrow$ Higher Evaporation.
- Higher Humidity, Higher Salinity (Soluble Solids) $\rightarrow$ Lower Evaporation.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Evaporation is the physical process by which liquid water is converted into water vapor.
The rate of evaporation is governed by Dalton's Law of Evaporation, which states that evaporation is proportional to the vapor pressure gradient between the water surface and the surrounding air.
Environmental factors like wind speed, water salinity, and atmospheric pressure also influence this rate.

Step 2: Detailed Explanation:
Let us analyze each of the given factors:
1. Difference of vapour pressure is increased (A):
According to Dalton's Law:
\[ E = C(e_s - e_a) \] where $(e_s - e_a)$ is the vapor pressure deficit.
Increasing this difference directly increases the driving force for evaporation.
Therefore, A is correct.
2. Velocity of wind is decreased (B):
As water evaporates, it saturates the air layer directly above the water surface, which slows down further evaporation.
Wind removes this saturated air layer and replaces it with drier air, maintaining a favorable vapor pressure gradient.
Therefore, decreasing wind velocity will decrease the evaporation rate, not increase it.
Hence, B is incorrect.
3. Concentration of soluble solids in water is decreased (C):
Dissolved salts and solids in water lower the chemical potential of the water molecules, reducing its saturation vapor pressure.
This means saline water evaporates more slowly than fresh water.
Reducing the concentration of dissolved solids (making the water purer) increases its vapor pressure, which increases the evaporation rate.
Therefore, C is correct.
4. Altitude is high (D):
At higher altitudes, the total atmospheric pressure is lower.
This lower air pressure reduces the resistance against water molecules escaping from the liquid surface into the air, which increases the rate of evaporation.
Therefore, D is correct.
Thus, the factors that increase the evaporation rate are A, C, and D.

Step 3: Final Answer:
The rate of evaporation increases when the vapor pressure deficit is increased (A), the concentration of soluble solids is decreased (C), and the altitude is high (D).
Hence, the correct option is (D).