The air has a limited capacity to hold water vapour at a given temperature.
Explanation
Evaporation is the physical process by which a liquid transitions into a vapour at any temperature below its boiling point. Unlike boiling, which occurs throughout the liquid, evaporation is strictly a surface phenomenon governed by the kinetic energy of surface molecules and atmospheric conditions.
Statement-wise Analysis:
- An increase in humidity increases the rate of evaporation. is Incorrect. Humidity refers to the amount of water vapour present in the air. Since air has a limited capacity to hold moisture at a specific temperature, an increase in humidity reduces the concentration gradient between the liquid surface and the air, thereby decreasing the rate of evaporation.
- An increase in wind speed decreases the rate of evaporation. is Incorrect. An increase in wind speed increases the rate of evaporation. Wind moves water vapour particles away from the liquid surface, maintaining a lower concentration of vapour near the surface and facilitating further evaporation.
- The air has a limited capacity to hold water vapour at a given temperature. is Correct. At any given temperature, a specific volume of air can only hold a definite amount of water vapour. This saturation limit dictates the rate of evaporation; as the air approaches this limit, the net rate of evaporation slows down.
- Evaporation is a bulk phenomenon rather than a surface phenomenon. is Incorrect. Evaporation is a surface phenomenon because only particles at the surface possess enough energy to overcome the forces of attraction and escape. Boiling is a bulk phenomenon, where particles from the entire volume of the liquid change into vapour.
Key Takeaway:
Evaporation is a surface phenomenon where the rate is directly proportional to surface area, temperature, and wind speed, but inversely proportional to ambient humidity.