Evaporation of sweat from human skin dissipates body heat.

Question

Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).

Assertion (A): Evaporation of sweat from human skin dissipates body heat.

Reason (R): High humidity on a hot day increases discomfort.

Select the correct answer from the code given below:

1. A. Both (A) and (R) are true, but (R) is not the correct explanation of (A) (Correct answer)
2. B. (A) is false, but (R) is true
3. C. (A) is true, but (R) is false
4. D. Both (A) and (R) are true and (R) is the correct explanation of (A)

Correct Answer

Option A — Both (A) and (R) are true, but (R) is not the correct explanation of (A)

Detailed Solution & Explanation

The correct answer is Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Key Points

• Assertion is true. The evaporation of sweat from the skin absorbs latent heat of vaporisation from the body, and so cools it.
• Reason is true. High humidity on a hot day does increase discomfort, because the air is already close to saturation.
• (R) does not explain (A). The reason explains why cooling fails in humid conditions — it is a statement about the *obstruction* of the process, not about *why* evaporation cools the body. The mechanism in the assertion is latent heat absorption, which the reason never mentions. Hence option (a).

Additional Information

• The latent heat of vaporisation of water is about 2,260 kJ/kg — unusually high, which is what makes sweating such an effective cooling mechanism.
• The same principle explains why water stays cool in an earthen pot (matka), why we feel cold stepping out of a bath, and how a desert (evaporative) cooler works. It is also why evaporative coolers work well in dry Rajasthan but poorly in humid coastal Kerala.
• Evaporation is faster with higher temperature, larger surface area, lower humidity and greater wind speed.
• The perceived temperature combining heat and humidity is reported as the heat index; the temperature at which air becomes saturated is the dew point.