Question

Assertion: Perspiration from the human body helps in cooling the body.
Reason: A thin layer of water on the skin enhanced its emissivity.
A. Both, Assertion and Reason, are true and Reason is the correct explanation of Assertion.
B. Both, Assertion and Reason, are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.

Answer 1

In this type of question, we have to check for the statement given and the reason for the given statement are right or wrong and also for the correct explanation. To solve this, we make use of the concept of exchange of heat and emissivity. Perspiration is a process of thermoregulation of the human body.

Complete answer:
Let us first know about perspiration. Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin. It involves the exchange of heat from body to surrounding. In the human body, sweating is primarily a means of thermoregulation.

A thin layer of water is present on the skin of the human body. The water layer takes heat from the body and gets converted into vapour and the human body cools down.But, the thin layer of water reduces the emissivity. The emissivity of the surface of human skin is its effectiveness in emitting energy as thermal radiation. So, Perception from the human body helps in cooling the body, but the thin layer of water on skin reduces its emissivity.

Hence, option C is correct.

Note: When water leaves the body through perspiration, the content of molecules remaining in the body decreases, therefore temperature also decreases. And, Emissivity changes with temperature because of energy that is tied up in the behavior of the molecules that form the surface.