Question

Assertion: The normal boiling point of water is 100°C and standard boiling point of water is 99.6°C.
Reason: The temperature at which vapour pressure of liquid is equal to the external pressure is called boiling temperature at that pressure.
A. Both assertion and reason are correct and Reason is the correct explanation for Assertion.
B. Both assertion and reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

In order to answer this question you must recall the definition of Boiling point which is explained in terms of atmospheric pressure and vapour pressure. And at least once, go through the standard values of atmospheric pressure with reference to the Melting point and Boiling point and you must be aware of basic applications of vapour pressure. And then by putting all these concepts in this question you will reach out to the correct answer.

Complete step by step answer:
Step 1: In this step we will figure out the points which we have learned from the concept of atmospheric pressure and vapour pressure by keeping in mind the direction of question.
And we conclude that, when the pressure is 1 atm then the boiling point is known as normal boiling point and when the atmospheric pressure is 1 bar then the boiling point is known as standard boiling point of any liquid. And since we know that 1 atm pressure is greater than 1 bar pressure and therefore the standard boiling point of a liquid is slightly lower than the normal boiling point.
Hence, the given assertion is correct.

Step 2: In the previous step we proved that the assertion is correct, but the concept of vapour pressure is never used. Which tells that it does not depend upon the vapour pressure.
Hence the given reason is incorrect.
Since, the assertion is correct and the reason is not the correct explanation for assertion.

Therefore, the correct answer is option B.

Note: The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding environmental pressure. A liquid in a partial vacuum has a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high pressure has a higher boiling point than when that liquid is at atmospheric pressure.