Question

Assertion: The meniscus of a liquid disappears at the critical temperature.
Reason: The density of a liquid and its gaseous phase become equal at the critical temperature.
(A) Both assertion and reason are correct and the reason is the correct explanation for assertion
(B) Both assertion and reason are correct but the reason is not the correct explanation for assertion
(C) The assertion is correct but the reason is incorrect
(D) Both assertion and reason are incorrect

Answer 1

Hint : At the critical temperature, the states cannot be distinguished. Their properties also have the same values. They do not change. The critical temperature is the temperature when the substances can no longer be liquefied. The meniscus is the curve over the liquid.

Complete Step By Step Answer:
The critical temperature of a substance is the highest temperature at which the substance can exist in a liquid state. At this temperature, the substance can no longer liquefy, no matter how much pressure is applied.
At the critical temperature, it becomes difficult to state whether the substance is in the gaseous state or the liquid state. Indeed, at the critical temperature both the states cannot be distinguished. The surface present initially between the liquid and the gas disappears at the critical temperature. And so the meniscus of the liquid disappears at the critical temperature. At this point, the various physical properties such as density, refractive index, etc. have the same values and they become for both the states.
Thus because of the above statement, we can say that both assertion and reason are correct. And the reason is the correct explanation for assertion.
Hence the correct option is A.

Note :
Critical temperature is different for different substances. The meniscus is a curve in the surface of a liquid when it touches another material. It may be concave or convex. The critical temperature of gas provides an insight into the strength of the intermolecular force of attraction that its particles are subjected to.