Question

Assertion The surface of water in the capillary tube is concave.
ReasonThe pressure difference between two sides of the top surface of water is $\frac{2S}{a}\cos \theta$.

• A. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
• B. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.
• C. Assertion is correct but Reason is incorrect.
• D. Assertion is incorrect but Reason is correct.

Answer 1

Answer: (A)

The pressure difference between the two sides of the top surface of a liquid in a capillary tube is given by
$p_i-p_o=\frac{2S}{r}=\frac{2S}{a\sec \theta }=\frac{2S}{a}\cos \theta$
where, $\theta =$ angle of contact.
In case of water is taken in the capillary tube, the contact angle between water and glass is acute. Thus, the pressure of water inside the tube, just at the meniscus (air-water interface) is less than the atmospheric pressure. So, the surface of water in the capillary is concave as shown below

Therefore, Assertion and Reason are correct and Reason is the correct explanation of Assertion.