JEE Physics 2025 Question with Solution

Given: Two liquids A and B have contact angles $\theta_A$ and $\theta_B$ in a capillary tube, with

Find: The correct statement about the nature of their meniscus.

The shape of a meniscus depends on the contact angle:

• If $\theta < 90^\circ$, then $\cos\theta > 0$, so the meniscus is concave.
• If $\theta > 90^\circ$, then $\cos\theta < 0$, so the meniscus is convex.

When $K$ is negative,

so $\cos \theta_A$ and $\cos \theta_B$ must have opposite signs.

That means one liquid has $\theta < 90^\circ$ and the other has $\theta > 90^\circ$. Hence, one meniscus is concave and the other is convex.

From the solution statement provided:

• $\cos \theta_A > 0$ implies liquid A has concave meniscus.
• $\cos \theta_B < 0$ implies liquid B has convex meniscus.

Also, if $K = 0$, then

so liquid A would have a flat meniscus, not convex. Therefore option D is incorrect.

Therefore, the correct option is C.

Given:

Find: Which statement about liquids A and B is correct.

For a liquid in a capillary tube, the sign of $\cos\theta$ determines the meniscus nature:

Now examine the sign of $K$:

A ratio is negative only when numerator and denominator have opposite signs. Therefore,

So exactly one liquid must have a concave meniscus and the other must have a convex meniscus. This eliminates options A and B, because both of those assign the same meniscus type to both liquids.

The extracted solution concludes the matching as:

• liquid A: concave meniscus
• liquid B: convex meniscus

For completeness, if

then

which corresponds to a flat meniscus. Hence the zero case does not support option D.

Therefore, the correct statement is: if $K$ is negative, then liquid A has concave meniscus and liquid B has convex meniscus. The correct option is C.