If a tube of small radius opened at both ends is placed in a liquid, the liquid will

Analyze the question: This is about capillarity, which is the rise or fall of a liquid in a narrow tube. It depends on whether the liquid wets the walls of the tube.

Concept:

• If the liquid wets the glass (e.g. water in glass), the adhesive forces between liquid and glass are stronger than the cohesive forces within the liquid. The contact angle is less than 90°, the meniscus is concave, and the liquid rises above the outside level.
• If the liquid does not wet the glass (e.g. mercury in glass), the cohesive forces are stronger. The contact angle is greater than 90°, the meniscus is convex, and the liquid is depressed (falls) below the outside level.

The height of rise or fall is given by:

h = 2T cosθ / (rρg)

When θ < 90°, cosθ is positive, so h is positive (rise). When θ > 90°, cosθ is negative, so h is negative (fall).

Checking each option:

• A. Wrong. The level changes in both cases, depending on wetting.
• B. Wrong. A wetting liquid rises, not falls.
• C. Correct. A non-wetting liquid (like mercury) is depressed below the outside level.
• D. Wrong. A non-wetting liquid does not rise.

Answer: C.

Exam tip: Remember "Water Wets, Walks up; Mercury Mopes, goes down." Wetting liquids rise (concave meniscus), non-wetting liquids fall (convex meniscus). Also, a narrower tube gives a larger rise or fall, since h is proportional to 1/r.