Meniscus of mercury in an open tube In an open capillary or glass tube, the free surface of pure mercury (clean glass, air above) assumes which shape?
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AConvex meniscus (curved downward at the walls)
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BConcave meniscus (curved upward at the walls)
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CPerfectly horizontal plane surface
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DRandomly varying curvature
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EFlat at centre and concave near walls
Answer
Correct Answer: Convex meniscus (curved downward at the walls)
Explanation
Introduction / Context:Surface tension and wetting determine the shape of a liquid's free surface near a solid boundary. Mercury and water show opposite wetting behavior on clean glass, producing different meniscus shapes that directly affect capillary rise or depression.
Given Data / Assumptions:
- Open tube with air above the liquid.
- Clean glass surface.
- Liquid is mercury (high surface tension, non-wetting on glass).
Concept / Approach:
Contact angle greater than 90 degrees (non-wetting) yields a convex meniscus (bulging upward in the middle, depressed near the glass). For mercury on glass, the contact angle is obtuse, causing capillary depression and a convex meniscus.
Step-by-Step Solution:
Identify wetting: mercury does not wet glass ⇒ large contact angle.Large contact angle implies convex meniscus.Therefore, the free surface is convex, appearing curved downward near the walls relative to the centre.Verification / Alternative check:
Water on clean glass (wetting) yields a concave meniscus and capillary rise, the opposite of mercury's behavior. Observations in manometers confirm mercury's capillary depression.
Why Other Options Are Wrong:
Concave is characteristic of wetting liquids like water; perfectly horizontal would require zero capillary effects; random curvature contradicts deterministic surface tension physics.
Common Pitfalls:
Confusing “curved upward” and “curved downward” descriptions; always tie the shape to the contact angle and wetting behavior.
Final Answer:
Convex meniscus (curved downward at the walls)