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?

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:

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)