Capillarity Experiment — Why Use a Small-Diameter Glass Tube for Water? In a capillary-rise experiment with water, why is a small-diameter <em>glass</em> tube preferred?

Introduction / Context: Capillary phenomena are central to soil mechanics, ink pens, and microfluidics. Measuring capillary rise reliably requires careful choice of tube diameter and material to obtain a clear, measurable meniscus movement.

Given Data / Assumptions:

• Working fluid: water (wetting fluid on clean glass).
• Tube is narrow and clean with a circular cross-section.
• Contact angle θ for water–glass is acute (cos θ ≈ positive).

Concept / Approach: The capillary rise formula is h = 4 σ cos θ / (ρ g d), showing h ∝ 1/d for a given fluid–solid pair. Therefore, using a smaller internal diameter increases the rise, improving measurement accuracy and reducing relative reading error against the scale.

Step-by-Step Reasoning:

Verification / Alternative check: Doubling d halves the capillary rise, often making readings less distinct; conversely, halving d doubles h, validating the experimental preference.

Why Other Options Are Wrong: Contact angle is material-dependent and cleaning-dependent; small d does not force θ to zero. Metal tubes can exhibit capillarity with water depending on surface condition. Meniscus curvature is inherent and not eliminated by small d.

Common Pitfalls: Using contaminated tubes (θ changes); ignoring temperature effects on σ; misreading the meniscus (bottom for water).

Final Answer: Because capillary rise h is inversely proportional to tube diameter d, so a smaller d gives a larger, easier-to-measure rise.