Which one of the following liquids typically shows capillary fall rather than capillary rise in a clean glass tube?

Introduction / Context:
Capillary action is a familiar phenomenon in daily life, observed when liquids rise or fall in thin tubes. Whether a liquid rises or falls depends on the interplay between cohesive forces within the liquid and adhesive forces between the liquid and the tube material. This question asks which liquid shows capillary fall, not rise, in a clean glass tube, which is a standard conceptual point in fluid mechanics.

Given Data / Assumptions:

• The tube is made of clean glass.
• The liquids considered are common ones such as water, alcohol, mercury, and kerosene.
• We are focusing on the direction of capillary movement relative to the general liquid surface.

Concept / Approach:
If adhesive forces between the liquid and the tube are stronger than the cohesive forces within the liquid, the liquid wets the tube, forming a concave meniscus and rising in the capillary. If cohesive forces dominate, the liquid does not wet the tube, forms a convex meniscus, and experiences capillary depression or fall. Water and most organic liquids wet clean glass and rise. Mercury, however, does not wet glass and therefore shows capillary fall.

Step-by-Step Solution:
Step 1: Recall that water rises in a thin glass tube, forming a concave meniscus due to strong adhesive forces with glass. Step 2: Alcohols and kerosene also generally wet glass and show a rise rather than a fall in narrow tubes. Step 3: Mercury has strong cohesive forces among its own atoms and poor adhesion to glass, so it forms a convex meniscus and the level inside the capillary is lower than the outside level. Step 4: This lowering of the mercury column is called capillary depression or capillary fall.

Verification / Alternative check:
In standard physics experiments, students observe that a thin glass capillary dipped in water shows a column of water higher than the outer water level, whereas a capillary dipped in mercury shows the mercury level inside the tube lower than that in the reservoir. This direct observation supports the idea that mercury exhibits capillary fall in glass tubes.

Why Other Options Are Wrong:
Water: It wets glass strongly, leading to capillary rise and a concave meniscus. Alcohol: It also generally wets glass and tends to rise in capillary tubes. Kerosene: Like many organic liquids, it shows capillary rise in glass rather than fall.

Common Pitfalls:
A student may guess water because it is the liquid used in many capillary examples, but the question specifically asks about capillary fall, not rise. Another mistake is to assume that all liquids behave the same in glass tubes. Remember the key rule: if the liquid wets the surface, you get rise and a concave meniscus; if it does not wet the surface, you get fall and a convex meniscus.

Final Answer:
The liquid that shows capillary fall in a clean glass tube is mercury.