The rise or fall of a liquid in a narrow tube is because of the

1. Analyze the question The question asks what causes a liquid to rise (like water) or fall (like mercury) in a narrow tube. This phenomenon is called capillarity (or capillary action).

2. Explain the concept Capillarity comes from two forces working together:

• Cohesion: attraction between the liquid's own molecules.
• Adhesion: attraction between the liquid molecules and the walls of the tube.

These forces determine the shape of the liquid surface (the meniscus), and surface tension acts along that curved surface to pull the liquid up or push it down.

• Water in glass: adhesion is greater than cohesion. The meniscus curves upward (concave), the contact angle is less than 90°, and surface tension pulls the liquid up the tube.
• Mercury in glass: cohesion is greater than adhesion. The meniscus curves downward (convex), the contact angle is greater than 90°, and the liquid is depressed.

The height of rise or fall is given by:

h = 2T cosθ / (rρg)

where T is the surface tension, θ is the contact angle, r is the tube radius, ρ is the liquid density, and g is the acceleration due to gravity.

3. Check each option

• A. Viscosity: This is resistance to flow. It affects how fast the liquid moves, not how high it finally rises. Not correct.
• B. Surface tension: T appears directly in the formula for h. Correct.
• C. Friction between the walls and the liquid: Friction is not what drives the rise. Not correct.
• D. Osmotic pressure: This relates to solvent movement across a semipermeable membrane. Not correct.

4. Answer The rise or fall of a liquid in a narrow tube is because of the surface tension of the liquid, which is option B.

5. Exam tip Remember: "Narrow tube = capillarity = surface tension." Also note that h is inversely proportional to the radius, so a narrower tube gives a bigger rise. Questions on this topic often ask about that relationship too.