When the cohesion between molecules of a fluid is greater than adhesion between fluid and the glass, then the free level of fluid in glass tube dipped in the glass vessel will be:
Try it first: pick an option, then read the explanation.
Correct answer: B. Lower than the surface of the fluid
Asked in SSC JE 1 MARCH 2017 Evening Shift
Detailed Explanation
The meniscus profile and capillary elevation inside a vertical tube reflect the dominant intermolecular force. Non-wetting liquids behave differently from wetting liquids.
Why “Lower than the surface of the fluid” Is Correct
When cohesion between liquid molecules is stronger than adhesion between the liquid and the glass wall (as with mercury in glass), the liquid depresses to minimize contact, resulting in a level lower than the surrounding fluid surface.
Why the Other Options Are Wrong
- A. Same as the surface of the fluid: The level is identical only when tube diameter is very large or contact angle is exactly 90°.
- C. Higher than the surface of the fluid: The liquid rises higher only when adhesion is greater than cohesion (wetting, e.g. water in glass).
- D. Dependent on atmospheric pressure: Capillary rise and depression are caused by intermolecular surface tension, not atmospheric pressure.
Key Point
Cohesion > Adhesion produces capillary depression: tube liquid level falls below vessel liquid level.
Key Concepts for Students
- Capillary depression formula: h = 4T cosθ / (ρgd), where cos θ < 0 for θ > 90°.
- Mercury in glass forms a convex upward meniscus.
- Capillary depression corrections must be applied when reading mercury manometers.
Practice more: this is question 63 in Properties of Fluid MCQs – Part 5. Try the whole set and check every answer instantly.