The phenomenon of rise or fall of liquid level in a capillarity tube is called as capillarity. In non-wetting liquids (e.g. mercury), what is the relation between cohesion of mercury and adhesion between mercury and capillary tube?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 16.11.2022
Detailed Explanation
Capillarity is governed by the competition between cohesive intermolecular forces (attraction between like liquid molecules) and adhesive intermolecular forces (attraction between liquid molecules and the solid wall).
Why “Cohesion is greater than adhesion” Is Correct
When cohesion is greater than adhesion, liquid molecules prefer sticking to one another rather than spreading onto the solid glass. This results in an obtuse contact angle (θ > 90°), a convex upward meniscus, and capillary depression (fall of liquid level).
Why the Other Options Are Wrong
- B. Cohesion is equal to adhesion: If cohesion equaled adhesion, the meniscus would be flat with contact angle 90°, producing neither rise nor fall.
- C. Cohesion is lesser than adhesion: When cohesion is less than adhesion (adhesion > cohesion), the liquid wets the tube and experiences capillary rise, as in water on glass.
- D. No relation between cohesion and adhesion.: Capillary behavior is directly determined by the balance between cohesion and adhesion.
Key Point
Non-wetting (capillary depression): Cohesion > Adhesion (θ > 90°). Wetting (capillary rise): Adhesion > Cohesion (θ < 90°).
Key Concepts for Students
- Mercury on glass has a contact angle θ ≈ 128° to 130° (obtuse angle).
- Water on clean glass has θ ≈ 0° (acute angle, complete wetting).
- Capillary depression h is negative because cos θ is negative for θ > 90°.
Practice more: this is question 20 in Properties of Fluid MCQs – Part 2. Try the whole set and check every answer instantly.