Concept:Capillary action is the upward movement of a liquid through a narrow space against gravity, governed by the interplay between adhesive and cohesive forces.
Formula:$$ h = \frac{2 \gamma \cos \theta}{\rho g r} $$
Solution:- Cohesive forces are the intermolecular attractions between identical molecules (e.g., water clinging to water via hydrogen bonds).
- Adhesive forces are the attractions between different substances (e.g., polar water molecules clinging to the polar silicates of a glass tube).
- For water to climb up the walls of a glass capillary tube, the adhesive pull from the glass must be physically stronger than the cohesive pull trying to keep the water molecules clustered together.
- Therefore, the adhesive forces must exceed the cohesive forces.
Why other options are incorrect:If cohesive forces were greater, the liquid would exhibit capillary depression (like Mercury), forming a convex meniscus and moving downward, not upward. If they were equal, the liquid surface would remain flat.
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