Surface tension makes the surface of a liquid behave like a stretched elastic sheet. What produces it?
ACohesive forces pulling surface molecules inward
BAdhesive forces pulling surface molecules outward
CGravity pressing down on the whole surface
DAir pressure squeezing the liquid surface
Answer & Solution
Correct answer: A. Cohesive forces pulling surface molecules inward
1. A molecule deep inside a liquid has neighbours on all sides, so the attractions on it cancel and it feels zero net force.
2. A molecule at the surface has neighbours only below and beside it, so the cohesive forces pull it inward.
3. That inward pull contracts the surface to the smallest possible area, and the effect is called surface tension.
4. The cause is cohesion between like molecules, not adhesion to something else.
5. Gravity and air pressure act on the liquid too, but neither explains why a surface tries to shrink its own area.
_Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action_
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