NDA (II) 2025 General Ability Test · Question 66 of 147

Correct — B, surface tension decreases when temperature increases. Surface tension arises from the net inward cohesive pull on the molecules sitting at a liquid's surface. Heating the liquid gives its molecules more kinetic energy, which weakens these cohesive bonds and lets surface molecules escape more easily, so the surface tension falls. It drops to zero at the critical temperature, where the liquid-vapour boundary disappears altogether.

• (a)It increases when temperature increases — This is the reverse of the truth. Adding heat increases molecular motion and weakens cohesion, so surface tension goes down, not up. This is the common misconception the question is testing.
• (c)It will remain same with increase or decrease of temperature — Surface tension is not temperature-independent — it varies measurably and almost linearly with temperature, falling as the liquid warms.
• (d)It will always decrease with increase or decrease of temperature — Self-contradictory: a quantity cannot decrease when temperature both rises and falls. Cooling a liquid actually raises its surface tension.

Surface tension is the cohesive force per unit length acting along a liquid's surface, which makes the surface behave like a stretched elastic film. It weakens as temperature rises because thermal agitation reduces the net cohesive attraction between molecules, and it falls to zero at the liquid's critical temperature.

Option (a) matches a loose intuition that heating strengthens things, but the opposite is true. In fact hot water cleans better partly because its lower surface tension lets it spread and wet surfaces more readily. The rule to remember is simple: warmer liquid means weaker surface tension.

• Surface tension is caused by the unbalanced inward cohesive force on the molecules at a liquid's surface.
• It decreases roughly linearly as temperature rises and becomes zero at the critical temperature.
• For water it falls from about 0.076 N/m near 0 degrees Celsius to about 0.059 N/m at 100 degrees Celsius.
• Adding a detergent (a surfactant) also lowers surface tension, independently of temperature.

• Assuming that heating a liquid increases its surface tension (it decreases it).
• Confusing surface tension with viscosity — both fall with temperature for a liquid, but they are different properties.

Asked as the temperature dependence of surface tension, or through examples such as water striders staying afloat and detergents lowering the tension.

• practice — not a real PYQ

  Adding detergent to water changes its surface tension by

  • (a)increasing it
  • (b)decreasing it, so the water spreads and wets surfaces better
  • (c)keeping it unchanged
  • (d)first increasing then decreasing it

  Answer(b) decreasing it — surfactants lower surface tension, letting water wet surfaces more effectively.

• practice — not a real PYQ

  As a liquid is heated towards its critical temperature, its surface tension

  • (a)increases steadily
  • (b)stays constant
  • (c)approaches zero
  • (d)becomes infinite

  Answer(c) approaches zero — at the critical temperature the liquid-vapour boundary vanishes and surface tension is zero.