NDA (I) 2022 General Ability Test · Question 53 of 150

Correct — A, the temperature is high and the surface area of the liquid is large. Evaporation is a surface phenomenon in which the faster-moving molecules escape from the liquid's surface. Raising the temperature gives more molecules enough energy to escape, and a larger exposed surface area lets more molecules leave in a given time. Both factors speed up evaporation, so high temperature together with a large surface area gives the fastest rate.

• (b)the temperature is low and the surface area of the liquid is large — A large surface area does help, but a low temperature slows evaporation because fewer molecules have enough energy to escape.
• (c)the temperature is low and the surface area of the liquid is small — Both conditions slow evaporation, so this gives the slowest rate — the opposite of what is asked.
• (d)the temperature is high and the surface area of the liquid is small — High temperature helps, but a small surface area limits how many molecules can escape, so the rate is slower than with a large surface area.

Evaporation is the change of a liquid to vapour at its surface, and it occurs at all temperatures (unlike boiling). Its rate rises with higher temperature, larger surface area, lower humidity and faster air movement (wind). Because the most energetic molecules leave, evaporation cools the remaining liquid.

Each option pairs two conditions. Pick the pair that both make it easier for surface molecules to escape — a high temperature (more energetic molecules) and a large surface area (more molecules exposed).

• Evaporation happens only at the surface and at any temperature, unlike boiling which occurs throughout at the boiling point.
• It speeds up with higher temperature, larger surface area, lower humidity and faster wind.
• Evaporation is a cooling process because it removes the fastest-moving molecules.
• Wet clothes dry faster when spread out (large area) on a hot, windy day.

• Confusing evaporation (surface, any temperature) with boiling (throughout, at the boiling point).
• Forgetting that low humidity and faster wind also speed up evaporation.

Pairs two variables per option; choose the pair that both increase the escape of surface molecules — high temperature and large surface area.

No directly related past PYQ was found.

• practice — not a real PYQ

  Evaporation of a liquid generally causes

  • (a)heating of the surroundings
  • (b)cooling of the surroundings
  • (c)no change in temperature
  • (d)an increase in the liquid's temperature

  Answer(b) cooling of the surroundings — the fastest molecules leave, lowering the average energy.

• practice — not a real PYQ

  Which condition makes wet clothes dry the fastest?

  • (a)humid, still air
  • (b)hot, dry, windy air
  • (c)cold, still air
  • (d)cold, humid air

  Answer(b) hot, dry, windy air — high temperature, low humidity and wind all speed up evaporation.