Evaporation of a liquid causes cooling because
Correct answer: D. the fastest moving molecules escape, lowering the average kinetic energy of those left behind
- A. the surface of the liquid contracts
- B. cold air replaces the escaping vapour
- C. the vapour formed is colder than the liquid
- D. the fastest moving molecules escape, lowering the average kinetic energy of those left behind
Explanation
Only molecules with above average kinetic energy can break free from the surface, so the average energy of the remaining liquid falls and the temperature drops with it. This is why sweating cools the body and why water in an earthen pot stays cool. Evaporation happens at all temperatures, unlike boiling, which occurs only at one specific temperature for a given pressure.
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About Evaporation, Boiling Point and Vapour Pressure
Evaporation is the escape of higher-energy molecules from a liquid surface, while vapour pressure is the pressure exerted by vapour in equilibrium with that liquid. The topic distinguishes evaporation from boiling and relates both to temperature, intermolecular forces, surface area, atmospheric pressure and the boiling point.
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