Which one of the following statements is correct ?
- A.Both boiling and evaporation are surface phenomena.
- B.Boiling is a surface phenomenon, but evaporation is a bulk phenomenon.
- C.Both boiling and evaporation are bulk phenomena.
- D.Boiling is a bulk phenomenon, but evaporation is a surface phenomenon.
Correct answer
Explanation
The correct answer is D, Boiling is a bulk phenomenon, but evaporation is a surface phenomenon. In boiling, bubbles of vapour form throughout the liquid; in evaporation, only molecules at the surface escape.
Evaporation takes place at any temperature below the boiling point, slowly and quietly, and it cools the liquid left behind, which is why sweat cools the body and water stays cool in an earthen pot. It is faster with a larger surface area, higher temperature, lower humidity and more wind. Boiling happens at one fixed temperature for a given pressure, when the vapour pressure of the liquid equals the pressure above it.
A is wrong: boiling takes place in the whole mass of the liquid, not only at its surface.
B is wrong: it reverses the two; evaporation is the surface process.
C is wrong: evaporation is limited to the surface, so it is not a bulk process.
Exam tip: boiling = whole liquid, fixed temperature; evaporation = surface only, any temperature, causes cooling.
Practice Questions
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Show answer
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