If heat is to be supplied continuously to a liquid at its boiling point, the temperature remains constant because:
Correct answer: B. High energy molecules continuously leaving the surface of liquid
- A. Extra heat is spread in the air
- B. High energy molecules continuously leaving the surface of liquid
- C. The forces of attractions become stronger
- D. The vapour pressure of the boiling liquid becomes greater than the external pressure
Explanation
At the boiling point, supplied heat is used as latent heat to overcome intermolecular attractions and convert liquid into vapour, not to increase the average kinetic energy of the molecules. Therefore the temperature remains constant while high-energy molecules continuously escape from the liquid. The likely mistake is choosing d, but boiling begins when vapour pressure equals external pressure, not when it becomes greater because of the supplied heat.
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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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