Asked by kevin
Explain why the temperature of boiling water remains constant as the water evaporates.
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Boiling is a phase change where liquid becomes vapor. Temperature measures the average kinetic energy of molecules. During boiling, added heat is not used to increase molecules’ kinetic energy (so the average kinetic energy — the temperature — does not rise); instead the heat supplies the latent heat of vaporization, the energy required to overcome intermolecular attractions and convert liquid into gas.
Two linked points make the temperature stay constant while evaporation proceeds:
- At the boiling point the liquid’s vapor pressure equals the external pressure, so vapor forms throughout the liquid and molecules escape rapidly.
- Every joule of heat supplied goes into breaking intermolecular bonds (increasing the molecules’ potential energy) to produce vapor (the latent heat), not into raising kinetic energy. Therefore the temperature stays at the boiling point until all the liquid has vaporized.
(If pressure changes, the boiling temperature changes accordingly; after all the liquid is gone, further heating raises the vapor temperature.)
Two linked points make the temperature stay constant while evaporation proceeds:
- At the boiling point the liquid’s vapor pressure equals the external pressure, so vapor forms throughout the liquid and molecules escape rapidly.
- Every joule of heat supplied goes into breaking intermolecular bonds (increasing the molecules’ potential energy) to produce vapor (the latent heat), not into raising kinetic energy. Therefore the temperature stays at the boiling point until all the liquid has vaporized.
(If pressure changes, the boiling temperature changes accordingly; after all the liquid is gone, further heating raises the vapor temperature.)
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