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When liquid water is at 100°C under 1 atm pressure, any additional heat instead of increasing kinetic energy of water molecules increase the potential energy between water molecules, leading to phase transition.

However, at the same 1 atm pressure but with water at 60°C, the same amount of added heat primarily increases the kinetic energy, raising the temperature.

How does the added heat "know" when to increase potential energy versus kinetic energy? What's the fundamental mechanism that dictates this behavior?

Tobias Fünke
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THE SAFE
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1 Answers1

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The molecules do not know if they should increase potential energy or kinetic energy, in fact the kinetic energy contributes to the heat when the particles are moving in a chaotic way with zero mean momentum.

Evaporation at 1 atm can happen at temperatures lower than 100ºC but when the boiling temperature is achieved, the evaporation process happens at such a high rate that any additional energy gained by the liquid will be transferred to water particles leaving the liquid, so the liquid water will remain at 100ºC.

If the pressure is lower than 1 atm, the gas particles are less abundant outside the liquid, therefore it becomes easier for the liquid water to pass to the atmosphere (to evaporate) and therefore the boiling temperature is reached faster. The opposite would happen for pressures higher than 1 atm.

Sergio Prats
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