Why the temperature remains constant at the boiling point?

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Discussion Overview

The discussion revolves around the phenomenon of boiling and why the temperature remains constant at the boiling point of liquids. Participants explore the concepts of vaporization, equilibrium, and the formation of vapor bubbles, addressing both theoretical and conceptual aspects of the topic.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why liquids vaporize at the boiling point instead of experiencing a gradual increase in temperature alongside vapor pressure, suggesting a potential link to vapor bubble formation.
  • Another participant proposes that the system is in equilibrium between liquid and gas phases, arguing that any slight temperature increase would lead to more atoms leaving the liquid, thus cooling it back to the boiling point.
  • A different viewpoint asserts that the rate of evaporation increases with temperature, but boiling is distinct because vapor bubbles can form throughout the liquid, unlike evaporation which occurs only at the surface below the boiling point.
  • This participant also challenges the idea of equilibrium, stating that if the liquid is evaporating slowly, it is not in equilibrium, and emphasizes that equilibrium for water occurs at 100% relative humidity.
  • Another participant clarifies that at boiling temperature, there is 100% relative humidity above the liquid, and while heat addition causes a net migration from liquid to gas, this is small compared to the overall equilibrium process.

Areas of Agreement / Disagreement

Participants express differing views on the concepts of equilibrium and the nature of boiling versus evaporation. There is no consensus on the relationship between temperature, vaporization, and equilibrium conditions.

Contextual Notes

Some assumptions regarding the definitions of equilibrium and relative humidity are not fully explored, and the discussion does not resolve the complexities of the boiling process versus evaporation.

Tahmeed
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Why all liquids vaporize on boiling point instead of the temperature gradually increasing along with vapor pressure and more liquid atoms evaporating? Does it have anything to do with formation of vapor bubbles?
 
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To a good approximation, the system is in equilibrium between atoms going from liquid to gas and atoms going from gas to liquid. If the temperature would rise a tiny bit, we would quickly have more atoms leaving the liquid, taking away energy, and cooling it down to the boiling point again. Same with the opposite direction.

Why would you expect a dependency on the amount of liquid present?
 
The rate of evaporation does increase gradually as you raise the temperature. Vaporization is much more rapid at boiling since bubbles can form throughout the liquid, whereas below the boiling point, vaporization only happens at the surface.

To contradict mfb, there's no reason to believe the system is close to equilibrium. If the liquid is slowly evaporating, then it's not in equilibrium. In case of water, there is equilibrium when the relative humidity is 100%, and water no longer evaporates. Anything less than 100% relative humidity is not equilibrium.

This thread may help explain the difference between evaporation and boiling:
https://www.physicsforums.com/threads/does-pressure-in-phase-diagram-refer-to-vapor-pressure.876217/
 
You have 100% relative humidity (actually, 100% water vapor) above the surface if your liquid is at boiling temperature. If you keep adding heat there is a net migration from liquid to gas, but that is tiny compared to the overall equilibrium reaction.
 
Oh I see you were answering the question in the title. I didn't even notice that question, so I was just answering the question in the body.
 

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