Why the temperature remains constant at the boiling point?

[Tahmeed]

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?

 

[mfb]

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?

 

[Khashishi]

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/

 

[mfb]

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.

 

[Khashishi]

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.