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Question 1: If we have an open container which has water in it and is heated. If there is no lid over the container then how can this water ever boil as there will be no vapor pressure created over the water surface.

Question 2: What is the logic in the fact that boiling starts when vapor pressure becomes equal to atmospheric pressure.

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    $\begingroup$ related chemistry.stackexchange.com/questions/23566/… $\endgroup$
    – Mithoron
    Commented Oct 8, 2015 at 12:48
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    $\begingroup$ as there will be no vapor pressure created over the water surface. This part is not really true. There will be vapor pressure created over the water's surface. It might not ever reach atomspheric pressure, but it will rise as the temperature of the pot of water is raised. Exactly how will depend on a complex balance of convective and conductive fluid transport mechanisms (e.g. is there a fan blowing across the surface of the water or not). $\endgroup$
    – Curt F.
    Commented Dec 7, 2015 at 15:58

2 Answers 2

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Boiling is not about the upper surface at all. What happens there is but of little interest to us. Boiling means that bubbles are forming at the bottom and probably within the bulk of the liquid. Guess what is inside these bubbles? It is pure water vapor. That's where the equilibrium between vapor and water actually comes into play.

As for the second question, the bubbles must withstand the external pressure, otherwise they would not be able to form in the first place (or if they will, they will be instantly squeezed into nothing). That's why they actually appear only when vapor pressure becomes equal to atmospheric pressure.

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    $\begingroup$ Vapour pressure itself is defined as the pressure that the vapour of a substance apply on the liquid phase of the substance. And you are saying that vapour pressure help those bubbles to come to the surface. How ? Doesn't it sound like a contradiction. $\endgroup$
    – Tushar
    Commented Jul 13, 2021 at 6:24
  • $\begingroup$ @Ivan I think OP’s confusion is how to relate Q. 1 and Q. 2. Equilibrium vapour pressure is defined in a closed container whereas for boiling, the vapour pressure of bubble of vapour formed (the pressures exerted on surrounding liquid) must be more than the ambient pressure. This is pretty confusing! $\endgroup$
    – Apurvium
    Commented Apr 7 at 3:28
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Question 1: If we have an open container which has water in it and is heated. If there is no lid over the container then how can this water ever boil as there will be no vapor pressure created over the water surface.

It's the other way around: in an open container, boiling will not cease provided you (1) continue to heat the solution so as to keep it at the boiling temperature and (2) insufficient liquid boils to allow equilibration of the vapor with the liquid. In the end there is no liquid left, precisely because the vapor never achieves equilibrium with the liquid, with the pressure of the vapor remaining throughout below the saturation vapor pressure.

The second question is nicely addressed in another answer.

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