Vapor pressure and boiling point for solutions in open containers

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.

• – Mithoron Oct 8 '15 at 12:48
• 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). – Curt F. Dec 7 '15 at 15:58

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.

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.