Could you help me with vapour pressure & boiling point?

Question

Suppose a container with a fluid is kept somewhere on the earth. Now, due to the gravity of the earth, the atmosphere will exert a force on the vapour just above the fluid, which is its weight (of magnitude F, say). Then, according to Newton's Third Law of Motion, the vapour just above the fluid will exert an equal & opposite force on the atmosphere (of magnitude F). Since the areas on which the two forces are acting is the vapour of the fluid just above the fluid surface, the pressure exerted by both the forces is the same, 'coz pressure=force/area.

According to definition, boiling point of a fluid is the temperature at which vapour pressure = atmospheric pressure. But we have found (in the above statement) that vapour pressure = atmospheric pressure at all conditions.

What this logic also implies is that any fluid will boil at any condition of pressure & temperature,even at absolute 0, and in gravity-less space.

Where have I gone wrong with my reasoning? Please specify.

Answer 1

You don't have in mind that vapour has no surface. The vapour is molecules that left the fluid so the pressure of the atmosphere is all around them and not at a surface around the total vapour but in every single molecule.

Answer 2

Your reasoning is OK, but the boiling point is when the vapor pressure of the liquid exceeds the external pressure.

There is a full discussion of it here:

http://en.wikipedia.org/wiki/Boiling_point