Science Paradox: Vapor at 0psia, 72 F

I was thinking about how one can create water vapor, for example, at room temperature by creating a full vacuum (0 psia) environment - such as placing the water in a bowl under a bell jar, and using a vacuum pump to suck out the air. BUT!

Paradoxically, don’t we define pressure by the resultant force of all molecules colliding with the walls of a vessel? So, if I have a vapor at 0psia in a bell jar at room temp (72 F), I would still have molecules colliding with the walls because I am not at absolute zero - where all molecular motion stops. Therefore, do we really have 0psia when T<> absolute zero (273 C)??? And if so, then is our definition of pressure correct?

This question just hit me out of the blue…next time, I’ll duck!

• Jinx, fighting ignorance one question at a time!

You cannot get to absolute 0 pressure keep pumping molecules out and pressure keeps dropping. If you could pump all molecules out, then pressure would be zero.

The temperature gives you the average speed of the molecules. The speed times the number of molecules hitting the wall per unit of area gives you the pressure. No contradiction that I can see.

You cannot get to absolute 0 pressure. Keep pumping molecules out and pressure keeps dropping. If you could pump all molecules out, then pressure would be zero.

The temperature gives you the average speed of the molecules. The speed times the number of molecules hitting the wall per unit of area gives you the pressure. No contradiction that I can see.

This topic was accidentally posted twice, so I’ll close this copy and direct further comment to the other, http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=133222