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Q & A: pressure above water

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Most recent answer: 09/20/2009
Q:
a column of water in a sealed glass tube of 40 feet will have ~ a 7 foot void at the top (a vacuum if you will) but since water would evaporate due to the lower pressure is it a steam void above the column?
- Russell (age 22)
A:
Yes.

I'm not sure I follow the geometry here, but it sounds like the tube has no air in it, just water. If none of the water evaporated, there would be a vacuum above it. Some water does evaporate, however, until the vapor is in equilibrium with the liquid. At room temperature (22C), that would be at a pressure of roughly 1/40 of an atmosphere. The value depends strongly on temperature. at 100C, it's 1 atm.

Mike W.

(published on 09/20/2009)

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