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Author Topic: How effective is lowering ambient pressure on the evaporation rate of liquid?  (Read 2968 times)

Atomic-S

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How effective is lowering ambient pressure on the evaporation rate of liquid?
« on: 11/07/2008 01:59:44 »
Suppose that a pan of water is placed inside a very large chamber containing dry air at room temperature and pressure. Then in another very large identical chamber at the same temperature, another like pan of water is placed; however in the second chamger, the pressure is lower than atmospheric, but not so low as to induce boiling in the water. Question:  will the water in one pan evaporate at a significantly faster rate than the other?

cconley

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How effective is lowering ambient pressure on the evaporation rate of liquid?
« Reply #1 on: 11/07/2008 22:10:09 »
First, if you put "dry air" over a pan of water, it won't be dry for very long.

To the point of your question, as you lower the pressure, the water will evaporate faster and faster.  If you do this in a bell jar, you can see the water re-condensing on the walls of the jar during the process.  Simply put, there is less force holding the water molecules together, so it becomes easier for them to move away from each other and become a gas.

Eventually you'll get to a pressure of about 20 torr (assuming room temp) and all the water molecule will be able to separate and the water will begin to boil.

-Chris: CT,USA

Atomic-S

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How effective is lowering ambient pressure on the evaporation rate of liquid?
« Reply #2 on: 12/07/2008 07:38:38 »
Interesting. Now let us suppose that the experiment is altered so that in each chamber a fan blows the air across the face of the water at considerable speed. The temperatures are the same again but the pressures are not. Will the evaporation rates be equal or not? (The answer to this question is related to the concept that the reason why the water evaporates more slowly in higher pressure air is not that there is "more force" on the surface but because the higher density of air causes escaping water molecules to accumumate in the vicinity of the surface, thereby in many cases returning thereunto, retarding evaporation.)

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How effective is lowering ambient pressure on the evaporation rate of liquid?
« Reply #2 on: 12/07/2008 07:38:38 »