Pressure and humidity

Does pressure/vacuum affect water vapor differently than other gases and hence humidity?

For example, imagine I have a vacuum bell at atmospheric (mean sea level) pressure and the air in it is 50% humidity. If I apply a vacuum to the bell and reduce the pressure to, say, 300mm of Hg (Torr), will the humidity of the air inside the bell change or remain the same?

If it changes, that would seem to suggest that for some reason the water molecules are preferentially being sucked out by the vacuum pump. Why would that be?

• Possible duplicate of Will a cloud form in a depressurized room. – David Hammen Jun 24 '15 at 8:51
• @DavidHammen The question has nothing to do with condensation or droplet formation. – Ambrose Swasey Jun 24 '15 at 10:50
• The two questions are very closely related, and both hinge on what I think is a misconception on your part of what "relative humidity" represents. – David Hammen Jun 24 '15 at 12:14
• @DavidHammen Since I never used the term relative humidity, which you quoted, as though I had used it, maybe it's YOU who have the misconceptions. As should be obvious from my question, by humidity, I mean the proportion of water molecules to all the gaseous molecules in the bell. – Ambrose Swasey Jun 24 '15 at 13:58
• The absolute humidity -- the ratio of water molecules to air molecules -- will remain constant unless you have a very special pump indeed. – tfb May 17 '16 at 6:13

Will the humidity of the air inside the bell change or remain the same?

Assuming there is no change in temperature, the relative humidity will decrease.

If it changes, that would seem to suggest that for some reason the water molecules are preferentially being sucked out by the vacuum pump. Why would that be?

You may have a misunderstanding of what relative humidity means. From your question, it appears you are thinking relative humidity is the ratio of water molecules to other molecules. That is not what relative humidity is. Relative humidity is the ratio of the partial pressure of water vapor present in air to the saturation partial pressure of water vapor at the same temperature, expressed as a percentage. Removing water vapor while keeping the temperature the same reduces relative humidity.

• The vacuum pump does not just remove water molecules, it removes air molecules also. – Ambrose Swasey Jun 23 '15 at 14:00
• "Assuming there is no change in temperature." I described the experiment. You tell me: will the temperature change? There is no need to assume anything. What you can assume is that I hooked up a vacuum pump turned it on until it read 300mm, then turned it off. – Ambrose Swasey Jun 23 '15 at 14:03
• @TylerDurden - There will be a slight temperature drop due to the Joule-Thomson effect. This will be negligible compared to the large drop in pressure. The assumption is valid. – David Hammen Jun 23 '15 at 14:53

Looking at natural aspects of the world we can see first that a lack of pressure in no way means water won't and some point coalesce amongst themselves given the ability (eg clouds). So does; by creating a lack of pressure do we also decrease the relative amount of water molecules? NO, but pulling out the molecules via vacuum pump will decrease the overall molecules in the space. However this is usurped by the vessel and the outside environment. If the relative humidity outside a permeable vessel remains higher coupled with a vacuums relative temperature being slightly lower than the outside environment water will permeate through. By achieving a perfect vacuum are you removing all molecules from within the free space, and thus all the humidity, unless the bell is holding water within it and is able to release them once the other molecules have left.