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