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If a room is depressurized will a visible cloud or mist form?

I would expect it would, but when I apply a vacuum to a flask I see no cloud form inside the flask. Would a room behave differently than the flask?

I am more interested in the case of sudden evacuation than a gradual depressurization.

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  • $\begingroup$ Your question is not showing the research effort and details. So I shall vote to close it. $\endgroup$
    – user73555
    Feb 26, 2015 at 13:12
  • $\begingroup$ In a vacuum, there's no fluids. Clouds are made of fluids. Therefore, a cloud cannot form in a vacuum and if it did, it wouldn't be a vacuum $\endgroup$
    – Jim
    Feb 26, 2015 at 16:15
  • $\begingroup$ I use a product with my wine which is a rubber stopper whose structure allows for functioning as a one-way valve. When I use the pump with it and create a vaccum in the bottle (with sufficient vigor) the inside of the wine bottle becomes rather misty. Upon searching for why this occurs consistently, I found myself here, and it seems to be empirical evidence for the phenomenon that you specify, in particular the case where your room contains an ample amount of (mostly) water. $\endgroup$
    – Steven Lu
    Feb 20, 2018 at 5:37

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The air in the room must be sufficiently laden (ideally, saturated) with water vapor (or another substance, such as alcohol, that can be absorbed by the air). Then, if you drop the pressure suddenly, so that the air can expand adiabatically, the temperature can drop below the dew point of the vapor, and the vapor has an opportunity to condense into a cloud. (The condensation is more likely in the presence of a little dust or some other points of nucleation; in perfectly clean, smooth conditions, I believe it's possible for the air to just become supersaturated.)

One way to do this is to hold the saturated air in one chamber while evacuating a neighboring chamber. After the second chamber has been pumped out, quickly open a connection between the chambers by opening a valve or removing a partition. Another way is to have the saturated air in a chamber fitted with a diaphragm, and suddenly to tug on it, expanding the volume of the chamber. That's the approach used by Wilson in his cloud chamber, invented c. 1911 and for which he won the Nobel Prize in 1927; see: http://en.m.wikipedia.org/wiki/Cloud_chamber

(Millikan was originally trying to perfect this technique to measure the elementary charge by studying the surface of a cloud in an electric field, but he, Wilson, and others were frustrated by complicating effects like re-evaporation of the cloud and its unsteady surface. That's when Millikan discovered that with a large enough electric field he could actually disperse the cloud, leaving behind a small number of charged droplets suspended in the field. If I recall correctly, Millikan's technique for dropping the pressure and producing the cloud was to open a valve to a connected low-pressure chamber.)

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  • $\begingroup$ There's a big difference between opening a connection to an evacuated chamber and using a diaphragm to expand the gas. The latter involves work. The former is free expansion (Joule expansion) and involve no work. There will be a slight bit of cooling due to the Joules-Thomson effect, but this will be slight compared to the cooling due to adiabatic expansion. $\endgroup$
    – David Hammen
    Jun 24, 2015 at 8:45
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When you reduce the pressure in a vessel by pumping air out you reduce the temperature of the air. If the temperature falls below the dew point then water will condense out of the air and a mist will form.

The trouble is that the dew point depends on the pressure, and it falls as the pressure reduces. So whether a mist forms or not is dependant of how close you are to the dew point when you start pumping. If the air in your vessel is already close to satured with water a mist is likely to form, but if the air is fairly dry a mist is unlikely to form. It also takes time for a mist to nucleate and the droplets grow to a visible size. If you pump too rapidly it's likely you will remove all the air and water vapour before a visible mist has time to form.

You might want to try putting a small amount of water in your flask and leaving it a few minutes for the water to evaporate into the air in the flask. Then try pumping out the flask slowly.

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  • $\begingroup$ Actually, I think a sudden evacuation would be more likely to produce a cloud than slow pumping. For example, have two chambers separated by a valve. Pump one out, then quickly open the valve. This is the technique conventionally used in cloud chambers. The advantage is that the gas is more likely to cool adiabatically. With slow pumping it is continually being warmed by conduction from the walls. $\endgroup$
    – pwf
    Feb 26, 2015 at 16:33
  • $\begingroup$ @pwf: a sudden evacuation would leave nothing in the flask to condense. However I agree that a sudden partial pressure drop with no further pumping is probably the best way to get a mist. Do you want to post this as an answer? If not I'll edit my answer to include your comment. $\endgroup$
    – John Rennie
    Feb 26, 2015 at 16:39
  • $\begingroup$ I am more interested in the case of sudden evacuation. For example, imagine a hatch blew on the International space station, would a mist or cloud form in the rooms? Maybe it depends how much moisture there is or possibly other factors. $\endgroup$
    – Ambrose Swasey
    Feb 26, 2015 at 16:44
  • $\begingroup$ @TylerDurden: it depends on the rate of pressure drop. It a hatch blows then pretty soon there will be nothing left in the ISS - no atmosphere and no mist. But it takes time for all the air to escape so the pressure drop will be gradual. A mist will form if the rate of pressure drop and the initial temperature and humidity are just right. However I'm not sure what just right means in hard numbers. $\endgroup$
    – John Rennie
    Feb 26, 2015 at 16:47
  • $\begingroup$ @JohnRennie I know that if you emit a moisture-laden gas into space a cloud of tiny crystals instantly appear and just hang there in space next to the capsule. Maybe that is similar. The question would be whether the moisture turns to a liquid/solid before the vapor exits the vessel. $\endgroup$
    – Ambrose Swasey
    Feb 26, 2015 at 16:51
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If you have a syringe, close the outlet with your finger, push the plunger down hard, then let it go. A cloud will form inside the syringe, & will come out like smoke as you push the plunger back down. I discovered this when I was a kid. It didn't make sense to me at the time, why the fog occurred when it was depressurised rather than when pressurised, because I had seen somebody bleeding water out of a compressor tank, and I had heard that water boils at a lower temperature in low pressure & therefor, should evaporate more, and form a fog when pressurised.

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