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I am using some Dust Off brand cans to clean out a computer after an accident with concrete dust... long story.

Inside one of these cans is a flurocarbon which quickly turns into a gas at room temperature. It is not ordinary air as that would be too difficult or expensive to store compressed.

Anyway, I noticed that when orientated with the spray head upwards, the stuff which exited the nozzle was mostly vaporised immediately with a small amount of liquid residue. However, when orientated downwards some kind of liquid which quickly froze exited the nozzle. In fact such ice is still frozen on my computer case and is slowly melting.

What causes the two distinct states? And why does the frozen stuff that sprays out upside down take ages to melt, but the frozen stuff which exits when orientated upwards melts, and subsequently evaporates very quickly?

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Compressed air? Or fluorocarbon? ""ordinary air as that would be too difficult or expensive to store compressed. "" Aha... –  Georg May 27 '11 at 8:34
    
@Georg it's called compressed air but it's not actually 78% nitrogen, 21% oxygen etc. –  Thomas O May 27 '11 at 13:17
    
Either its air or it is a fluorocarbon, tertium non datur: en.wikipedia.org/wiki/Dust-Off –  Georg May 27 '11 at 13:23
    
@Georg Yes, it's a flurocarbon. Or similar. Don't get hung up on my terminology, I'm only an entry-level physics student. –  Thomas O May 27 '11 at 13:29

1 Answer 1

The fluorocarbon in the can is a liquid under pressure. When you spray the can upright, only the fluorocarbon vapor at the top of the can is released. When you turn the can upside-down, the liquid is forced out instead.

As you have noted, this liquid quickly evaporates because is is a gas at room temperature. In other words, it begins to boil. This carries heat away from the liquid very rapidly, cooling it to its boiling point.

This also cools whatever you are spraying the liquid on, thus condensing water out of the air and freezing it.

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""in the can is a liquid under pressure."" That is true, but not a sufficient reason. The liquid has to have a vapor pressure greater than one atmosphere, ie have a boiling point below ambient. –  Georg May 27 '11 at 9:48
    
@George: "The liquid has to have a vapor pressure greater than one atmosphere, ie have a boiling point below ambient." Didn't I say that in my second paragraph? –  Colin K May 27 '11 at 14:28

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