If you’ve ever used a can of compressed
air (also called a gas duster), to, say, clean crumbs out of your computer keyboard , you’re
probably aware that after a little while, the air coming out of the can and even the
can itself get really really cold. Like, cold enough they put frostbite warnings
on the can! And for good reason! It’s tempting to think that compressed air
cans get cold because when the gas comes out of the can it expands and thus cools off. But that’s not exactly right - whether an
expanding gas gets hotter or colder (and how much hotter or colder it gets) depends on
the exact manner in which the gas expands. And if we apply the relevant equation for
“normal” gas expansion , we predict that the gas inside the compressed air can should
drop from room temperature to around 100 degrees celsius below zero , which is, um, WAY colder
than what comes out of a compressed air can. So the gas can’t be expanding in the normal
way gases expand. . And here’s why: that would be like cutting
the top off the can and letting the gas expand freely in all directions. But the gas is actually being squeezed out
through a tiny valve. This difference is key; the gas passing through
a valve isn’t simply expanding - it’s also being pushed through by the rest of the
gas behind it! And that compression from behind gives the
gas enough heat energy to essentially counteract the cooling from expansion. In terms of the gas law, this means the volume
goes up by the same factor that the pressure goes down, so pressure times volume is pretty
much constant and the temperature stays about constant. But not exactly - most gases at room temperature
do get slightly colder when passing through a valve . A good demo of this is to let the
air out of a bike tire; the valve gets colder , but not crazy cold. Similarly, the gas leaving a can of compressed
air cools a little bit passing through the nozzle . But this can’t be the only contributor
to the cooling. I mean, the can itself cools off by significantly
more than can be explained by expansion through a valve , and it’s not like it’s even
being sprayed by the air coming out. No, the real cooling power is hinted at by
the warning labels on cans of compressed air telling you to not to shake them or spray
them upside down - if you DO shake one, you’ll realize right away that it’s not just gas
inside - there’s liquid in there, too! Liquid like 1,1-difluoroethane, which is a
gas at normal temperatures and pressures, but a liquid once you pressurize it to around
6 times atmospheric pressure. And it’s the essential component of these
compressed air cans. Inside the can, 1,1-difluoroethane exists
as both a liquid and a gas, in equilibrium - just enough of the liquid boils off to maintain
six atmospheres of pressure in the top of the can, a pressure high enough that rest
stays liquid. Because it’s at six times atmospheric pressure,
when you open the valve the difluoroethane rushes out in a steady stream. But this then means that the inside of the
can is no longer pressurized enough to keep the liquid from boiling - so more of it boils
off until the gas reaches six atmospheres of pressure again and a new equilibrium is
reached with slightly less liquid in the can. This is how the can is able to keep blowing
a stream of consistent strength even when mostly empty. But more importantly to our temperature conundrum,
changes from liquid phase to gas phase require a TON of energy, and that energy has to come
from somewhere. Just like how the evaporation of sweat removes
energy from your skin, cooling you off, inside a can of compressed air, vaporization - aka
boiling - is what steals energy from the liquid and cools it off. Significantly! Spraying out 10% of the contents will cool
the entire remainder of the can by around 20 degrees celsius! If it seems counterintuitive that a boiling
substance cools itself off, look no further than the humble pressure cooker . Water normally
boils above 100 degrees celsius, but by sealing in steam, the pressure rises, enabling the
water in the pot to remain a liquid well beyond water’s normal boiling point - just like
the difluoroethane in a can of compressed air. And releasing water vapor out of the nozzle
of a pressure cooker lowers the pressure inside, allowing a bit more water to boil off as steam
and lowering the temperature of the remaining water - just like the difluoroethane in a
compressed air can. And if you keep letting off steam, eventually
the water will cool all the way back down to its regular boiling point of 100 degrees
, just like how if you keep spraying a can of compressed air, the difluoroethane inside
will cool all the way back down to its regular boiling point of negative 25 degrees . A can of compressed air is quite literally
a 1,1-difluoroethane pressure cooker. And just like you shouldn’t shake a pressure
cooker or turn it upside down (unless you want to spray superheated water everywhere),
cans of compressed air don’t work very well sideways or upside down: instead of spraying
out gas, you’ll spray out the liquid that was only being kept liquified by the high
pressure inside the can , so it immediately vaporizes and drastically cools down whatever
it’s contacting . INSTANT ICE! (though difluoroethane can dissolve in water and is poisonous, so
definitely don’t use this ice for anything food-related). In conclusion, the cause for the coldness
of cans of compressed air can be clarified by comprehending the consequent clue: they
aren’t actually cans of compressed air. They’re cans of pressure-liquified 1,1-difluoroethane,
and lowering the pressure inside by spraying them allows more liquid to boil off, cooling
what remains. I love learning about the physics of regular
stuff; I mean, black holes and quantum mechanics are cool, too, but they’re not quite as
tangible or relatable as the things we interact with on a regular basis. And if you, too, want to dive deeper into
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