## Subtitles section Play video

• 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 fornormalgas 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 the physics of everyday objects, look no further

• than Brilliant, this video's sponsor.

• Brilliant has a whole course on the physics of everyday objects, including fridges and

• water towers and bikes - bikes are great! - and Brilliant also has fun, short daily

• challenges and puzzles to learn about stuff like regression to the mean and fluids and

• thermodynamics without the huge time commitment it would take to learn enough about Joule-Thompson

• expansion through a valve to make a whole youtube video about it

• Brilliant continues to be an incredible supporter of MinutePhysics and they're offering 20%

• off of a premium subscription to the first 200 of you who go to brilliant.org/minutephysics,

• which gives you full access to all of Brilliant's courses, puzzles and daily challenges.

• Again, that's brilliant.org/minutephysics for 20% off a premium subscription, and to

• let Brilliant know you came from here.

If you've ever used a can of compressed air (also called a gas duster), to, say, clean

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 compressed gas liquid pressure air valve

# Why Do Compressed Air Cans Get Cold?

• 10 2
Summer posted on 2021/05/19
Video vocabulary