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When you hold a can of spray and press on it to let out some spray, you feel the can is getting colder.

How does this happen?

For temperature to decrease you have to take energy out of the system, how does the temperature of a spray can decrease by simply letting some air out?

Many explanations I have heard don't make sense to me, like "molecules are more spaced apart so heat is more spread out". Temperature isn't a measure of energy density right? Temperature is the average kinetic energy of the molecules.

Energy is removed from the system because some particles exit the system, but shouldn't the average kinetic energy remain constant? PV=nRT, less n and less P give us constant V and T.

bubakazouba
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2 Answers2

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When the contents of the can were originally pressurized, they got hot. Then, the heat flowed out through the can walls and the can cooled down to ambient temperature, while still under pressure.

Then, when you release the pressure in the can by pressing on the spray valve, the gas expands and it cools off below ambient. This effect occurs in the expelled products and inside the can, as its pressure decreases. (With the release of pressure, the volatile liquid in the bottom of the can then vaporizes, also absorbing heat from its surroundings.)

This by the way is how open cycle refrigeration (as used in jet aircraft) works.

niels nielsen
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It is very similar to the process of a cryocooler which makes liquid oxygen like a piston compresses the helium in the cavity and then let's a heat sink disapate the heat and the the compressed helium is given space to expand and hence cool air around to such cryogenic temperatures that it changes phase/state.l hope that helps

MIHIR SANGHVI
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