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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.

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    $\begingroup$ The gas that remains in the can at any time has done work to expand, and force the gas ahead of it out of the can. So this is equivalent to doing work against a piston, and this results in a decrease in its internal energy of the gas remaining in the can. $\endgroup$
    – Chet Miller
    Commented Sep 15, 2020 at 12:16

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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

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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.

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  • $\begingroup$ I think the OP is asking about the can, not the expelled products. $\endgroup$
    – Winston
    Commented Sep 15, 2020 at 7:20
  • $\begingroup$ niels, most refrigeration cycles condense their refrigerant, then take a large pressure drop across an expansion valve to cause that liquid to boil at a low temperature. Your answer can be interpreted to mean that there is only high pressure vapor in a pressurized spray can, which may or may not be true, and it's definitely not true for most refrigeration cycles. $\endgroup$
    – David White
    Commented Sep 15, 2020 at 19:08
  • $\begingroup$ will edit. -niels $\endgroup$
    – niels nielsen
    Commented Sep 15, 2020 at 21:20

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