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I have a compressed gas duster with difluoroethane as the gas. When I shake the bottle, I feel sloshing, so I assume that it is a vapor liquid mixture inside.

When I expel the gas, the can gets cold to the touch.

I thought that liquid vapor mixtures have constant temperature during phase transition. Why does the bottle get cold?

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    $\begingroup$ Can you provide a source for "liquid vapour mixtures have constant temperature during phase transition"? Perhaps there is some assumption involved (e.g. closed system, adiabatic etc.) that is not applicable to this case. $\endgroup$
    – AJN
    Feb 23 at 7:52

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Put simply for the liquid in the can to become a gas it needs to absorb heat.

This can be seen often in many applications your deodorant spray if you use a lot, furniture polish cans if you clean big tables etc.

Even the gas used on welding machines can show this. I was welding with a big industrial welder and it started to spatter - the professional welder training me just said coffee time. I said but we have to fix it - he said we will "coffee time". Eventually he said look at the gas bottle and it had a big frost ball covering the pressure regulator. Once that melted and it all warmed up it was fine again.

This effect is also used in the expansion valve in refrigeration systems prior to the evaporator.

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I will expand on Solar Mike's answer.

The physics/chemistry here is based on the Latent heat of Evaporation

As you let you air out of the can, the pressure drops, which causes Boiling Point to decrease. This causes the liquid at the bottom of the can to boil into gas. Boiling consumes heat, which cools the can.

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