# Why does spray deodorant come out of the can so cold, even though it is kept at room temperature?

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Why does spray deodorant come out of the can so cold, even though it is kept at room temperature?

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the gas in the can expands while going out of it… with the expansion temperature goes down.

When gas expands it cooles down – this is the reversal of why your bike pump heats up when you pump up the tire of your bike. The gas in the deodorant expands to bring the deodorant.

There is a second effect – the deodorant is wet, which cooles you down just as if you sprayed water on yourself

***Gay-Lussac’s law of pressure–temperature*** describes the relationship between the pressure and temperature of a fixed mass of gas kept at a constant volume.

> *The pressure of a gas of fixed mass and fixed volume is directly proportional to the gas’s absolute temperature.*

> **P** = ***k***•**T**

As the pressure of a gas rises/falls, the temperature does the same.

Decrease the pressure (spray the deodorant) and the temperature decreases.
Increase the pressure (pump up a bike tire) and there’s an increase in temperature.

Worth noting the gas in your canister isn’t necessarily just expanding, it may also be evaporating. The pressure in the bottle holds it in liquid form at room temp, and a reduction of pressure as it exits causes the liquid to vaporise.

This furthers the cooling effect as vaporisation is highly endothermic (absorbs a bunch of heat).

Your AC unit works on the same principle – by releasing compressed liquid into a very cold gas, using it to cool the room then compressing it back into a liquid (the liquid will be hotter than at the start). The compressor is the noisy and power hungry bit in your AC.