Why does a can of compressed air get so cold?

Question

I assume it may be nitrogen or something, but if someone could please explain it to me, that would be much appreciated.

Thanks

Answer 1

When the air is compressed it gives off heat.  Then when the pressure is reduced it is cold and wants to absorb heat.

Answer 2

Compressing a substance does not cause it's temperature to drop, actually the opposite is true.  Compression actually causes temperature to rise.

I believe what you are observing is that when compressed air is released, it feels cold because it is being "uncompressed."  As the pressure decreases, so does the temperature.  

If you are interested in the physics of this process, check out "Boyle's Law" and the "Ideal Gas Law"  a quick overview of these physical laws can be found on Wikipedia.  For more details check out the answers in the Physics category.

Also, compressed molecules may change state.  Liquids for example take up less space than gas molecules.  This change of state requires energy and so the temperature drops and energy is consumed.

Hopefully that makes some sense!

Answer 3

The compressed "air" is actually a fluorinated hydrocarbon (usually tetrafluoroethane, if memory serves).  This can be compressed to a liquid at room temperature, and so it exists as a liquid inside of the can (shake the can to hear it slosh around).  When the pressure is relieved, it boils rapidly, consuming energy, and as other answerers have stated, cools further upon expanding as a gas.

Answer 4

Temperature is felt as the total Kinetic Energy of the molecules of air in this case.  Kinetic Energy is the motion of the air particles.  When you compress them they have less room to move, less Kinetic Energy and so they feel cold.