[jtroutma]

Divided: If I understand what you are talking about correctly... (compressed air canisters?) then that is not entirely accurate.

Those canisters that you pick up at a local Computer shop or Staples are actually filled with CO2 in a liquid form. CO2 by nature wants to be a gas at say room temperature. So when you use them, you are releasing the CO2 that is a gas in the canister to the outside and "boiling" off new CO2 in the canister at the same time. Hence the reason why if you turn the canister upside-down and use it, it shoots out more of a liquid than air. CO2 will evaporate very quickly without pressure or in a warm environment (like above -60C, if I am not mistaken). When you "boil" off the CO2 it is taking the heat from the can with it; AKA the can gets very cold! Simmiar to a phase exchange system where you take a liquid and let it "boil" in a evaporator to produce the chilling effect.

There may also be some other gases or chemicals in the canister to aid in the "boiling" effect but not much, say 5% of the whole container.

Hope that helps.