h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The process for cooling air is a phisical, not chemical, process, though it does use chemicals. Having worked in the HVAC field for a while several years ago, I was corrected on this myself. The whole process works on pressure, or more accurately, on friction. I'm not going to go into a long technical description of the phisics involved with air-conditioning, but will try to provide an overview.

Low pressure gas/liquid in a copper (it's always copper, the aluminum fins only help) tube will lower the temperature of the air passing over it, while a high pressure gas in another part of the copper tube (at these pressures it's usually gas) will add temperature to air passing over it. The whole trick is to regulate which point of the operation these sections of copper tubing are at. At it's core, it is simply transfering the friction of the molecules that make up air that makes A/C work.

See, I told you I'd keep it short, and I never started on the whole 'there is no such thing as cold' arguement.