This strange substance-ice so cold it burns- is an excellent refrigerant, of course, and also a useful source for the ghoulish vapors in vampire and mystery movies. Dry ice is actually composed of carbon dioxide, which at normal temperatures sublimes, or becomes a gas.
The carbon dioxide is stored and shipped as a liquid in tanks at pressures approaching 1,073 pounds per square inch. In order to make dry ice, the liquid is withdrawn from a tank and allowed to evaporate at a normal pressure in a porous bag. This rapid evaporation consumes so much heat that part of the liquid CO2 freezes to a temperature of – 109 degrees Fahrenheit. Looking something like snow, the frozen liquid is then compressed by machine into blocks, which can be shipped and sold, and which will, in time, melt away to a gas.
From: How Do They Do That? by Caroline Sutton