Dry ice

Dry ice at normal pressures does not melt into liquid carbon dioxide but rather sublimates directly into carbon dioxide gas. Hence it is called "dry ice" as opposed to normal "wet (water) ice".

It is used for:

• cooling
• producing "dry ice fog" for special effects: when dry ice is put into contact with water, the frozen carbon dioxide sublimates into a mixture of cold carbon dioxide gas and cold humid air. This causes condensation and a fog.
• cleaning: shooting tiny dry ice pellets at a surface cools the dirt and causes it to pop off. This is analogous to sandblasting with the advantage that the abrasive agent in this case, dry ice, sublimates to nothing.
• increasing precipitation from existing clouds or decreasing cloud thickness by cloud seeding
• Producing carbon dioxide gas where needed in such odd systems as the fuel tank inerting system in the B-47 aircraft

Dry ice is produced by compressing carbon dioxide gas to a liquid form, removing excess heat, and then letting the liquid carbon dioxide expand quickly. This expansion causes a drop in temperature so that some of the CO2 freezes to "snow" which is then compressed.