PHYSICAL DESCRIPTION Dry ice is frozen carbon dioxide, a standard part of our earth’s atmosphere. It is the gas that we exhale during breathing and the gas that plants use in photosynthesis. It is also the same gas added to water to make soda water. Dry ice is beneficial for freezing and keeping things frozen because of its frigid temperature: -109.3°F or -78.5°C. Dry ice readily freezes items and is widely used because it is easy to handle using insulated gloves. Dry ice changes directly from a solid to a gas – called sublimation – without going through a wet liquid stage.
HOW TO MAKE DRY ICE The first step in making dry ice is to compress carbon dioxide gas until it liquefies, removing excess heat. The C02 gas will liquefy at a pressure of approximately 870 pounds per square inch at room temperature. Next, reduce the pressure of the liquid carbon dioxide by sending it through an expansion valve into an empty chamber. The liquid will flash, turning some into gas, causing the remainder to cool. As the temperature drops to -109.3°F, the temperature of frozen CO2, some will freeze into dry ice snow. This dry ice snow is compressed under a large press to form blocks or extruded into pellets. Dry ice is much heavier than traditional ice, weighing double.
DRY ICE MAKERS Dry ice machines are available in all sizes and use liquid CO2. Hand-held ones make soft dry ice that dissipates quickly. Large commercial machines use hydraulic presses to compress the dry ice snow with up to 60 tons of pressure. It can produce a 55-pound block in under 60 seconds.