Dry ice is the solid form of carbon dioxide (chemical formula CO2), comprising two oxygen atoms bonded to a single carbon atom. It is colorless, with a sour zesty odor, non-flammable, and slightly acidic.[1]

 

 

Phase diagram of carbon dioxide

At pressures below 5.13 atm and temperatures below −56.4 °C (−69.5 °F) (the triple point), CO2 changes from a solid to a gas with no intervening liquid form, through a process called sublimation.[note 1] The opposite process is called deposition, where CO2 changes from the gas to solid phase (dry ice). At atmospheric pressure, sublimation/deposition occurs at −78.5 °C (−109.3 °F).

The density of dry ice varies, but usually ranges between about 1.4 and 1.6 g/cm3 (87–100 lb/ft3).[2] The low temperature and direct sublimation to a gas makes dry ice an effective coolant, since it is colder than water ice and leaves no residue as it changes state.[3] Its enthalpy of sublimation is 571 kJ/kg (25.2 kJ/mol).

Dry ice is non-polar, with a dipole moment of zero, so attractive intermolecular van der Waals forces operate.[4] The composition results in low thermal and electrical conductivity.[5]