<P> Dry ice sublimates at 194.65 K (− 78.5 ° C; − 109.3 ° F), at Earth atmospheric pressures . This extreme cold makes the solid dangerous to handle without protection due to burns caused by freezing (frostbite). While generally not very toxic, the outgassing from it can cause hypercapnia (abnormally elevated carbon dioxide levels in the blood) due to buildup in confined locations . </P> <P> Dry ice is the solid form of carbon dioxide (CO), a molecule consisting of a single carbon atom bonded to two oxygen atoms . Dry ice is colorless, non-flammable, with a sour zesty odor, and can lower the pH of a solution when dissolved in water, forming carbonic acid (H CO). </P> <P> At pressures below 5.13 atm and temperatures below − 56.4 ° C (− 69.5 ° F) (the triple point), CO changes from a solid to a gas with no intervening liquid form, through a process called sublimation . The opposite process is called deposition, where CO changes from the gas to solid phase (dry ice). At atmospheric pressure, sublimation / deposition occurs at − 78.5 ° C (− 109.3 ° F) or 194.65 K . </P> <P> The density of dry ice varies, but usually ranges between about 1.4 and 1.6 g / cm (87 and 100 lb / cu ft). 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 . Its enthalpy of sublimation is 571 kJ / kg (25.2 kJ / mol). </P>

What changes carbon dioxide gas back into carbon in solid form