<P> The redox potential for this reaction near pH 7 is about − 0.53 V versus the standard hydrogen electrode . The nickel - containing enzyme carbon monoxide dehydrogenase catalyses this process . </P> <P> Carbon dioxide is colorless . At low concentrations the gas is odorless however, at sufficiently - high concentrations, it has a sharp, acidic odor . At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg / m, about 1.67 times that of air . </P> <P> Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below − 78.5 ° C (− 109.3 ° F; 194.7 K) and the solid sublimes directly to a gas above − 78.5 ° C. In its solid state, carbon dioxide is commonly called dry ice . </P> <P> Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 5.1 bar (517 kPa) at 217 K (see phase diagram). The critical point is 7.38 MPa at 31.1 ° C. Another form of solid carbon dioxide observed at high pressure is an amorphous glass - like solid . This form of glass, called carbonia, is produced by supercooling heated CO at extreme pressure (40--48 GPa or about 400,000 atmospheres) in a diamond anvil . This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium dioxide . Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released . </P>

What is the freezing point of carbon dioxide