carbon dioxide (CO₂)

Carbon dioxide molecule

A chemical compound composed of one carbon and two oxygen atoms joined together by double covalent bonds. Carbon dioxide, a colorless, odorless, noncombustible gas, is a minor component of Earth's atmosphere (0.03 percent by volume) but one that is essential to terrestrial life. It is the major component of the thick atmosphere of Venus and of the thin atmosphere of Mars, and is an important greenhouse gas. Below -78°C it forms a solid, commonly known as dry ice (see below), and is found in this state, together with water ice, in the martian polar caps and in comets.

Carbon dioxide is taken in by plants during photosynthesis and produced by animal respiration. The building of an organism requires a way of making organic molecules from carbon dioxide. This is achieved during the Calvin cycle of photosynthesis in green plants. By means of this cycle, plants incorporate carbon dioxide into sugars which form the basis for all of the organic molecules synthesized by cells. Carbon dioxide is also an important greenhouse gas.

Carbon dioxide is prepared in the laboratory by reacting a carbonate with acid; industrially it is obtained by calcining limestone, burning coke in excess air, or from fermentation.

Liquid carbon dioxide, formed under pressure, is used in fire extinguishers. Carbon dioxide is also used to make carbonated drinks. When dissolved in water an equilibrium is set up, with carbonate, bicarbonate, and hydrogen ions formed, and a low concentration of carbonic acid (H₂CO₃).

density (at 0°C)	1.98 g/dm³
melting point	-56.6°C
boiling point	-78.5°C
molar mass	44.0095 g/mol

Carbon dioxide and global warming

Carbon is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.

Carbon dioxide contributes about 60% of the potential global warming effect of manmade emissions of greenhouse gases. Although living organisms naturally emits this gas, these emissions are offset by the uptake of carbon dioxide by plants during photosynthesis; they therefore tend to have no effect on atmospheric concentrations. The burning of fossil fuels, however, releases carbon dioxide fixed by plants many millions of years ago and thus increases its concentration in the atmosphere.

The global warming potential (GWP) of other greenhouse gases is measured in relation to that of carbon dioxide, which by international scientific convention is assigned a value of 1.

Carbon dioxide equivalent

Carbon dioxide equivalent is the amount of carbon dioxide by weight emitted into the atmosphere that would produce the same estimated radiative forcing as a given weight of another radiatively active gas. Carbon dioxide equivalents are computed by multiplying the weight of the gas being measured (for example, methane) by its estimated global warming potential (which is 21 for methane). "Carbon equivalent units" are defined as carbon dioxide equivalents multiplied by the carbon content of carbon dioxide (i.e., 12/44).

Dry ice

Solid carbon dioxide, or dry ice, is used as a refrigerant for transporting perishables. It is made by compressing carbon dioxide gas to a pressure of about - 7MN/m² at -57°C, when it liquefies; it is then expanded adiabatically to atmospheric pressure and cools, solid carbon dioxide "snow" separating. This is compressed into blocks. Dry ice sublimes at -78.5°C;.

Carbon dioxide as a possible solvent for alien life

It has been suggested that on some worlds carbon dioxide, rather than water, might serve as a suitable solvent for life. Although this may sound unlikely, when carbon dioxide is subjected to pressures of 90 atmospheres or more, such as are found on planets such as Venus or Neptune, it enters a quasi-liquid, supercritical state. In this unfamiliar form, carbon dioxide is as heavy as water but almost as easy to move through as air because molecules of carbon dioxide don't stick together as tightly as the molecules in most liquids. When certain enzymes (biological catalysts) are placed in supercritical carbon dioxide, they work as well as they do in other substances that have been suggested as alternative solvents for life, such as hexane and ether.

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