Carbon dioxide (chemical formula CO2) is a natural chemical composed of two oxygen atoms. It is a gas at standard temperature and pressure and exists in Earth’s atmosphere in this state, as a trace gas at a concentration of 0.039 per cent by volume.
As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate energy for themselves and oxygen as a waste product. But in darkness photosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced. Carbon dioxide is also produced by combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. In addition it is emitted from volcanoes, hot springs, geysers and other places where the earth’s crust is thin; and is freed from carbonate rocks by dissolution. CO2 is also found in lakes, at depth under the sea, and commingled with oil and gas deposits.
The environmental effects of carbon dioxide are of significant interest. In the earth’s atmosphere, it acts as a greenhousegas which is believed to play a major role in global warming and anthropogenic climate change. Also a major source of oceanacidification is CO2 which dissolves in water forming carbonic acid, which is a weak acid, because CO2 molecule ionization in water is incomplete.
CO2 + H2O H2CO3
Structure and bonding
The carbon dioxide molecule is linear and centrosymmetric. The two C-O bonds are equivalent and are short (116.3 pm), consistent with double bonding. Since it is centrosymmetric, the molecule has no electrical dipole. Consistent with this fact, only two vibrational bands are observed in the IR spectrum – an antisymmetic stretching mode at 2349 cm−1 and a bending mode near 666 cm−1. There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the Raman spectrum. Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor.
At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.5 times that of air.
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 dryice.
Liquid carbon dioxide forms only at pressures above 5.1 atm; the triplepoint of carbon dioxide is about 518 kPa at −56.6 °C. 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.
Carbon dioxide was considered a totally different substance from air. In the 17th century, chimistul Jan Baptist van Helmont observed during his experiment that when burning charcoal in a closed vessel, the mass of the resulting ash is much less than that of the original charcoal. He also gave a self interpretation of what happened, that the rest of the charcoal had been transmuted into an invisible substance he termed a “gas” or “wild spirit”.
In the 1750s, the Scottish psysician Joseph Black, thought about studying the properties of carbon dioxide in detail. He found this way that limestone (calcium carbonate) could be heated or treated with acids to produce a gas he called “fixed air.” Black also found that when bubbled through an aqueous solution of lime, it would precipitate calcium carbonate. He used this phenomen to show people that carbon dioxide is produced by animal respiration and microbial fermentation.
In 1772, English chemist Joseph Priestley published a paper in which he described a process of dripping sulfuric acid on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas. This was the invention of Soda water.
Humphry Davy and Michael Faraday have first liquefied carbon dioxide in 1823. Charles Thilorier gave the first description of solid carbon dioxide
Several methods are used in order to produce industrial carbon dioxide. In its main route, carbon dioxide is produced as a side product of the industrial production of ammonia and hydrogen. These processes begin with the reaction of water and natural gas.
Carbon dioxide usually results from combustion of fossil fuels and wood (can be compared to fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages). It can also result from thermal decomposition of limestone, CaCO3, in the manufacture of lime (Calcium oxide, CaO).
Carbon dioxide is used by the food industry, the oil industry and the chemical industry.