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carbon dioxide

The Columbia Encyclopedia, Sixth Edition | 2008 | The Columbia Encyclopedia, Sixth Edition. Copyright 2008 Columbia University Press. (Hide copyright information) Copyright

carbon dioxide chemical compound, CO 2 , a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. It does not burn, and under normal conditions it is stable, inert and nontoxic. It will however support combustion of magnesium to give magnesium oxide and carbon. Although it is not a poison, it can cause death by suffocation if inhaled in large amounts. It is a fairly stable compound but decomposes at very high temperatures into carbon and oxygen. It is fairly soluble in water, one volume of it dissolving in an equal volume of water at room temperature and pressure; the resultant weakly acidic aqueous solution is called carbonic acid . The gas is easily liquefied by compression and cooling. If liquid carbon dioxide is quickly decompressed it rapidly expands and some of it evaporates, removing enough heat so that the rest of it cools into solid carbon dioxide "snow." A standard test for the presence of carbon dioxide is its reaction with limewater (a saturated water solution of calcium hydroxide ) to form a milky-white precipitate of calcium hydroxide.

Carbon dioxide occurs in nature both free and in combination (e.g., in carbonates ). It is part of the atmosphere , making up about 1% of the volume of dry air. Because it is a product of combustion of carbonaceous fuels (e.g., coal, coke, fuel oil, gasoline, and cooking gas), there is usually more of it in city air than in country air. The natural balance of carbon dioxide in the atmosphere is growing from its stable level of 0.13% to a predicted 0.14% by the year 2000. It is anticipated that this extra carbon dioxide will fuel the greenhouse effect, warm the atmosphere, and further disrupt the natural carbon dioxide cycle (see global warming ).

In various parts of the world—notably in Italy, Java, and Yellowstone National Park in the United States—carbon dioxide is formed underground and issues from fissures in the earth. Natural mineral waters such as Vichy water sparkle (effervesce) because excess carbon dioxide that dissolved in them under pressure collects in bubbles and escapes when the pressure is released. The chokedamp (see damp ) of mines, pits, and old, unused wells is largely carbon dioxide. Carbon dioxide is a raw material for photosynthesis in green plants and is a product of animal respiration . It is also a product of the decay of organic matter.

Carbon dioxide has varied commercial uses. Its greatest use as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water. Formed by the action of yeast or baking powder, carbon dioxide causes the rising of bread dough. The compound is also used in water softening, in the manufacture of aspirin and lead paint pigments, and in the Solvay process for the preparation of sodium carbonate. In some fire extinguishers carbon dioxide is expelled through a nozzle and settles on the flame, smothering it. It also has numerous nonchemical uses. It is used as a pressurizing medium and propellant, e.g., in aerosol cans of food, in fire extinguishers, in target pistols, and for inflating life rafts. Because it is relatively inert, it is used to provide a nonreactive atmosphere, e.g., for packaging foods, such as coffee, that can be spoiled by oxidation during storage. Solid carbon dioxide, known as dry ice, is used as a refrigerating agent.

There are three principal commercial sources for carbon dioxide. High-purity carbon dioxide is produced from some wells. The gas is obtained as a byproduct of chemical manufacture, as in the fermentation of grain to make alcohol and the burning of limestone to make lime. It is also manufactured directly by burning carbonaceous fuels. For commercial use it is available as a liquid under high pressure in steel cylinders, as a low-temperature liquid at lower pressures, and as the solid dry ice.

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carbon dioxide

A Dictionary of Ecology | 2004 | | © A Dictionary of Ecology 2004, originally published by Oxford University Press 2004. (Hide copyright information) Copyright

carbon dioxide(CO2) The product of the complete oxidation of carbon and the compound most involved in the transport of carbon through the carbon cycle. Carbon dioxide is utilized by autotrophs in the process of photosynthesis. When organic matter decomposes its carbon is oxidized to CO2 and released into the atmosphere. Carbon that enters long-term ‘storage’, as carbonate rocks (e.g. limestone) or fossil fuels, can be oxidized on exposure to oxygen (e.g. when fossil fuels are burned). Apart from water vapour, carbon dioxide is the most important greenhouse gas, absorbing long-wave radiation at wavelengths of about 5 μm and 18 μm.

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MICHAEL ALLABY. "carbon dioxide." A Dictionary of Ecology. 2004. Encyclopedia.com. 9 Jul. 2009 <http://www.encyclopedia.com>.

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carbon dioxide

The Oxford Companion to the Body | 2001 | | © The Oxford Companion to the Body 2001, originally published by Oxford University Press 2001. (Hide copyright information) Copyright

carbon dioxide When the body ‘burns’ food the end products are mainly water and carbon dioxide, together with some nitrogenous chemicals such as urea. The carbon dioxide enters the bloodstream, is carried to the lungs, and is excreted in the expired air of breathing. The atmospheric air we inhale contains virtually no carbon dioxide, whereas there is about 5% in the air we breathe out.

Carbon dioxide reacts in the blood to form carbonic acid and bicarbonate and, if it were allowed to accumulate, would cause acidosis. This condition is particularly harmful to the cells of the brain. Carbon dioxide diffuses into the liquid in the brain, the cerebrospinal fluid (CSF); any excess makes it more acid, and this in turn stimulates neural receptors in the brain stem that increase breathing. The result is that the carbon dioxide is blown off in the lungs and the acidity of the blood and brain are kept close to normal levels. Carbon dioxide is the main chemical stimulus to breathing, which is regulated primarily to keep blood and brain acidity at healthy values. If the carbon dioxide in the lungs increases by only 0.2%, from a normal level of about 5%, then breathing is doubled. Breathholding accumulates carbon dioxide in the body, which leads to an irrepressible desire to breathe (lack of oxygen is also a stimulus, but far weaker than carbon dioxide). Conversely, if we voluntarily hyperventilate, the level of carbon dioxide in the blood will decrease, and breathing may be inhibited until more carbon dioxide accumulates. Hyperventilation can have harmful effects because of the pronounced reduction in blood and CSF acidity. Since decreases in carbon dioxide and acidity constrict blood vessels, particularly in the brain, one effect is to reduce the blood supply to the brain.

Carbon dioxide was identified, but not understood chemically, in about 1600 ad by van Helmont, who called it ‘gas sylvestre’, the gas produced by combustion. He showed that it would not support life. Later Joseph Black, who had a lifelong interest in chemistry and was Professor of Medicine in Glasgow from 1757 to 1766, called it ‘fixed acid’, because it was absorbed by lime solution, and he showed that it was produced in respiration. The story goes that in 1764 Black climbed to the ceiling of a church in Glasgow, occupied for 10 hours of religious devotions by a congregation of 1500, and measured the ‘fixed acid’ that was exhaled by the diligent and sleepy congregation. But it was Lavoisier (1743–94) who definitely established the excretion of carbon dioxide after its formation in metabolism, although he erroneously believed that it was formed in the lungs. Lavoisier was guillotined, and it was said that ‘it took but a second to cut off his head; a hundred years will not suffice to produce one like it.’ Lavoisier concluded that any series of lectures in an auditorium extending over 3 hours would leave the audience in a soporific state due to the accumulation of carbon dioxide. In theory he was right. Carbon dioxide in excess can act as an anaesthetic and, in animals, major surgery has been performed under its influence alone. Some human lung diseases such as chronic bronchitis may leave the patient drowsy or even comatose because of the build up of carbon dioxide in the body. It is claimed, probably incorrectly, that in social environments yawning and weariness are due to an accumulation of carbon dioxide. Van Helmont investigated a Grotto del Cane (cave of dogs) in Italy in which it was claimed, rather implausibly, that a tall dog owner would survive while his lowly dog would perish, due to the depressant effect of carbon dioxide, held to the ground because of its greater density than air. Perhaps Black's Glasgow congregation was fortunate.

John Widdicombe


See also acid–base homeostasis; blood; respiration.
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COLIN BLAKEMORE and SHELIA JENNETT. "carbon dioxide." The Oxford Companion to the Body. Oxford University Press. 2001. Retrieved July 09, 2009 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O128-carbondioxide.html

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