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Carbon Dioxide

Carbon dioxide

Carbon dioxide was the first gas to be distinguished from ordinary air, perhaps because it is so intimately connected with the cycles of plant and animal life. Carbon dioxide is released during respiration and combustion. When plants store energy in the form of food, they use up carbon dioxide. Early scientists were able to observe the effects of carbon dioxide long before they knew its function.

About 1630, Flemish scientist Jan van Helmont discovered that certain vapors differed from air that was then thought to be a single substance or element. Van Helmont coined the term gas to describe these vapors and collected the gas given off by burning wood, calling it gas sylvestre. Today, it is known that this gas is carbon dioxide, and van Helmont is credited with its discovery. In 1756, Joseph Black proved that carbon dioxide, which he called fixed air, is present in the atmosphere and that it combines with other chemicals to form new compounds. Black also identified carbon dioxide in exhaled breath, determined that the gas is heavier than air, and characterized its chemical behavior as that of a weak acid. The pioneering work of van Helmont and Black soon led to the discovery of other gases. As a result, scientists began to realize that gases must be weighed and accounted for in the analysis of chemical compounds, just like solids and liquids.

In 1783, French physicist Pierre Laplace (17491827) used a guinea pig to demonstrate quantitatively that oxygen from the air is used to burn carbon stored in the body and produce carbon dioxide in exhaled breath. Around the same time, chemists began drawing the connection between carbon dioxide and plant life. Like animals, plants breathe using up oxygen and releasing carbon dioxide. Plants, however, also have the unique ability to store energy in the form of carbohydrates, our primary source of food. This energy-storing process, called photosynthesis, is essentially the reverse of respiration. It uses up carbon dioxide and releases oxygen in a complex series of reactions that also require sunlight and chlorophyll (the green substance that gives plants their color). In the 1770s, Dutch physiologist Jan Ingen Housz established the principles of photosynthesis.

English chemist John Dalton guessed in 1803 that the molecule contains one carbon atom and two oxygen atoms (CO2); this was later proved correct. The decay of all organic materials produces carbon dioxide slowly, and Earth's atmosphere contains a small amount of the gas (about 0.033%). Spectroscopic analysis has shown that in our solar system , the planets of Venus and Mars have atmospheres rich in carbon dioxide. The gas also exists in ocean water , where it plays a vital role in marine plant photosynthesis.

In modern life, carbon dioxide has many practical applications. For example, fire extinguishers use CO2 to control electrical and oil fires that cannot be put out with water. Because carbon dioxide is heavier than air, it spreads into a blanket and smothers the flames. Carbon dioxide is also an effective refrigerant. In its solid form, known as dry ice , it is used to chill perishable food during transport. Many industrial processes are also cooled by carbon dioxide, which allows faster production rates. For these commercial purposes, carbon dioxide can be obtained from either natural gas wells, fermentation of organic material, or combustion of fossil fuels .

Recently, carbon dioxide has received negative attention as a greenhouse gas. When it accumulates in the upper atmosphere, it traps the Earth's heat, eventually causing global warming . Since the beginning of the industrial revolution in the mid 1800s, factories and power plants have significantly increased the amount of carbon dioxide in the atmosphere by burning coal and other fossil fuels. This effect was first predicted by Svante August Arrhenius , a Swedish physicist, in the 1880s. Then in 1938, British physicist G. S. Callendar suggested that higher CO2 levels had caused the warmer temperatures observed in America and Europe since Arrhenius's day. Modern scientists have confirmed these views and identified other causes of increasing carbon dioxide levels, such as the clearing of the world's forests . Because trees extract CO2 from the air, their depletion has contributed to upsetting the delicate balance of gases in the atmosphere.

In rare circumstances, carbon dioxide can endanger life. In 1986, a huge cloud of the gas exploded from Lake Nyos, a volcanic lake in northwestern Cameroon, and quickly suffocated more than 1,700 people and 8,000 animals. Scientists have attempted to control this phenomenon by slowly pumping the gas up from the bottom of the lake.

See also Atmospheric chemistry; Atmospheric composition and structure; Atmospheric pollution; Forests and deforestation; Global warming; Greenhouse gases and greenhouse effect

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Carbon Dioxide

Carbon Dioxide


Carbon dioxide (CO2) is a nontoxic, odorless, and colorless gas present in trace concentrations in the atmosphere. The molecule is linear with a central carbon atom doubly bonded to two oxygen atoms (O=C=O). Natural sources of CO2 include volcanic outgassing, animal respiration, biomass decay, and oceanic evaporation. Removal processes include photosynthesis and dissolution into the oceans .

CO2 is a long-lived gas, with an atmospheric lifetime of more than one hundred years. It is a natural greenhouse gas and plays an important role in regulating Earth's climate. Like water vapor, CO2 traps outgoing infrared radiation emitted by Earth into space. By absorbing this energy, the atmosphere warms the earth, a process known as the natural greenhouse effect. Without carbon dioxide and water in the atmosphere, the earth's average surface temperature would be below 0°F, turning oceans into ice and dramatically altering life as it is known.

CO2 is also an anthropogenic greenhouse gas, ranked number one for its contributions to global warming. At the beginning of the Industrial Era (around 1750), CO2 concentrations worldwide were approximately 280 parts per million (ppm); by 1999 concentrations reached 367 ppm. (One ppm equals one molecule of CO2 for every million molecules of air, or 0.0001 percent.) CO2 emissions continue to rise; the average rate of increase since 1980 is 0.4 percent per year.

The recent rise in anthropogenic CO2 is attributed largely to fossil fuel combustion (73 percent) and land use conversion resulting from deforestation (25 percent). When oil, coal, or natural gas is burned to generate energy, the by-products are CO2 and water. Due to heavy fossil fuel consumption, the United States leads the world in anthropogenic CO2 emissions (see table). In 1996 the United States contributed more than 50 percent of the 1.027 × 1016 grams of total global CO2 emissions.

As concentrations of CO2 increase in the atmosphere, more outgoing infrared energy is trapped (energy that would have escaped to space), warming the earth's atmosphere and surface. The Intergovernmental Panel on Climate Change estimates that the global surface temperature has increased by 1.1°F since the late nineteenth century, due to increases in CO2 and other greenhouse gases.

International efforts are underway to reduce CO2 and other greenhouse gases. As of December 2001, 186 countries ratified the Kyoto Protocol, an international agreement to reduce CO2 and other greenhouse gas emissions. CO2 emissions can be reduced by reforestation and afforestation efforts by changing cropland management practices such as tilling, and by reducing the combustion of fossil fuels. The United States did not sign the protocol, but promotes voluntary development of climate-friendly technologies (i.e., renewable energy sources) coupled with changes in land use and forestry practices. Examples of the latter include decreased deforestation, increased reforestation, and agricultural practices designed to increase soil carbon.

see also Electric Power; Emissions Trading; Global Warming; Greenhouse Gases; Petroleum; Vehicular Pollution.

Bibliography

Turco, Richard P. (1997). Earth under Siege: From Air Pollution to Global Change. New York: Oxford University Press.


Internet Resources

Intergovernmental Panel on Climate Change, Working Group I. "The Carbon Cycle and Atmospheric Carbon Dioxide." In Climate Change 2001: The Scientific Basis. Available from http://www.ipcc.ch.

United Nations Framework Convention on Climate Change. "Texts of the Convention and the Kyoto Protocol." Available from http://unfccc.int/resource.

United Nations Framework Convention on Climate Change, Conference of the Parties. Fourth session, Buenos Aires, November 213, 1998, Information 9. "Summary Compilation of Annual Greenhouse Gas Emissions Inventory Data from Annex I Parties." Available from FCCC/COP/1998/INF.9.

Marin Sands Robinson

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carbon dioxide

carbon dioxide, chemical compound, CO2, 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. For the last 800,000 years the carbon dioxide in the atmosphere has oscillated over tens of thousands of years between 180 to 280 parts per million (ppm), but since the Industrial Revolution it has steadily increased above 280 ppm in a relatively brief time, reaching 400 ppm in 2013. This extra carbon dioxide fuels the greenhouse effect, warming the atmosphere and further disrupting 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

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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Carbon Dioxide

Carbon dioxide

Carbon dioxide is a chemical compound consisting of one part carbon and two parts oxygen and represented by the chemical formula CO2. For a number of reasons, carbon dioxide is one of the most important gases on Earth. Plants use carbon dioxide to produce carbohydrates (sugars and starches) in the process known as photosynthesis. (In photosynthesis, plants make use of light to break down chemical compounds and produce energy.) Since humans and all other animals depend on plants for their food, photosynthesis is necessary for the survival of all life on Earth.

Carbon dioxide in the atmosphere is also important because it captures heat radiated from Earth's surface. That heat keeps the planet warm enough for plant and animal (including human) life to survive. Increasing levels of carbon dioxide in the atmosphere may be responsible for long-term changes in Earth's climate. Those changes may have both beneficial and harmful effects on human and other forms of life on the planet.

History

Credit for the discovery of carbon dioxide goes to Flemish scientist Jan Baptista van Helmont (c. 15801644; some sources give death date as 1635). Around 1630, van Helmont identified a gas given off by burning wood and gave it the name gas sylvestre ("wood gas"). Today we know that gas is carbon dioxide. Van Helmont's discovery was important not only because he first recognized carbon dioxide but also because he first understood that air is a combination of gases, not a single gas.

Some of the most complete studies of carbon dioxide were conducted by Scottish chemist Joseph Black (17281799). In 1756, Black proved that carbon dioxide (which was then called "fixed air") occurred in the atmosphere and that it could form other compounds. He also identified carbon dioxide in the breath exhaled by humans.

The first practical use of carbon dioxide can be traced to an invention made by English chemist Joseph Priestley (17331804) in the mid-1700s. Priestley found that by dissolving carbon dioxide in water he could produce a fresh, sparkling beverage with a pleasant flavor. Since Priestley's discovery lacks only sugar and flavoring to make it a modern soda pop or cola drink, he can properly be called the father of the soft drink industry.

Properties and uses

Carbon dioxide is a colorless, odorless, noncombustible gas with a density about twice that of air. It can be converted to a solid known as dry ice rather easily. Dry ice has the interesting property that it sublimesthat is, changes directly from a solid to a gas without first melting into a liquid. All of these properties explain the most important commercial and industrial uses of carbon dioxide.

Among those uses is the one discovered by Priestley, the manufacture of carbonated ("soft") drinks. The presence of carbon dioxide provides the slightly tart and tingly flavor that makes such beverages so refreshing. Carbon dioxide is also used widely as a coolant, a refrigerant, and an ingredient in the manufacture of frozen foods. Carbon dioxide fire extinguishers are often used to control electrical and oil fires, which cannot be put out with water. Because the gas is more dense than air and does not catch fire, it spreads like a blanket over burning material and smothers the flames. In addition, carbon dioxide is sometimes used as a gaseous blanket to prevent substances from decaying.

In rare circumstances, carbon dioxide can be a threat to life. In 1896, a huge cloud of the gas exploded from Lake Nyos, a volcanic lake in northwestern Cameroon, a nation in western Africa. The cloud spread quickly and suffocated more than 1,700 people and 8,000 animals. Today, scientists are trying to control this phenomenon by slowly pumping carbon dioxide gas from the bottom of the lake.

[See also Carbon family; Combustion; Greenhouse effect; Photosynthesis; Pollution ]

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carbon dioxide

carbon dioxide A colourless odourless gas, CO2, which dissolves in water to give carbonic acid. It occurs in the atmosphere (0.04% by volume) but has a short residence time in this phase as it is both consumed by plants during photosynthesis and produced by respiration and by combustion.

The level of carbon dioxide in the atmosphere has increased by some 12% in the last 100 years, mainly because of extensive burning of fossil fuels and the destruction of large areas of rainforest. This has been postulated as the main cause of the average increase of 0.5°C in global temperatures over the same period, through the greenhouse effect. Atmospheric CO2 concentration continues to rise, in spite of some tentative steps to control emissions, giving the prospect of accelerated global warming in the foreseeable future.

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carbon dioxide

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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carbon dioxide

carbon dioxide (CO2) Colourless, odourless gas that occurs in the atmosphere (0.03%) and as a product of the combustion of fossil fuels and respiration in plants and animals. In its solid form (dry ice) it is used in refrigeration; as a gas it is used in carbonated beverages and fire extinguishers. Research indicates that its increase in the atmosphere leads to the greenhouse effect and global warming. Properties: m.p. −56.6°C (−69.9°F); sublimes −78.5°C (−109.3°F).

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carbon dioxide

carbon dioxide (kar-bŏn dy-ok-syd) n. a colourless gas formed in the tissues during metabolism and carried in the blood to the lungs, where it is exhaled (an increase in the concentration of this gas in the blood stimulates respiration). It forms a solid (dry ice) at –75°C (at atmospheric pressure) and in this form is used as a refrigerant. Formula: CO2.

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carbon dioxide

car·bon di·ox·ide • n. a colorless, odorless gas, CO2, produced by burning carbon and organic compounds and by respiration. It is naturally present in air (about 0.03 percent) and is absorbed by plants in photosynthesis.

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