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CARBON DIOXIDE |
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Carbon dioxide is an atmospheric gas comprised of one carbon and two oxygen atoms. A very widely known chemical compound, it is frequently called by its formula CO2. In its solid state, it is commonly known as dry ice.
Carbon dioxide derives from multiple sources including volcanic outgassing, the combustion of organic matter and respiration processes of living aerobic organisms. It is also produced by various microorganisms from fermentation and cellular respiration. Plants utilize carbon dioxide during photosynthesis, using both the carbon and the oxygen to construct carbohydrates. In addition, plants also release oxygen to the atmosphere, which is subsequently used for respiration by heterotrophic organisms, forming a cycle. It is present in the Earth's atmosphere at a low concentration and acts as a greenhouse gas. It is a major component of the carbon cycle. Chemical and physical properties
Carbon dioxide is a colorless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid.
Its density at 25 °C is 1.98 kg m−3, about 1.5 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole. As it is fully oxidized, it is not very reactive and, in particular, not flammable.
At temperatures below −78 °C, carbon dioxide condenses into a white solid called dry ice. Liquid carbon dioxide forms only at pressures above 5.1 atm; at atmospheric pressure, it passes directly between the gaseous and solid phases in a process called sublimation.
Water will absorb its own volume of carbon dioxide, and more than this under pressure. About 1% of the dissolved carbon dioxide turns into carbonic acid. The carbonic acid in turn dissociates partly to form bicarbonate and carbonate ions.
Test For Carbon Dioxide. When a lighted splint is inserted into a test tube containing carbon dioxide, the flame is immediately extinguished, as carbon dioxide does not support combustion. (Certain fire extinguishers contain carbon dioxide to extinguish the flame). To further confirm that the gas is carbon dioxide, the gas may be bubbled into calcium hydroxide solution. The calcium hydroxide turns milky because of the formation of calcium carbonate. Uses
Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical.
Carbon dioxide is used to produce carbonated soft drinks and soda water. Candy called Pop Rocks is pressurized with carbon dioxide gas at about 600 PSI. When you put the candy in your mouth, it melts (just like other hard candy) and releases the gas bubbles with an audible "pop". Traditionally, the carbonation in beer and sparkling wine comes about through natural fermentation, but some manufacturers carbonate these drinks artificially.
The leavening agents used in baking produce carbon dioxide to cause dough to rise. Baker's yeast produces carbon dioxide by fermentation within the dough, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or exposed to acids.
Carbon dioxide is often used as an inexpensive, nonflammable pressurized gas. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Steel capsules are also sold as supplies of compressed gas for airguns, paintball markers, for inflating bicycle tires, and for making seltzer. Rapid vaporization of liquid CO2 is used for blasting in coal mines.
Carbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide also finds use as an atmosphere for welding, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are brittler than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium.
Liquid carbon dioxide is a good solvent for many organic compounds, and is used to remove caffeine from coffee. First, the green coffee beans are soaked in water. The beans are placed in the top of a column that's seventy feet high. The carbon dioxide fluid at about 93 degrees Cel. enters at the bottom of the column. The caffeine diffuses out of the beans and into the carbon dioxide.
Carbon dioxide has begun to attract attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. It's used by some dry cleaners for this reason. (See green chemistry.)
Plants require carbon dioxide to conduct photosynthesis, and greenhouses may enrich their atmospheres with additional CO2 to boost plant growth. It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into biodiesel fuel. High levels of carbon dioxide in the atmosphere effectively exterminate many pests. Greenhouses will raise the level of CO2 to 10,000 ppm (1%) for several hours to eliminate pests such as whiteflies, spider mites, and others.
In medicine, up to 5% carbon dioxide is added to pure oxygen for stimulation of breathing after apnea and to stabilize the O2/CO2 balance in blood.
A common type of industrial gas laser, the carbon dioxide laser, uses carbon dioxide as a medium.
Carbon dioxide is commonly injected into or adjacent to producing oil wells. It will act as both a pressurizing agent and, when dissolved into the underground crude oil, will significantly reduce its viscosity, enabling the oil to flow more rapidly through the earth to the removal well. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.
Handling
Because of its particular characteristics, dry ice requires special precautions when handling. It is extremely cold and there should be no direct contact with skin (i.e., wear proper insulating gloves). It is constantly sublimating to carbon dioxide gas, so it cannot be stored in a sealed container as the pressure buildup will quickly cause the container to explode. The sublimated gas must be ventilated; otherwise, it may fill the enclosed space and create a suffocation hazard. Special care for ventilating vehicles is needed as well because of the small space. People who handle dry ice should also be aware that carbon dioxide is heavier than air and will sink to the floor. Some markets require those purchasing dry ice to be of 18 years of age or older.
Biology
Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars or fats with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it's exhaled.
Carbon dioxide content in fresh air is approximately 0.04%, and in exhaled air approximately 4.5%. When inhaled in high concentrations (about 5% by volume), it is toxic to humans and other animals. This is sometimes known as choke damp, an old mining industry term, and was the cause of death at Lake Nyos in Cameroon, where an upwelling of CO2-laden lake water in 1986 covered a wide area in a blanket of the gas, killing nearly 2000.
Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. The decreased binding to oxygen in the blood due to increased carbon dioxide levels is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect.
According to a study by the USDA [1], an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.
CO2 is carried in blood in three different ways. Most of it (about 80%–90%) is converted to bicarbonate ions HCO3− by the enzyme carbonic anhydrase in the red blood cells. 5%–10% is dissolved in the plasma and 5%–10% is bound to hemoglobin as carbamino compounds. The exact percentages vary depending whether it is arterial or venous blood.
The CO2 bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.
Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue.
Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO2 in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.
It is interesting to note that although it is oxygen that the body requires for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (e.g., pure nitrogen) leads to loss of consciousness without subjective breathing problems. This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others—otherwise one risks going unconscious without being aware of the imminent peril.
Plants remove carbon dioxide from the atmosphere by photosynthesis, which uses light energy to produce organic plant materials by combining carbon dioxide and water. This releases free oxygen gas. Sometimes carbon dioxide gas is pumped into greenhouses to promote plant growth. Plants also emit CO2 during respiration, but on balance they are net sinks of CO2.
OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods. The U.S. National Institute for Occupational Safey and Health limits brief exposures (up to ten minutes) to 3% and considers concentrations exceeding 4% as "immediately dangerous to life and health." People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7%–10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.
As of 2004, the earth's atmosphere is about 0.038% by volume (380 µL/L or ppmv) or 0.057% by weight CO2. This represents about 2.94 × 1012 tonnes of CO2. Because of the greater land area, and therefore greater plant life, in the northern hemisphere as compared to the southern hemisphere, there is an annual fluctuation of about 5 µL/L, peaking in May and reaching a minimum in October at the end of the northern hemisphere growing season, when the quantity of biomass on the planet is greatest.
Despite its small concentration, CO2 is a very important component of Earth's atmosphere, because it absorbs infrared radiation and enhances the greenhouse effect.
The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity; this was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year. Volcanic releases are about 1% of the amount which is released by human activities.
Since the start of the Industrial Revolution, the atmospheric CO2 concentration has increased by approximately 110 µL/L or about 40%, most of it released since 1945. Monthly measurements taken at Mauna Loa [2] since 1958 show an increase from 316 µL/L in that year to 376 µL/L in 2003, an overall increase of 60 µL/L during the 44-year history of the measurements. Burning fossil fuels such as coal and petroleum is the leading cause of increased man-made CO2; deforestation is the second major cause. In 1997, Indonesian peat fires may have released 13%–40% as much carbon as fossil fuel burning does. Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO2 remains in the atmosphere; some is absorbed in the oceans or biosphere. The ratio of the emitted CO2 to the increase is atmospheric CO2 is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically 57% over longer (5 year) periods.
The Global Warming Theory (GWT) predicts that increased amounts of CO2 in the atmosphere tend to enhance the greenhouse effect and thus contribute to global warming. The effect of combustion-produced carbon dioxide on climate is called the Callendar effect. Variation in the past
The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO2 levels were about 260–280µL/L immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 650,000 years before the present. [4] During this time, the atmospheric carbon dioxide concentration has varied between 180–210 µL/L during ice ages, increasing to 280–300 µL/L during warmer interglacials.
Some studies have disputed the claim of stable CO2 levels during the present interglacial (the last 10 kyr). Based on an analysis of fossil leaves, Wagner et al. argued that CO2 levels during the period 7–10 kyr ago were significantly higher (~300 µL/L) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO2. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO2 values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO2 measurements.
On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO2 concentrations (>3,000 µL/L) between 600 and 400 Myr BP and between 200 and 150 Myr BP.[8] On long timescales, atmospheric CO2 content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO2. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO2 response to emissions over the next hundred years. In more recent times, atmospheric CO2 concentration continued to fall after about 60 Myr BP, and there is geochemical evidence that concentrations were <300 µL/L by about 20 Myr BP. Low CO2 concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Myr BP. Although contemporary CO2 concentrations were exceeded during earlier geological epochs, present carbon dioxide levels are likely higher now than at any time during the past 20 million years [9] and at the same time lower than at any time in history if we look at time scales longer than 50 million years.
Oceans
The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions—much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:
CaCO3 + CO2 + H2O ⇌ Ca2+ + 2 HCO3-
Reactions like this tend to buffer changes in atmospheric CO2. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO2. Over hundreds of millions of years this has produced huge quantities of carbonate rocks. If all the carbonate rocks in the earth's crust were to be converted back into carbon dioxide, the resulting carbon dioxide would weigh 40 times as much as the rest of the atmosphere.
The vast majority of CO2 added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.
History
Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the 17th century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre).
Carbon dioxide's properties were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he termed "fixed air." He observed that the fixed air was denser than air and did not support either flame or animal life. He also found that it would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, Joseph Priestley used carbon dioxide produced from the action of sulfuric acid on limestone to prepare soda water, the first known instance of an artificially carbonated drink.
Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphrey Davy and Michael Faraday. The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.
References
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