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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), and controlling the carbon dioxide produced by human activities is key to limiting global warming and the disruptive effects of climate change.
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.
(also carbonic anhydride, carbonic acid gas), CO2, carbon (IV) oxide, the highest oxide of carbon. In 1756, J. Black demonstrated that a gas, which he called “fixed” air, is liberated upon decomposition of magnesium carbonate. The gas’s composition was established in 1789 by A. Lavoisier.
Carbon dioxide is a colorless gas with a faintly pungent odor and acid taste; it has a density of 0.0019 g/cm3 (at 0°C and 0.1 meganewtons per sq m [MN/m2]), a melting point of -56.6°C, a boiling point of -78.5°C, a critical temperature of 31°C, and a critical pressure of 7.62 MN/m2, or 76.2 kilograms-force per sq cm (kgf/cm2). At atmospheric pressure and a temperature of -78.5°C, carbon dioxide hardens into a white, snowlike mass known as dry ice, thus bypassing the liquid state. Liquid carbon dioxide exists at room temperature only when the pressure exceeds 5.85 MN/m2 (58.5 kgf/cm2). The density of liquid CO2 is 0.771 g/cm3 at 20°C, while that of the solid form is 1.512 g/cm3. In the gaseous phase, the carbon dioxide molecule has the symmetrical form O=C=O, with a distance between the carbon and oxygen atoms of 1.162 angstroms (Å). Solid CO2 crystallizes in a face-centered cubic lattice, with a = 5.62 Å.
Carbon dioxide is thermostable, dissociating into carbon monoxide and oxygen only at a temperature above 2000°C. The compound is noticeably soluble in water, forming solutions of 0.335 percent (by weight) at 0°C and 0.169 percent at 20°C; it partially reacts with water to form carbonic acid (H2CO3). Carbon dioxide dissolves in the following organic solvents: acetone, benzene, chloroform, and alcohols. It reacts vigorously with bases to yield carbonates. CO2 does not burn and does not support combustion. It is reduced only by very highly reactive metals at high temperatures, for example, by magnesium at 600°C and by calcium at 700°C. Carbon dioxide reacts with red-hot coal: CO2 + C = 2CO, a reaction having great importance in metallurgy. It also reacts with ammonia at a temperature of 160°-200°C and a pressure of 10–40 MN/m2 (100–400 kgf/cm2): CO2 + 2NH3= CO(NH2)2 + H2O. Carbon dioxide reacts with hydrogen in the presence of cupric oxide, forming methane.
Carbon dioxide is a component of air, constituting 0.03 percent by volume; the total content is 2.3 × 1012 tons. In the hydrosphere, there are 1.4 × 1014 tons of carbon dioxide in equilibrium with the atmosphere. Carbon dioxide is formed and introduced into the atmosphere upon the combustion of fuels, the decay of organic matter, the process of fermentation, and the respiration of humans and animals. As a result of industrial pollution, the amount of carbon dioxide in the air of industrial cities greatly exceeds permissible levels. Measures have been taken to reduce this level in a number of industrially developed countries, including the USSR. Carbon dioxide is necessary for the growth of plants, which absorb the compound from the atmosphere in the process of photosynthesis. The atmospheres of the planets Mars and Venus contain carbon dioxide as their major component.
Carbon dioxide is produced industrially mainly by roasting limestone at 900°-1300°C, a process that also yields lime; the compound is purified through its absorption by solutions of soda, potash, or ethanolamine. It is stored and transported in the liquefied state under a pressure of 6 MN/m2 (60 kgf/cm2) in steel cylinders. In the laboratory, CO2 is usually obtained by the reaction of hydrochloric acid with marble.
Carbon dioxide is used in the production of soda water, beer, and sugar. Dry ice is used for the preservation and transport of perishable food products. In the chemical industry, CO2 is consumed in producing soda, urea, and hydroxycarboxylic acids; in graphite-moderated reactors, it functions as a heat carrier. Carbon dioxide is also used in extinguishing fires and transporting flammable substances.
B. A. POPOVKIN
In agriculture, carbon dioxide is used as a fertilizer. An insufficiency of carbon dioxide in the air, which frequently occurs when the ground is shielded, as is especially the case with hydroponic cultivation, lowers the rate of photosynthesis and the crop yield. Gaseous carbon dioxide (from cylinders) or purified products (containing up to 15 percent CO2) of the catalytic combustion of natural gas and solid fuel are introduced during the daytime into hothouses and greenhouses to improve the carbon supply to plants. Solid carbon dioxide (dry ice) can be used as a source of gaseous carbon dioxide; here, pieces of the solid are distributed about an area. Organic and mineral fertilizers that liberate carbon dioxide upon decomposition may also be used as sources. The efficiency of carbon dioxide fertilizers depends on the mineral supply available to plants, the illumination, and the temperature of the soil and air.
In humans and animals, carbon dioxide, together with bicar-bonates, forms an important buffer system of the blood. An increase in the partial pressure of carbon dioxide in the blood strengthens the bond of oxygen to hemoglobin. By acting, both directly and indirectly, on the centers of the medulla oblongata, carbon dioxide figures in the regulation of respiration and blood circulation. A mixture of 95 percent oxygen and 5 percent carbon dioxide (Carbogen) is used in medicine in the case of toxic dosages of narcotics and carbon monoxide poisoning. In high concentrations, carbon dioxide is toxic, inducing hypoxia. Breathing carbon dioxide for a period of several days, even in concentrations of 1.5–3 percent, causes headache, vertigo, and nausea. At concentrations greater than 6 percent (critical level), a person becomes drowsy and unable to work, and there is a weakening of respiratory and cardiac activity, posing a threat to life. An accumulation of carbon dioxide in the air with a concomitant decrease in the oxygen content is seen in enclosed, poorly ventilated spaces, for example, areas in mines and sewers, and in places, such as breweries, where fermentation is occurring. First aid calls for removing the victim into the fresh air and applying artificial respiration. Carbon dioxide does not reach critical levels in the air in residential and public buildings. The concentration of carbon dioxide serves as an environmental indicator of air purity.
V. F. KIRILLOV
REFERENCESRemy, H. Kurs neorganicheskoi khimii, vol. 1. Moscow, 1972. (Translated from German.)
Nekrasov, B. V. Osnovy obshchei khimii, 3rd ed., vols. 1–2. Moscow, 1973.
Akhmetov, N. S. Neorganicheskaia khimiia, 2nd ed. Moscow, 1975.