Carbon Dioxide is a colorless, odorless, and slightly acid-tasting gas, sometimes called carbonic acid gas, the molecule of which consists of one atom of carbon joined to two atoms of oxygen (CO2). It was called “fixed air” by the Scottish chemist Joseph Black, who obtained it through the decomposition of chalk and limestone and recognized that it entered into the chemical composition of these substances. The French chemist Antoine Lavoisier proved that it is an oxide of carbon by showing that the gas obtained by the combustion of charcoal is identical in its properties with the “fixed air” obtained by Black. Carbon dioxide is about 1.5 times as dense as air. It is soluble in water, 0.9 volume of the gas dissolving in 1 volume of water at 20° C (68° F).
Carbon dioxide is produced in a variety of ways: by combustion, or oxidation, of materials containing carbon, such as coal, wood, oil, or foods; by fermentation of sugars; and by decomposition of carbonates under the influence of heat or acids. Commercially, carbon dioxide is recovered from furnace or kiln gases; from fermentation processes; from reaction of carbonates with acids; and from reaction of steam with natural gas, a step in the commercial production of ammonia. The carbon dioxide is purified by dissolving it in a concentrated solution of alkali carbonate or ethanolamine and then heating the solution with steam. The gas is evolved and is compressed into steel cylinders. The atmosphere contains carbon dioxide in variable amounts, usually 3 to 4 parts per 10,000, and has been increasing by 0.4 percent a year. It is used by green plants in the process known as photosynthesis, by which carbohydrates are manufactured.
Carbon dioxide is used in the manufacture of sodium carbonate, Na2CO3· OH2O (washing soda); sodium bicarbonate, NaHCO3 (baking soda); and basic carbonate of lead, Pb3 (OH)2(CO3)2 (white lead). Dissolved under a pressure of 2 to 5 atmospheres, carbon dioxide causes the effervescence in carbonated beverages. Carbon dioxide does not burn and does not support ordinary combustion, and because of these properties it is used for extinguishing fires. The CO2 extinguisher is a steel cylinder filled with liquid carbon dioxide, which, when released, expands suddenly and causes so great a lowering of temperature that it solidifies into powdery “snow.” This snow volatilizes (vaporizes) on contact with the burning substance, producing a blanket of gas that cools and smothers the flame. Solid carbon dioxide, known as dry ice, is widely used as a refrigerant. Its cooling effect is almost twice that of water ice; its special advantages are that it does not melt as a liquid but turns into gas, and that it produces an inert atmosphere that reduces bacterial growth. The presence of carbon dioxide in the blood stimulates breathing. For this reason, carbon dioxide is added to oxygen or ordinary air in artificial respiration and to the gases used in anesthesia.
Carbon Monoxide is a chemical compound of carbon and oxygen with the formula CO. Carbon monoxide melts at -205°C (-337°F) and boils at -191.5°C (-312.7°F). It is a colorless, odorless gas, about 3 percent lighter than air, and is poisonous to all warm-blooded animals and to many other forms of life. When inhaled it combines with hemoglobin in the blood, preventing absorption of oxygen and resulting in asphyxiation. Carbon monoxide is formed whenever carbon or substances containing carbon are burned with an insufficient air supply. Even when the amount of air is theoretically sufficient, the reaction is not always complete, so that the combustion gases contain some free oxygen and some carbon monoxide.
An incomplete reaction is especially probable when it takes place quickly, as in an automobile engine; for this reason, automobile-exhaust gases contain harmful quantities of carbon monoxide, sometimes several percent, although antipollution devices are intended to keep the level below 1 percent. As little as 1/1000 of 1 percent of carbon monoxide in air may produce symptoms of poisoning and as little as a fraction of 1 percent may prove fatal in less than 30 min. Carbon monoxide is a major component of air pollution in urban areas. In addition to being present in automobile exhaust, carbon monoxide also occurs in cigarette smoke.
Because it is odorless, carbon monoxide is an insidious poison. It produces only mild symptoms of headache, nausea, or fatigue, followed by unconsciousness. An automobile engine running in a closed garage can make the air noxious within a few minutes; a leaking furnace flue may fill a house with unsuspected poison. Fuel gas, which may contain as much as 50 percent carbon monoxide, often has small quantities of unpleasant-smelling sulfur compounds purposely added to make leaks noticeable.
Carbon monoxide is an important industrial fuel because it contains more than two-thirds of the heating value of the carbon from which it was formed. It is a constituent of water gas, producer gas, blast furnace gas, and coal gas. In smelting iron ore carbon monoxide formed from coke used in the process acts as a reducing agent, that is, it removes oxygen from the ore. Carbon monoxide combines actively with chlorine to form carbonyl chloride, or phosgene, and it combines with hydrogen, when heated in the presence of a catalyst, to form methyl alcohol. The direct combination of carbon monoxide with certain metals, forming gaseous compounds, is used in refining those metals, particularly nickel.
About the Author
Dr.Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, India.
(ArticlesBase SC #628347)
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