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methane (mĕthˈān), CH4, colorless, odorless, gaseous saturated hydrocarbon; the simplest alkane. It is less dense than air, melts at −184℃, and boils at −161.4℃. It is combustible and can form explosive mixtures with air. Methane occurs naturally as the principal component of natural gas; it is formed by the decomposition of plant and animal matter. When this decomposition occurs underwater in swamps and marshes, marsh gas is released. The firedamp of coal mines is chiefly methane. In the atmosphere methane is a significant greenhouse gas, helping to trap infrared radiation and warm the earth (see also global warming); its greenhouse effects are much greater than carbon dioxide in the short term. Major sources of atmospheric methane from human activities include landfills, dairy farms, and the oil and gas industry. Methane, in the form of icelike methane hydrate (composed of methane and frozen water), also is stored in seabed sediments at ocean depths where sufficiently low temperatures and high pressures prevail.
Methane can be prepared in the laboratory by heating sodium acetate with sodium hydroxide, by the reaction of aluminum carbide with water, by the direct combination of carbon and hydrogen, or by the destructive distillation of coal or wood. As natural gas, methane is widely used for fuel. It is also used for carbonizing steel. It is unaffected by many common chemical reagents but reacts violently with chlorine or fluorine in the presence of light and is therefore important as a starting material for the synthesis of solvents, e.g., methylene chloride, chloroform, and carbon tetrachloride, and of some of the Freon refrigerants.
(also marsh gas or firedamp), CH4, the first member of the homologous series of saturated hydrocarbons; a colorless, odorless gas. Boiling point, — 164.5°C, melting point, — 182.5°C; density (relative to air), 0.554 at 20°C. It burns with a virtually colorless flame. Heat of combustion, 50.08 megajoules per kg (11,954 kcal/kg).
Methane is the main component of natural gas (77–99 percent by volume), casinghead gas (31–90 percent), and mine gas (34–40 percent). It is also found in volcanic gases and is produced continuously by the decomposition of organic matter under the action of methane-forming bacteria with a limited supply of air (marsh gas and gases in irrigated fields). The atmospheres of Saturn and Jupiter are composed largely of methane. Methane is produced during thermal refining of petroleum and petroleum products (10–57 percent by volume), as well as during the coking and hydrogenation of coal (24-34 percent). Laboratory methods of methane preparation include fusion of sodium acetate with an alkali and the action of water on methyl magnesium iodide or aluminum carbide.
Methane forms explosive mixtures with air. Methane evolved in underground workings during the mining of mineral deposits, as well as in coal-washing and briquetting plants and sorting installations, is the most dangerous. For example, at a methane content of 5–6 percent in air, methane will burn near a heat source (flash point, 650°-750°C); it will explode when the content is 5–6 to 14–16 percent. If the methane content is more than about 16 percent, the gas may burn if there is an oxygen flow from an external source; a reduction in methane concentration at this stage can lead to an explosion. In addition, a substantial increase in the methane concentration in air induces suffocation (for example, 43 percent methane corresponds to 12 percent O2).
Explosive combustion propagates at a rate of 500–700 m/sec; the gas pressure upon explosion in a closed space is 1 meganewton per sq m.
After initial contact with the heat source, the combustion of methane is somewhat delayed. This property is the basis for the manufacture of permissible explosives and explosion-proof electrical equipment. Gas conditions are implemented in areas that are hazardous because of the presence of methane (particularly coal mines).
Methane is the most thermally stable saturated hydrocarbon. It is widely used as a household and industrial fuel, and also as an industrial raw material. For example, methyl chloride, methylene chloride, chloroform, and carbon tetrachloride are all prepared by chlorination of methane. Partial combustion of methane yields carbon black, and catalytic oxidation produces formaldehyde. Carbon disulfide is formed by the reaction of methane and sulfur. Important industrial methods used in the preparation of acetylene include thermal-oxidation cracking and electrocracking of methane. The industrial manufacture of hydrocyanic acid is based on catalytic oxidation of a methaneammonia mixture. Methane is used as a hydrogen source in the preparation of ammonia, as well as in the manufacture of water gas (synthesis gas) according to the formula CH4 + H2O → CO + 3H2; water gas is used in the industrial synthesis of hydrocarbons, alcohols, and aldehydes. Nitromethane is an important methane derivative.