methane mĕth´ān [key]
, colorless, odorless, gaseous saturated hydrocarbon; the simplest alkane
. It is less dense than air, melts at −184°C, and boils at −161.4°C. 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.
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