or quicksilver [from the Roman god Mercury], metallic chemical element; symbol Hg [Lat. hydrargyrum=liquid silver]; at. no. 80; at. wt. 200.59; m.p. −38.842 degrees Celsius; b.p. 356.58 degrees Celsius; sp. gr. 13.55 at 20 degrees Celsius; valence +1 or +2. Mercury was discovered in antiquity, and was known to the ancient Chinese, Hindus, and Egyptians, but was not recognized as an element. It was used as a medicine by Paracelsus. It was first recognized as a chemical element (in the modern sense) by A. L. Lavoisier about the end of the 18th cent.

Properties

Mercury is the only common metal existing as a liquid at ordinary temperatures. The pure metal has a silver-white mirrorlike appearance. Mercury is below cadmium in group IIb of the periodic table. It is relatively stable in dry air, but in moist air slowly forms a gray oxide coating. Mercury has high surface tension; when spilled, it breaks up into tiny beads which often become lodged in cracks.

Compounds

Mercury forms numerous compounds, assuming +1 valence in mercurous compounds and +2 valence in mercuric compounds. Mercury is not attacked by dilute hydrochloric or sulfuric acid. It reacts with hot nitric acid to form mercuric nitrate, Hg(No ₃ ) ₂. An excess of mercury reacts with nitric acid to form mercurous nitrate, HgNO ₃. Mercury reacts with hot concentrated sulfuric acid to form mercuric sulfate, HgSO ₄ ; with excess mercury, mercurous sulfate, Hg ₂ SO ₄, is formed. Mercury reacts directly with the halogens to form mercuric salts. At elevated temperatures mercury reacts slowly with oxygen to form mercuric oxide, HgO. A mercurous oxide may be formed chemically but is unstable, decomposing to a mixture of mercury and mercuric oxide.

Natural Occurrence and Uses

Mercury occurs uncombined in nature to a limited extent. The metal is obtained commercially from cinnabar, a mercuric sulfide ore; it is easily separated by roasting the ore in air. The metal is usually purified by repeated vacuum distillation.

Mercury metal has many uses. Because of its high density, it is used in barometers and manometers. Because it has a high rate of thermal expansion that is fairly constant over a wide temperature range, it is used extensively in thermometers. Mercury is important as a liquid contact material for electric switches. It is used in mercury-vapor lamps, which emit light rich in ultraviolet radiation; various kinds of such lamps are used for street lighting, as sun lamps, and in "black lights" (see lighting ). Mercury is used as an electrode in the production of chlorine and sodium hydroxide. It is also used in certain electric batteries. With some other metals mercury forms a special type of alloy called an amalgam ; a special amalgam (mostly mercury, silver, and tin) is used in dentistry for filling teeth.

Mercury compounds have many uses. Calomel ( mercurous chloride, Hg ₂ Cl ₂ ) is used as a standard in electrochemical measurements and in medicine as a purgative. Mercuric chloride (corrosive sublimate, HgCl ₂ ) is used as an insecticide, in rat poison, and as a disinfectant. Mercuric oxide is used in skin ointments. Mercuric sulfate is used as a catalyst in organic chemistry. Vermilion, a red pigment, is mercuric sulfide; another crystalline form of the sulfide (also used as a pigment) is black. Mercury fulminate, Hg(CNO) ₂, is used as a detonator. Mercury forms many organic compounds. Mercurochrome (in 2% aqueous solution) is used in medicine as a topical antiseptic. Mercury compounds were formerly used in the treatment of syphilis.

See also mercury poisoning.

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