Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Mercury

.
Mercury (element) (Latin, hydrargyrum, “liquid silver”), symbol Hg, metallic element that is a free-flowing liquid at room temperature. Mercury is one of the transition elements of the periodic table (see Periodic Law). The atomic number of mercury is 80.

Mercury, once known as liquid silver and as quicksilver, was studied by the alchemists (see Alchemy). It was first distinguished as an element by French chemist Antoine Laurent Lavoisier, who burned mercury and other substances in his experiments to determine the composition of air.

PROPERTIES AND OCCURRENCE

At ordinary temperatures mercury is a shining, mobile liquid, silvery-white in color. Slightly volatile at room temperature, mercury becomes solid when subjected to a pressure of 7,640 atmospheres (7.7 million millibars), and this pressure is used as a standard in measuring extremely high pressures. The metal dissolves in nitric or concentrated sulfuric acid but is resistant to alkalis. When cooled to sufficiently low temperatures, mercury has superconductivity (the ability to conduct electrical currents with negligible resistance). Mercury has a freezing point of about -39°C (about -38°F), a boiling point of about 357°C (about 674°F), and a density of 13.59 grams per cu cm. The atomic weight of mercury is 200.59.

Mercury ranks about 67th in natural abundance among the elements in crustal rocks. It occurs in its pure form or combined with silver in small amounts but is found most often in the ore cinnabar, a mineral consisting of mercuric sulfide (HgS). The metal is obtained from cinnabar by heating the ore in air until the mercuric sulfide breaks down, yielding pure mercury metal.

USES

Mercury is used in thermometers because its coefficient of expansion is nearly constant—that is, the change in volume for each degree of rise or fall in temperature is the same. It is also used in other types of scientific apparatuses, such as vacuum pumps, barometers, and electric rectifiers and switches. Mercury-vapor lamps are used as a source of ultraviolet light and for sterilizing water. Because of the extremely toxic effects of mercury, the use of the metal and its compounds has been reduced in several industries, including pharmaceuticals, dentistry, and agriculture. The use of mercury in fluorescent lamps and mercury batteries has also been significantly reduced as alternatives have been developed. Perhaps most significant is the substitution of diaphragm cells for traditional mercury cells in chlorine-alkali production, which once accounted for a large percentage of total mercury consumption.

Mercury combines with all the common metals except iron and platinum to form alloys that are called amalgams. In one method of extracting gold and silver from their ores, the metals are combined with mercury to make them dissolve; the mercury is then removed by distillation. This method is no longer commonly used, however.

Mercury forms monovalent and divalent compounds. Among the commercially important compounds of mercury are mercuric sulfide, a common antiseptic also used as the pigment vermilion; mercurous chloride, or calomel, used for electrodes, and formerly used as a cathartic; mercuric chloride, or corrosive sublimate; and organic compounds used as disinfectants, germicides, and antiseptics, known as mercurials.