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subject = Chemistry title = Lead papers = Lead is a lustrous, silvery metal that tarnishes in the presence of air and becomes a dull bluish gray. Soft and flexible, it has a low melting point (327 °C). Its chemical symbol, Pb, is from plumbum, the Latin word for waterworks, because of lead's extensive use in ancient water pipes. Itsatomic number is 82; its atomic weight is 207.19. Lead and lead compounds can be highly toxic when eaten or inhaled. Although lead is absorbed very slowly into the body, its rate of excretion is even slower. Thus, with constant exposure, lead accumulates gradually in the body. It is absorbed by the red blood cells and circulated through the body where it becomes concentrated in the soft tissues, especially the liver and kidneys. Lead can cause damage in the central nervous system and apparently can damage the cells making up the blood-brain barrier that protects the brain from many harmful chemicals. Symptoms of lead poisoning include loss of appetite, weakness, anemia, vomiting, and convulsions, sometimes leading to permanent brain damage or death. Children who ingest chips of old, lead-containing paint or are exposed to dust from the deterioration of such paint may exhibit symptoms. Levels of environmental lead considered nontoxic may also be involved in increased hypertension in a significant number of persons, according to studies released in the mid-1980s. As a result, the U.S. Centers for Disease Control in recent years have been revising downward the levels of environmental lead that it would consider safe. At one time, lead poisoning was common among those who worked with lead, but such workplace hazards have been largely curtailed. Lead has been used by humans since ancient times. It was used in ancient Egypt in coins, weights, ornaments, utensils, ceramic glazes, and solder. Lead is mentioned in the Old Testament. The Romans conveyed drinking water in lead pipes, some of which are still in operation. Roman slaves extracted and prepared the lead, describes a disease among the slaves that was clearly lead poisoning. Because of their potential toxicity, lead water pipes are no longer being installed. The greatest single use of lead metal today is in the plates of storage batteries for automobiles. The protective oxidation layer formed by lead in contact with such substances as air, sulfuric acid, and fluorine makes it highly resistant to corrosion. For this reason, lead has been used to make drainage pipes and lead chambers in sulfuric acid factories. It is also used as a roofing material. The softness and malleability of lead make it useful for sheathing telephone and television cables. Lead is used in solder because of its low melting point. When combined with tin, lead forms solder alloys that are stronger than lead alone, with melting points lower than those of either original metal. Lead has the highest density of all metals in common use, which, for example, makes it useful as a counterweight in the keels of ships. Because of their high density, lead bullets and shot encounter little air resistance and thus achieve excellent striking power. Shot is produced by allowing molten lead to drip down from heights up to 38.10 m (125 ft). The drops become spherical and are condensed by the cooling action of the air before being collected in a tank filled with water or oil. Lead's density and softness also make it highly suitable for damping sound and vibrations. To isolate them from vibration, heavy machinery and even whole buildings are placed on lead blocks. Because the effectiveness of shielding against gamma and X rays depends largely on the density of the shield, lead is used in the protective shielding of X-ray machines and nuclear reactors. Tetraethyl lead or tetramethyl lead (PbEt4) has often been added to gasoline to improve engine efficiency and reduce gasoline consumption in automobiles. Because of the toxic effect of lead on the environment, however, plans call for phasing out this use. Lead azide is sensitive to striking and is highly explosive; it is frequently used as a detonator of explosives. Lead iodide is a light yellow substance that is used as a dye in such processes as coloring bronze. It has light-sensitive properties comparable to those of silver salts. More Uses ÂÂ

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