silver, metallic chemical element; symbol Ag [Lat. argentum]; at. no. 47; at. wt. 107.8682; m.p. 961.93C; b.p. 2,212C; sp. gr. 10.5 at 20C; valence +1 or +2. Pure silver is nearly white, lustrous, soft, very ductile, malleable, and an excellent conductor of heat and electricity. In many of its properties it resembles copper and gold, the elements above and below it in Group 11 of the periodic table. It is not a chemically active metal, being considerably below hydrogen in the electromotive series (see metal). It is, however, attacked by nitric acid (forming the nitrate) and by hot concentrated sulfuric acid. Silver is almost always monovalent in its compounds, but an oxide, a fluoride, and a sulfide of divalent silver are known. It does not oxidize in air but reacts with the hydrogen sulfide present in the air, forming silver sulfide (tarnish). Silver nitrate is the most important compound. Silver chloride, bromide, and iodide are used in still photography because of their sensitivity to light. Solutions of certain protein complexes containing silver are used as antiseptics. A mirror can be made by coating glass with metallic silver derived from the reaction of a solution of a silver ammonia complex with an organic reducing agent such as formaldehyde. Although silver can be found uncombined in nature, most silver used today is obtained from its ores. Among these the most important are argentite or silver glance (silver sulfide), which is found associated with other metal sulfides, e.g., galena; horn silver or cerargyrite (silver chloride); two ores composed of silver and antimony (in different proportions) called pyrargyrite (or ruby silver ore) and stephanite; and another ore composed of silver and arsenic sulfides called proustite. Mexico, the United States (Idaho, Montana, Arizona, Colorado, Utah, Nevada, California, New Mexico, and Texas), the former USSR, Peru, Australia, and Canada are the leading producers. The metal is prepared in various ways depending upon the nature of its occurrence; the greatest quantity is obtained in connection with the refining of lead and copper. It is separated from lead by the Parkes process, which is based upon the fact that silver is soluble in molten zinc whereas lead is not. The cyanide process has largely replaced an amalgam process in which silver is dissolved in mercury. Some of the silver produced today is used, as in the past, in making coins (see coin; money; bimetallism). Large quantities are used for silver utensils and jewelry, and in plating tableware electrolytically from a solution of sodium silver cyanide. Alloys of silver with copper, in which the copper adds hardness, are important. Coin silver is an alloy consisting of 90% silver and 10% copper. Sterling silver contains 92.5% silver and 7.5% copper. Silver alloys are used in dental amalgams and for electrical contacts. Silver was one of the first metals to be used by humans (see silverwork).
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