Silver is a soft, white metal. It was one of the first metals used by human beings. People have used silver for ornaments and for money since about 4000 B.C. Many beautiful objects, including jewelry, fine tableware, religious decorations, coins, and mirrors, are made of silver. Silver also plays an important role in dentistry, medicine, photography, and electronics.
Most countries of the world have deposits of silver and silver ore. However, mining silver is expensive, and the metal can be recovered economically in only a few places.
Silver in its pure form is called metallic, free, or native. Pure silver is extremely soft. As a result, a small amount of another metal—usually copper—is generally added to increase the silver’s hardness and strength. For example, sterling silver is an alloy (mixture) of 92.5 percent silver and 7.5 percent copper. Silver plate is an object made of a base metal, such as steel, that is coated with a thin layer of silver or silver alloy.
Uses of silver.
Silver, along with copper and gold, is called a coinage metal. It has been used to make coins for thousands of years. In the past, the United States had silver coins that contained 90 percent silver and 10 percent copper. But in 1965 and 1970, Congress passed laws to eliminate silver from new dimes, quarters, half dollars, and dollars. The U.S. Mint uses a very high silver content to make some commemorative coins for collectors and some bullion coins for investors.
Silversmiths craft art objects from silver. The metal is also used by the electrical and electronic equipment industry for wire and other items, because silver conducts electricity better than do other metals. Doctors use thin plates, wires, and drainage tubes made of silver during surgery, because silver helps kill bacteria. Dentists fill cavities with silver amalgam—a mixture of silver, tin, and mercury.
Silver compounds also have many uses. Compounds of silver include silver nitrate, silver bromide, and several silver oxides. Silver nitrate is one of the few water-soluble silver compounds, and is used to make silver plate and silver mirrors. Silver bromide plays an important role as the light-sensitive chemical in photographic film (see Photography (Exposing the film) ). Manufacturers of batteries use silver oxides to make small, powerful batteries that are used in calculators, hearing aids, and watches.
Properties of silver.
Silver has an atomic number (number of protons in its nucleus) of 47. Its relative atomic mass is 107.8682. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. Silver’s atomic symbol, Ag, comes from the Latin word for silver, argentum. Silver melts at 961 °C and boils at 2193 °C. At 20 °C, it has a density of about 10.49 grams per cubic centimeter (see Density ). Chemists classify silver as a transition metal . For information on the position of silver on the periodic table, see the article Periodic table .
Silver reflects 95 percent of the light that strikes it, making it the most lustrous (shiny) of the metals. Silver conducts heat and electric current better than any other metal does. It is second only to gold in ductility (the ability to be drawn out into fine wires) and malleability (the ability to be hammered into various shapes).
Silver, like gold, does not react chemically with most substances. However, the presence of sulfur compounds causes silver to develop a black or gray coating of silver sulfide called tarnish. Because polluted air contains these compounds, silver tarnishing is a greater problem today than in the past.
Sources of silver.
China, Mexico, and Peru lead the world in the production of silver. The main silver-producing states of the United States include Alaska and Nevada. The leading silver-mining provinces of Canada include British Columbia, Ontario, and Quebec. Zacatecas is Mexico’s leading silver-producing state.
Silver occurs in deposits of native metal and as silver ores. Native silver mines provide only a small amount of the world’s silver. The most common silver ores contain the mineral argentite or the compound silver sulfide. Silver often occurs along with such metals as copper, gold, lead, and zinc. Miners obtain about 80 percent of the world’s silver as a by-product of mining and processing these metals.
Extracting and refining silver.
There are several methods of extracting silver from ores. Most of the world’s silver is extracted from copper and lead ores. These ores are first crushed and then smelted, producing a mixture that contains the primary metal and small quantities of silver. In the process of refining copper, the silver is separated from the copper to form a mixture called sludge. The sludge is removed and treated with nitric acid to dissolve the silver. The silver is then recovered by electroplating (see Electroplating ).
Silver is extracted from metallic lead ore by the Parkes process. In this process, zinc is added to molten (melted) lead ore to form a solid alloy with the silver contained in the ore. This alloy, which is less dense than molten lead, floats to the surface and is raked off. Heating the alloy then removes the zinc from the silver.
Once extracted, silver is removed from the extracting solution. A process called electrolysis is then used to refine and purify the extracted silver. In electrolysis, two electric conductors called electrodes are submerged in a liquid. The impure silver serves as one of the electrodes, called the anode. A strip of pure silver metal serves as the other electrode, called the cathode. Refiners dip the two electrodes into a solution of silver nitrate and nitric acid. An electric current sent between the electrodes causes the anode, or impure silver, to dissolve. The impurities in the silver fall to the bottom of the solution, and pure silver crystals collect on the cathode. These crystals are scraped off, melted, and cast into bars of silver. See Electrolysis .
