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Ancient rock paintings

show the characteristic red color of cinnabar, a natu­rally occurring sulfide ore of mercury.



highly concentrated, small amounts in an ore called cinnabar. The ore is converted by heat to a metal. The chief producers of mercury are Spain, Italy, and the Soviet Union.

When it is heated or cooled, mercury ex­pands and contracts evenly. Through a wide range of temperatures, it remains in its liquid form. Because of these two properties, mer­cury is widely used in thermometers and some types of barometers. It is also used in long-life batteries for radios, watches, cam­eras, and calculators. Because mercury con­ducts electricity well, it is used in some elec­tric switches and relays. The liquid metal helps them operate silently and efficiently. Mercury vapor is employed in fluorescent tubes and in electric lamps, producing a high-intensity blu­ish light rich in ultraviolet radiation. An alloy of mercury—called amalgam—includes a mixture of silver and mercury. It is used by dentists to fill cavities in teeth.

Mercuric compounds are used in manufac­turing ammunition and in producing vermil­ion, a red pigment for paint. Medically, differ­ent mercuric compounds are used to treat kidney disease and to disinfect wounds. A common mercuric compound is the antiseptic Mercurochrome.

Mercury was once used so extensively in agriculture and industry that too much found its way into the environment, especially into plants and animals that people use for food. Mercury is poisonous. Although small amounts are relatively harmless, the body has difficulty in ridding itself of it. Thus, mercury


can gradually build up to dangerous levels— both in the human body and in the bodies of animals we use for food. Since 1972, the U.S. government has restricted the use of mercury in agriculture and industry.


Cadmium sulfideoccurs in several forms. The yellow variety is employed as a pig­ment (called cadmium yel­low) in paints, such as that used on the earthmovers.


 


Fact entries

Zinchas been known under various names for at least 2,500 years, usually as an alloy. The first complete study of zinc was published in 1746 by Andreas Marg-graf, a German chemist. At.


no. 30; at. mass 65.39; m.p. 419.58° C; b.p. 907° C

Cadmiumwas discovered in 1817 by the German chemist Friedrich Stromeyer (1776-1835). He


found it as an impurity in a type of zinc carbonate called cadmia, from which the element derives its name. At. no. 48; at. mass 112.41; m.p. 320.9° C; b.p. 765° C.


Mercurywas named after the Roman god Mercury. Its symbol, Hg, is derived from the Latin Hydrargyrum, meaning liquid silver. The element has been known since ancient times. Sam-


ples have been found in tombs dating from about 1500 B.C. At. no. 80; at. mass 200.59; m.p. -38.87° C; b.p. 356.58° C.


 
 

  3A 4A
 
  B c
  10.81 12.011
 
  Al Si
26.9815 28.0855
Zn Ga Ge
65.39 69.72 72.59
Cd In Sn
112.41 114.82 118,71
Hg Tt Pb
200.59 204 383 207.2

The boron groupof ele­ments makes up Group 3A of the periodic table. Boron and aluminum are the most important members. The re­maining three have few uses.




The boron group

Group 3A of the periodic table is the boron group of elements: boron (B), aluminum (AD, gallium (Ga), indium (In), and thallium (Tl). All except boron are metals.

Boron

Boron is a yellowish-brown crystal in its pure nonmetallic state. In nature, however, it is never found free—it is always combined with oxygen. Boron is sparsely distributed through­out the earth's crust. Although it is found in the volcanic regions of Italy, most of the world's known supplies are in the desert regions of southern California.

The principal compound of boron is borax, which has been known for over 2,000 years as a mild antiseptic, detergent, and water-soften­ing agent. Today, borax is also used to make porcelain enamels for sinks, stoves, and refrig­erators. It helps make a hard glaze for dishes and extra-strength glass for thermometers and glass cookware. There are many other indus­trial uses for borax.

Borazon is an artificial crystal made of boron and nitrogen. Aluminum boride is made


of aluminum and boron. Both these com­pounds can be used as substitutes for dia­mond in metal grinding and polishing. Bora­zon is the only substance that can scratch a diamond, although a diamond can also scratch borazon.

Boron is used in making certain parts for nuclear reactors and other nuclear devices. It is also used in the aerospace industry for jet nozzles and various rocket parts. Small amounts of boron are essential for good plant growth. Large amounts, however, are poison­ous.

Aluminum

Aluminum is a lightweight, silver-colored metal that is the most abundant metallic ele­ment, making up about 8 per cent of the earth's crust. After oxygen and silicon, it is the third most common element. Aluminum is a part of most clays, but the most important source is bauxite, an impure aluminum com­pound.

Very little aluminum is used in its pure form. It is too soft and has little strength. Pure aluminum is usually used for jewelry or for decorative trim on cars and appliances. Most often, aluminum is combined with up to 15 per cent of one or more various metals. These


 


               
   
 
   
 
   
 
 



Bauxite

Molten alumina and cryolite in

Li

Anode reaction 62--2eU0'' 0 + O -ˆ>,t


The principal stages in the industrial extraction of aluminumfrom its chief ore, bauxite, are shown in the flow diagram (right). The key stage is the electrolysis of molten alumina and cryo­lite. The details of this proc­ess are illustrated in the dia­gram bottom right. The process produces molten, pure aluminum. Electrolysis requires a large amount of electricity, so aluminum extraction plantsIbelow) are often sited in areas where electricity is cheap-near hydroelectric power stations, for example.


Pulverized bauxite heated with sodium : hydroxide to form | sodium aluminate solution

;Molten aluminum run ■ off and cast into ingots for subsequent fabrication into finished products


Mixture filtered to remove insoluble impurities (mainly silicates, iron oxides and titanium oxides)

Molten alumina and cryolite mixture electrolysed to produce pure aluminum (also molten)


 

Mixture filtered to separate out the hydrated alumina crystals

Addition of alumina crystals causes more hydrated alumina to crystallize out of the mixture

I

Cryolite

H JBBHKKUM

................... . '■'"■".'................................................................................

 

Pure alumina powder dissolved in molten cryolite (sodium aluminum fluoride)
' Cathode reaction Al3+ + 3e - Al

Molten, pure Molten aluminum Graphite aluminum out formed at cathode cathode

Hydrated alumina crystals heated to produce pure alumina (in the form of a white powder)



Major groups of elements: The boron group 35





f^l

include copper and magnesium (for increased strength and hardness), manganese (for resist­ance to rust), silicon (for ease in casting), tin (for easier shaping), and zinc (for added strength). Other metals may also be used for special purposes.

Aluminum, when alloyed with these metals, has several very valuable properties. It is one of the lightest metals, weighing about one-third as much as steel. It has replaced steel for many parts in trucks and cars, making lighter vehicles that use less fuel. Some aluminum al­loys are as strong as steel. They are used in the bodies of trucks and airplanes and for guard­rails along highways. Aluminum does not cor­rode. It is invaluable for outdoor use, such as for window frames, roofs, gutters, lawn furni­ture, and boats.

Although it conducts electricity only two-thirds as well as copper, aluminum weighs one-third as much. In the United States, most high-voltage power lines are made of alumi­num wire. Aluminum conducts heat very well and is used in cookware. It reflects about 80 per cent of the light that strikes it and is used in lighting fixtures. It reflects heat equally well and is used for roofing to keep buildings cooler in hot weather.


Date: 2015-12-11; view: 572


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