Gold & Silver

By peace | January 8, 2007

Gold is a yellow metal which does not oxidize in the air, but always stays shiny. Very few chemical agents attack it : only chlorine, bromine, aqua regia ( a mixture of hydrochloric acid and nitric acid). Gold is also the most malleable and ductile of all metals: it can be made into very thin, almost transparent sheets and into gold thread which weighs only half a milligram per metre. It is, therefore, particularly suitable for making necklaces and similar items.

Gold is also very dense and heavy and this makes it very suitable for coinage, because any attempt at counterfeiting using lighter metals can easily be discovered. It is even heavier than lead, with a specific gravity of 19.3 grams per cub centimetre (against 11.34 for lead). In addition, its thermal and electrical conductivities are amongst the highest of any metal, but its mechanical strenth is low and it is not a very hard metal. For these reasons, it is nearly alwasy used alloyed with other metals. Alloyed with copper, for example, it assumes a reddish or pinkish colour, with silver it becomes greenish and in conjunction with nickel or palladium it becomes very hard and white: it is then called white gold, which is often used in place of platinum.

The chemical and physical properties of gold, and specially of its alloys, suggest its use in many applications. Whenever possible, however, other, less expensive metals are substituted for it.

In dentistry, it is used(in the form of alloys with other metals) to make replacement teeth and fillings since it does not corrode and its chemically inert. Contacts in many electronic circuits are also made from gold, as are conductors and the internal contacts in the most sophisticated miniaturized integrated circuits. in this last case, the soldering of the gold wires is done under the microscope, using laser beams. At certain times in the past, gold has been considered as a medicament and even until the end of the last century a gold-based preparation was prescribed as an antidote to tuberculosis.

Silver is a chemical element with the symbol Ag (Latin: argentum). A soft white lustrous transition metal, it has the highest electrical and thermal conductivity of any metal and occurs in minerals and in free form. This metal is used in coins, jewelry, tableware, photography, and in mirrors.

Silver is a very ductile and malleable (slightly harder than gold) univalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost and tarnishability has prevented it from being widely used in place of copper for electrical purposes.

Pure silver also has the highest thermal conductivity, whitest color, the highest optical reflectivity (although it is a poor reflector of ultraviolet light), and the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for the effect of light upon them. This metal is stable in pure air and water, but does tarnish when it is exposed to ozone, hydrogen sulfide, or air containing sulfur. The most common oxidation state of silver is +1 (for example, silver nitrate; AgNO3); a few +2 (for example, silver(II) fluoride; AgF2) and +3 compounds (for example, silver(III) persulfate; Ag2(SO5)3) are also known.

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