COPPER, SILVER & GOLD
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5. Copper, Silver and Gold
(ISBN 1-869860-29-2)
Contents: Introduction; Copper ores; Reducing copper oxide; Mining and smelting copper ore; Electrical refining of copper; Copper as a metal; Copper as a conductor; Copper alloys: brass; Copper alloys: bronze; Copper in the environment; Copper sulphate; Copper colours; Silver; Silver and silver ores; Silver in photography; Everyday uses of silver; Gold in the Earth; Gold at the surface; Uses of gold; Gold alloys and gold plating; Key facts about copper, silver and gold; The Periodic Table; Understanding equations; Glossary of technical terms; Index |
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An element is a substance that cannot bebroken down into a simpler substance by any known means. Each of the 92 naturally occurring elements is therefore one of the fundamental materials from which everything in the Universe is made. Copper, silver and gold are often called the "coinage metals" because they are used to make most of the world's coins. One reason for this isthat none of the coinage metals is very reactive with other elements, and therefore they arevery resistant to corrosion. Copper Copper, a soft orangy-coloured metal, was one of the first metals to be used inthe ancient world. It has been exploited for at least 7000 years. Its name comes from the Latin, cuprum, which means "metal of Cyprus", an island in the Mediterranean Sea where the Romans had large copper mines. Copper is an excellent conductor of heat and electricity and is found in most of the flexible cables used in the world. Its softness also makes it suitable for tubing for water pipes and central heating systems, because itcan be soldered easily and readily bent tofit around corners. Above all, it can be mixed with other metals to make extremely useful alloys such as brass and bronze. Properties of Copper Silver Silver is a white, shiny, heavy metal. Its symbol is
Ag, after argentum, aLatin word meaning "white and
shining". Silver has been sought after since the earliest times and
is regarded as a precious metal, just as gems are
precious stones. Yet although silver is known for its precious
value, only 16% of all the silver used in the world is used
for coins and jewellery, while 40% goes to make photographic film. Much of the rest is used in
industry and health services. Mirrors for example, Properties of Silver Gold The chemical symbol for gold is Au, after the Latin word for gold, aurum. Gold is one of the rarest elements found on Earth and has been sought out by people since ancient times. Properties of Gold Reducing copper oxide Copper is mainly found in the form of a black ore, copper oxide, or a brassy-coloured ore, copper sulphide. In both cases the metal has tobe separated from its compound. The demonstration on this page shows how copper can be extracted from its compound. The ore is copper oxide, a compound of copper and oxygen. To obtain pure copper the oxygen has to be removed, using a process called reduction. The reducing agent used here is carbon monoxide The black copper oxide isplaced in a special
glass tube with a small hole near the rounded end. At the
start, the tube is full of air. This is swept away by pumping
in carbon monoxide gas. Reactions between elements and compounds are often very slow at room temperature. To speed up the rate of reaction, the copper oxide is heated using the flame from a Bunsen burner. The blue flame coming from the small hole in the
tube is produced by burning carbon monoxide gas. Notice
that the copper oxide
is glowing orange on the surface, which shows that
the oxygen has been removed. The copper oxide continues to be reduced,
andthe oxygen combines with the carbon monoxide to
form carbon dioxide. Carbon dioxide does not ignite, so
the flame goes out. Eventually all of the oxygen is removed from
the copper oxide powder, and only copper is left. Copper oxide + carbon monoxide Í copper + carbon dioxide CuO(s) + CO(g) Í Cu(s) +
CO2(g) Even the best of chemical reactions cannot completely remove all of the impurities in a metal, so ores refined in a furnace do not produce pure metals. This is why many metals are refined to their final stage of purity by electrical means in a process called electrolysis. Impure copper from the furnace is used as one of the electrodes of an electrolysis cell. The other electrode is made from a thin sheet of pure copper. The copper is then refined by placing the two electrodes in a copper sulphate bath and passing a current between them. The impure copper on the anode corrodes, and copper ions pass through the electrolyte, collecting on the cathode sheet as pure copper. When the cathode has acquired a sufficient thickness of pure copper, it is lifted from the electrolysis cell and replaced with a new electrode. Similarly, when the anode has corroded completely away, it is replaced with a new ingot of smelted metal. The cathodes are then melted down and made into wire and sheet metal. The laboratory demonstration of electrolysis and
the giant industrial equivalent are shown
on the book pages. A demonstration of electrolysis can be done using a beaker and two copper strips. A dry battery serves as the source of electrical current. The electrolyte is reagent quality copper sulphate solution. Signs of corrosion and plating are evident within minutes, and a completely corroded strip can be produced within a day or so. The process of electrolysis is shown here in a
laboratory demonstration using two copper electrodes and
copper sulphate solution as the electrolyte. An alloy is a mixture of metals. Copper forms alloys more easily than most other metals. Each of the alloying metals gives the alloy itsown special properties. Some metals make the alloy stronger, others change its colour, make it easier to machine or make it even more resistant to corrosion or wear. Themetal most often alloyed with copper is brass. Brass is one of the most widely used alloys. It is mainly copper, alloyed with between 5 and 40% zinc. Brass is often used for corrosion-resistant decorative purposes such as door furniture. It is much harder and stronger than copper and it will machine well. The most common mixture of brass contains 36% zinc and is known as common brass. The properties of brass can be altered significantly by adding small quantities ofother elements. Those most commonly used are lead, tin, aluminium, manganese, iron, nickel, arsenic and silicon. For example, by adding up to 3% lead the machinability of brass can be improved significantly. Copper-rich brasses have special uses, such as making the percussion caps of ammunition; those with between 10 and 20% zinc are called gilding metals and are used for decorative brasswork and jewellery. This form of brass will take an enamel well and is easy to braze. As the amount of zinc is increased still further, thebrass develops the property of being easily shaped when hot. This material is used to make inexpensive, but complex engineering shapes that are easy to machine. However, even higher proportions of zinc make the alloy more susceptible to corrosion when the brass is placed in water. To counteract this problem, arsenic is added to the alloy. Tin can also be added to brass to improve its corrosion resistance, and tin-zinc-copper brasses, in which there is1% tin, are known as admiralty brass because of their suitability for use on ships.
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