Copper is the metallic chemical element of atomic number 29, symbol Cu, atomic weight 63.55, specific gravity 8.96, melting point 1,985°F (1,085°C), and boiling point 4,645.4°F (2,563°C). It consists of two stable isotopes, of mass numbers 63 (69.1%) and 65 (30.9%).
Copper is one of only two metals that is not colored shades of silver or gray. Copper is reddish brown, while the metal gold is said to be a color of gold—which is a unique color that is sometimes loosely described as yellow but can be black or ruby in color when finely crushed. All other metals are silvery, with various degrees of brightness or grayness. Copper was traditionally a popular choice for coinage in many countries, including two-pence coins in England, one and two Euro cent coins, and one-cent (penny) coins in the United States. However, because a piece of copper the size of a penny has become more valuable than one cent, today's U.S. pennies are made of zinc, with just a thin coating of copper.
Copper is in group 11 of the periodic table, along with silver and gold. This trio of metals is sometimes referred to as the coinage metals, because they are relatively valuable, corrosion-free and pretty, which makes them excellent for making coins. Strangely enough, the penny is the only American coin that is not made from a copper alloy. Nickels, dimes, quarters, and half dollars are all made from alloys of copper with other metals. In the case of nickels, the main metal is, of course, nickel.
Copper is one of the elements that are essential to life in tiny amounts, although larger amounts can be toxic. About 0.0004% of the weight of the human body is copper. It can be found in such foods as liver, shellfish, nuts, raisins, and dried beans. Instead of the red hemoglobin in human blood, which has an iron atom in its molecule, lobsters and other large crustaceans have blue blood containing hemocyanin, which is similar to hemoglobin but contains a copper atom instead of iron.
History of copper
Copper gets its chemical symbol Cu from its Latin word, cuprum. It got that name from the island of Cyprus, the source of much of the ancient Mediterranean world's supply of copper.
However, copper was used long before the Roman Empire. It is one of the earliest metals known to humans. One reason for this is that copper occurs not only as ores (compounds that must be converted to metal), but occasionally as native copper—actual metal found that way in the ground. In prehistoric times, an early human could simply find a chunk of copper and hammer it into a tool with a rock. (Copper is very malleable, meaning that it can be hammered easily into various shapes, even without heating.)
Native copper was mined and used in the Tigris-Euphrates valley (modern Iraq) as long as 7,000 years ago. Copper ores have been mined for at least 5,000 years because it is easy to get the copper out of them. For example, if a copper oxide ore (CuO) is heated in a wood fire, the carbon in the charcoal can reduce the oxide to metal:
Making pure copper
Extremely pure copper (greater than 99.95%), called electrolytic copper, can be made by electrolysis. The high purity is needed because most copper is used to make electrical equipment, and small amounts of impurity metals in copper can seriously reduce its ability to conduct electricity. Even 0.05% of arsenic impurity in copper, for example, will reduce its conductivity by 15%. Electric wires must, therefore, be made of very pure copper, especially if the electricity is to be carried for many miles through high-voltage transmission lines.
To purify copper electrolytically, the impure copper metal is made the anode (the positive electrode) in an electrolytic cell. A thin sheet of previously purified copper is used as the cathode (the negative electrode). The electrolyte (the current-carrying liquid in between the electrodes) is a solution of copper sulfate and sulfuric acid. When current is passed through the cell, positively charged copper ions (Cu2+) are pulled out of the anode into the liquid, and are attracted to the negative cathode, where they lose their positive charges and stick tightly as neutral atoms of pure copper metal. As the electrolysis goes on, the impure copper anode dissolves away and pure copper builds up as a thicker and thicker coating on the cathode. Positive ions of impurity metals such as iron, nickel, arsenic, and zinc also leave the anode and go into the solution, but they remain in the liquid because the voltage is purposely kept too low to neutralize them at the cathode. Other impurities, such as platinum, silver, and gold, are also released from the anode, but they are not soluble in the solution and simply fall to the bottom, where they are collected as a very valuable sludge. In fact, the silver and gold sludge is usually valuable enough to pay for the large amount of electricity that the electrolytic process uses.
Uses of copper
By far the most important use of copper is in electrical wiring; it is an excellent conductor of electricity (second only to silver), it can be made extremely pure, it corrodes very slowly, and it can be formed easily into thin wires—it is very ductile.
Copper is also an important ingredient of many useful alloys—combinations of metals, melted together. Brass is copper plus zinc. If it contains mostly copper, it is a golden yellow color; if it is mostly zinc, it is pale yellow or silvery. Brass is one of the most useful of all alloys. It can be cast or machined into everything from candlesticks to cheap, gold-imitating jewelry that turns skin green. (When copper reacts with salt and acids in the skin, it produces green copper chloride and other compounds.) Several other copper alloys are common: bronze is mainly copper plus tin; German silver and sterling silver are silver plus copper; silver tooth fillings contain about 12% copper.
Probably the first alloy ever to be made and used by humans was bronze. Archaeologists broadly divide human history into three periods; the Bronze Age is the second one, after the Stone Age (prehistoric period) and before the Iron Age. During the Bronze Age, both bronze and pure copper were used for making tools and weapons.
Because it resists corrosion and conducts heat well, copper is widely used in plumbing and heating applications. Copper pipes and tubing are used to distribute hot and cold water through houses and other buildings.
Because copper is an extremely good conductor of heat, as well as of electricity (the two usually go together), it is used to make cooking utensils such as sauté and fry pans. An even temperature across the pan bottom is important for cooking, so the food does not burn or stick to hot spots. The insides of the pans must be coated with tin, however, because too much copper in our food is toxic.
Copper corrodes only slowly in moist air—much more slowly than iron rusts. First, it darkens in color because of a thin layer of black copper oxide, CuO. Then, as the years goes by, it forms a bluish-green patina of basic copper carbonate, with a composition usually given as Cu2(OH)2CO3. (The carbon comes from carbon dioxide in the air.) This is the cause of the green color of the Statue of Liberty, which is made of 300 thick copper plates bolted together. Without traveling to New York City, a person can see this color on the copper roofs of old buildings such as churches and city halls.
Compounds of copper
In its compounds, copper can have a valence of either +1 (cuprous compounds) or +2 (cupric compounds). Cuprous compounds are not stable in water, and when dissolved they turn into a mixture of cupric ions and metallic copper.
Copper compounds and minerals are often green or blue. The most common minerals include malachite, a bright green carbonate, and azurite, a blue-green basic carbonate. Among the major copper ores are cuprite (CuO), chalcopyrite (CuFeS2), and bornite (Cu5FeS4).
Copper is mined in more than 55 nations. The world's leading producers are Chile, China, Peru, United States, Russia, and Australia.
Cupric sulfate, CuSO4 • 5H2O, is also called blue vitriol. These poisonous blue crystals are used to kill algae in the purification of water, and as an agricultural dust or spray for getting rid of insects and fungi.