What is Gold?
An attractive and highly valued metal, gold has been known for at least 5500 years. Gold is sometimes found free in nature but it is usually found in conjunction with silver, quartz (SiO2), calcite (CaCO3), lead, tellurium, zinc or copper. There is roughly 1 milligram of gold dissolved in every ton of seawater, although extracting it currently costs more than the gold is worth. It has been estimated that all of the gold that has currently been refined could be placed in a cube measuring 20 meters on a side.
Gold is the most malleable and ductile of all known metals. A single ounce of gold can be beaten into a sheet measuring roughly 5 meters on a side. Thin sheets of gold, known as gold leaf, are primarily used in arts and crafts for gilding. One sheet of gold leaf can be as thin as 0.000127 millimeters, or about 400 times thinner than a human hair.
Pure gold is soft and is usually alloyed with other metals, such as silver, copper, platinum or palladium, to increase its strength. Gold alloys are used to make jewelry, decorative items, dental fillings and coins. The amount of gold in an alloy is measured with a unit called a karat. One karat is equal to one part in twenty-four, so an 18 karat gold ring contains 18 parts pure gold and 6 parts alloy material.
Gold is a good conductor of heat and electricity and does not tarnish when it is exposed to the air, so it can be used to make electrical connectors and printed circuit boards. Gold is also a good reflector of infrared radiation and can be used to help shield spacecraft and skyscrapers from the sun's heat. Gold coated mirrors can be used to make telescopes that are sensitive to infrared light.
A radioactive isotope of gold, gold-198, is used for treating cancer. Gold sodium thiosulfate (AuNa3O6S4) is used as a treatment for arthritis. Chlorauric acid (HAuCl4) is used to preserve photographs by replacing the silver atoms present in an image.