Gold is so soft that when jewellery is made from it, it is usually alloyed with one or a combination of copper, nickel, and silver. If gold jewellery is immersed in an individual acid, the gold itself will not dissolve, although the alloying metal does dissolve. If a chunk of metal is fake gold--if it consists of a metal sample in which there is very little to no gold--it will dissolve in nitric acid.
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Before treatment with acid, the density of the jewellery should be evaluated. Gold has a density of 19.3 grams per cubic centimetre of metal. Copper and nickel are less dense than that--8.96 and 8.91 respectively. Silver has a density of 10.49. If the density of the jewellery is not appreciably more than 9, the piece is fake. Even if it has a trace of gold in it, perhaps a plated covering, it is less than the 10-Karat designation of the lowest grade of solid gold jewellery.
Pure gold, called also 24-Karat gold, dissolves in alkali cyanide and in a few other unusual substances. It will not, however, dissolve in individual mineral acids. It does dissolve in certain combination acids, most notably "aqua regia" (a mixture of concentrated hydrochloric and nitric acids). Gold dissolved in aqua regia produces a dark amber-to-orange colour.
Acid Test--Nickel, Copper, and Silver
Nickel produces a green colour in nitric acid. After treatment--if there is no metal left--there was no real gold in the first place. The jewellery was fake. If the majority of the metal remains, the metal was precious.
Copper behaves in a similar fashion to produce a deeply blue solution. Again, if there is no metal left after treatment, there was no gold present. The jewellery was fake. If the majority of metal remains, the metal was precious.
Silver does not turn colour in nitric acid; however, it does dissolve readily.
Even lead is much lighter than gold. Many other metals that might be used to produce fake gold actually are more valuable than gold. Some are very hard to come by, such as spent uranium.
Very rarely, tungsten has been used to make fake gold and mostly in the form of electroplated bars. Unfortunately, tungsten dissolves only slowly in nitric acid.
The metals previously mentioned are the ones likely to be used for fake gold. Applying the density and acid tests will enable one to determine whether a piece is real or is fake.
Unless one is trained in the use of concentrated acid, it is best not to attempt the acid tests to determine whether or not a piece of gold is fake. Rather, it is advisable to seek a paid evaluation by one's local jeweller.
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