Physical and chemical properties of gold

Gold is a precious metal and was one of the first metals discovered and exploited by people. It has been around for more than 6,000 years. It is an important raw material for making jewelry and currency, so it is known as the “king of metal”. With the development of science and technology, gold has become more and more widely used and consumes more and more. It is one of the important war preparation materials, which has good physical and mechanical properties and high chemical stability.
Gold is the first group element in the periodic table of the chemical element, with an atomic number of 79 and an atomic weight of 197. Gold has a large proportion but is lighter than platinum . 's density 19.32 g / cm ^3, for example of 46 mm diameter gold balls, which have a weight of 1 kg. Relatively large density of the metal (g / cm ³⁾ arranged in a density: 21.15 platinum, gold, 19.32, 13.6 Hg, Pb 11.4 Ag 10.5.
The color of pure gold is golden yellow, which varies with the content of impurities; silver and platinum can make the color of gold lighter; copper can make the color of gold darker; colloidal gold shows different according to the degree of dispersion. colour.
Gold is a very soft metal with a hardness of 2 to 3.7, second only to aluminum and tin . With nails, streaks can be drawn on the surface of pure gold. This kind of property makes it easy to work like decorations and currency, but it is easy to wear, so that it loses its original appearance and luster, and even causes weightlessness. Therefore, when making jewelry and currency, it is necessary to add a certain amount of silver and Copper, to increase the hardness to overcome the above weaknesses.
Gold has good ductility and is higher than ordinary metals. 1 gram of pure gold can be drawn into a filament of 3.420 meters long and can be pressed into a gold foil having a thickness of 0.23 x 10-8 mm. If a small amount of impurities are mixed in the pure gold, the mechanical properties will change significantly. When 0.01% of lead is mixed in pure gold, it becomes brittle, and ductility and forgeability are greatly reduced.
The volatility of gold is very small. At 1000-1300 °C, the amount of volatilization of molten gold is small, but the amount of volatilization increases with the increase of temperature.
The melting point of gold is 1064.43 ° C (International Practical Temperature Scale in 1968). If the gold is heated close to the melting point, it can be welded like iron , and the gold particles can be fused into a gold nugget; at a lower temperature, it must be pressed to form a block.
Gold has good electrical and thermal conductivity. Gold's conductivity is second only to silver and copper, and ranks third among metals. The thermal conductivity of gold is 74% of silver.
The chemical nature of gold is very stable. Gold is not directly oxidized by oxygen when it is replaced by temperature or high temperature. At normal temperature, gold does not work with a single inorganic acid (such as hydrochloric acid, nitric acid, sulfuric acid), but aqua regia (three parts of hydrochloric acid and one part of nitric acid) and cyanide solution dissolve gold well.
Gold can not only be alloyed with other precious metals, but also alloys or compounds with many other metals, and can also be enriched in these metals. Common alloys are: gold and silver alloys, gold and copper alloys, gold and silver copper alloys and gold amalgam. The reason why gold forms alloys with many other metals is that the atomic radius of gold is very close to the atomic radius of these metals.
Kim often exists in the form of natural gold. Natural gold containing silver and copper impurities is substantially different from alloys of these metals. The alloy is a product obtained by melting and solidifying these metals, and has a homogeneous structure; natural gold is precipitated from an aqueous solution.
On the surface of natural gold, there is often a film of iron oxide or fine-grained gangue, called the wrap layer. At this time, the surface color of gold may be brown, dark brown or even black. On the one hand, this parcel affects the viewing of gold, and on the other hand it adds to the difficulty of recycling it.

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