Gold is an extremely inactive metal. Even in nature, it occurs mainly in the form of nuggets (in contrast to alkali and alkaline earth metals, which are exclusively found in minerals or other compounds). With a long exposure to air, it is not oxidized by oxygen (this noble metal is also valued for this). Therefore, it is rather difficult to find where gold dissolves, but it is possible.
Industrial method
When mining gold from the so-called gold sands, you have to work with a suspension of approximately equally small particles of gold and sand grains, which must be separated from each other. You can do this by washing, or you can use sodium or potassium cyanide - there is no difference. The fact is that gold forms a soluble complex with cyanide ions, and the sand does not dissolve and remains as it is.
The key point in this reaction is the presence of oxygen (what is contained in the air is enough): oxygen oxidizes gold in the presence of cyanide ions and the complex is obtained. With insufficient air or by itself without cyanide, the reaction does not occur.
Now it is the most common method of industrial production of gold. Of course, there are still many stages to obtaining the final product, but we are specifically interested in this stage: cyanide solutions are what gold is dissolved in.
Amalgam
The amalgamation process is also used in industry, only when working with ores and solid rocks. Its essence lies in the ability of mercury to form an amalgam - an intermetallic compound. Strictly speaking, mercury does not dissolve gold in this process: it remains in the amalgam in solid form.
With amalgamation, the rock is wetted with liquid mercury. However, the process of "pulling" gold into the amalgam is long, dangerous (mercury vapor is poisonous) and ineffective, therefore this method is rarely used anywhere.
Aqua regia
There are many acids that can corrode living tissues and leave terrible chemical burns (up to death). However, there is no single acid in which gold dissolves. Of all the acids, only the famous mixture - aqua regia - can act on it. These are nitric and hydrochloric (hydrochloric) acids, taken in a ratio of 3 to 1 by volume. The remarkable properties of this hellish cocktail are due to the fact that acids are taken in very high concentrations, which greatly increases their oxidizing ability.
Imperial vodka begins to act with the fact that nitric acid begins to oxidize hydrochloric acid first, and in the course of this reaction atomic chlorine is formed - a very reactive particle. It is she who attacks the gold and forms a complex with it - hydrochloric acid.
This is a very useful reagent. Very often, gold is stored in the laboratory in the form of crystalline hydrate of such an acid. For us, it serves only as confirmation that gold is dissolved in aqua regia.
It is worth paying attention once again that not one of the two acids oxidizes the metal in this reaction, but the product of their mutual reaction. So if you take, for example, just one “nitrogen” - a known acid-oxidizing agent - nothing will come of it. Neither concentration nor temperature can make gold dissolve in nitric acid.
Bleach
Unlike acids, in particular hydrochloric acid, individual substances can become where gold dissolves. A widely known household bleach is a solution of gaseous chlorine in water. Of course, you won’t do anything with a regular store solution, higher concentrations are needed.
Chlorine water acts as follows: chlorine dissociates into hydrochloric and hypochlorous acids. Hypochlorous acid under the light decomposes into oxygen and hydrochloric acid. In this decomposition, atomic oxygen is released: like atomic chlorine in reaction with aqua regia, it is very active and oxidizes gold for a sweet soul. As a result, again a complex of gold with chlorine is obtained, as in the previous method.
Other halogens
In addition to chlorine, other elements of the seventh group of the periodic table also oxidize gold well. It is difficult to say fully about them: "that in which gold dissolves".
Gold can react with fluorine in different ways: during direct synthesis (with a temperature of 300-400 ° C), gold fluoride III is formed, which immediately hydrolyzes in water. It is so unstable that it decomposes even when exposed to hydrofluoric (hydrofluoric) acid, although it should be comfortable among fluoride ions.
Also, the action of the strongest oxidizing agents: noble gas fluorides (krypton, xenon) can also produce gold fluoride V. Such fluoride generally explodes on contact with water.
With bromine, things are somewhat simpler. Under normal conditions, bromine is a liquid, and gold dissipates well in its solutions, forming soluble gold III bromide.
Gold also reacts with iodine when heated (up to 400 ° C), forming gold iodide I (this degree of oxidation is explained by the lower activity of iodine in comparison with other halogens).
Thus, gold undoubtedly reacts with halogens, however, whether gold dissolves in them is a controversial statement.
Lugol's solution
In fact, iodine (ordinary iodine I 2 ) is insoluble in water. And let's dissolve its complex with potassium iodide. This compound is called Lugol's solution - and it can dissolve gold. By the way, they often lubricate the throat with a sore throat, so not everything is so simple.
This reaction also goes through the formation of complexes. Gold forms complex anions with iodine. It is used, as a rule, for etching gold - a process in which interaction occurs only with the surface of the metal. Lugol's solution is convenient in this case, because unlike aqua regia and cyanides, the reaction is noticeably slower (and the reagents are more affordable).
Bonus
Speaking about the fact that single acids - this is what gold does not dissolve in, we lied a bit - in fact, there are such acids.
Perchloric acid is one of the strongest acids. Its oxidizing properties are extremely high. In a dilute solution, they appear poorly, but in large concentrations they work wonders. During the reaction, its salt of gold perchlorate is formed - yellow and unstable.
Of the acids in which gold is dissolved, there is still hot concentrated selenic acid. As a result, salt is also formed - red-yellow gold selenate.