Gold is an extremely inactive metal. Even in nature, it occurs mainly in the form of nuggets (as opposed to alkali and alkaline earth metals, which are found exclusively in minerals or other compounds). When exposed to air for a long time, it is not oxidized by oxygen (this noble metal is also valued for this). Therefore, it is quite difficult to find what gold dissolves in, but it is possible.
Industrial method
When extracting gold from the so-called gold sands, you have to work with a suspension of approximately equally small particles of gold and grains of sand, which must be separated from each other. You can do this by flushing, or you can use sodium or potassium cyanide - there is no difference. The fact is that gold forms a soluble complex with cyanide ions, while 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 a complex is obtained. With insufficient air or by itself without cyanideno reaction.
Now this is the most common way of industrial production of gold. Of course, there are still many stages before obtaining the final product, but we are specifically interested in this stage: cyanide solutions are what gold dissolves in.
Amalgam
The amalgamation process is also used in industry, only when working with ores and hard 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 solid in the amalgam.
During amalgamation, the rock is wetted with liquid mercury. However, the process of "pulling" gold into an amalgam is long, dangerous (mercury vapor is poisonous) and ineffective, so this method is rarely used anywhere.
Royal vodka
There are many acids that can corrode living tissue 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 wonderful properties of this infernal cocktail are due to the fact that acids are taken in very high concentrations, which greatly increases their oxidizing power.
Aqua regia begins to act with the fact that nitric acid begins to oxidize hydrochloric acid first, and during this reaction atomic chlorine is formed - a very reactive particle. It is she who goes to attack gold and forms a complex with it - chloroauric acid.
This is a very useful reagent. Very often, gold is stored in the laboratory in the form of a crystalline hydrate of such an acid. For us, it serves only as confirmation that gold dissolves in aqua regia.
It is worth paying attention once again to the fact that it is not one of the two acids that oxidizes the metal in this reaction, but the product of their mutual reaction. So if we take, for example, only "nitrogen" - a well-known oxidizing acid - nothing will come of it. Neither concentration nor temperature can make gold dissolve in nitric acid.
Chlorine
Unlike acids, in particular hydrochloric acid, individual substances can become what gold dissolves in. Widely known household bleach is a solution of gaseous chlorine in water. Of course, you can’t do anything with an ordinary store-bought solution, you need higher concentrations.
Chlorine water acts as follows: chlorine dissociates into hydrochloric and hypochlorous acids. Hypochlorous acid decomposes under light into oxygen and hydrochloric acid. In such decomposition, atomic oxygen is released: like atomic chlorine in the reaction with aqua regia, it is very active and oxidizes gold for a sweet soul. The result is again a complex of gold with chlorine, as in the previous method.
Other halogens
Except for chlorine,gold is also well oxidized by other elements of the seventh group of the periodic table. To fully say about them: "what gold dissolves in" is difficult.
Gold can react differently with fluorine: in direct synthesis (with a temperature of 300-400°C), gold III fluoride is formed, which is immediately hydrolyzed 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, by the action of the strongest oxidizing agents: fluorides of noble gases (krypton, xenon), gold fluoride V can also be obtained. Such fluoride generally explodes on contact with water.
Things are somewhat easier with bromine. Bromine is a liquid under normal conditions, and gold disperses well in its solutions, forming soluble gold bromide III.
Gold also reacts with iodine when heated (up to 400°C), forming gold iodide I (this oxidation state is due to the lower activity of iodine compared to other halogens).
Thus gold certainly reacts with halogens, but whether gold dissolves in them is debatable.
Lugol's solution
In fact, iodine (common iodine I2) is insoluble in water. 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 of those 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. used,as a rule, for gold etching - a process in which the interaction is 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 accessible).
Bonus
Saying that single acids are something in which gold does not dissolve, we lied a little - 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 high concentrations they work wonders. The reaction produces its gold perchlorate s alt - yellow and unstable.
Of the acids in which gold dissolves, there is also hot concentrated selenic acid. As a result, a s alt is also formed - red-yellow gold selenate.
