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Strong Acids
Six acids are considered "strong" acids. They are sulfuric acid, nitric acid, hydrochloric acid, perchloric acid, bromic acid and hydroiodic acid. None of these alone will dissolve gold, although they all are hazardous to handle with without proper safety precautions.
Aqua Regia
Aqua regia ("royal water") is a mixture of two strong acids -- 3 parts hydrochloric acid to 1 part nitric acid. Aqua regia not only retains the strong acidic properties of the strong acids, but it also releases free chlorine gas. It is the free chlorine that complexes with the gold to form gold chloride ion (AuCl) that allows the gold to become soluble.
Chlorine Gas
Adding gold to a solution of water with chlorine gas being pumped into it also can dissolve gold. As with the aqua regia solution, the chlorine ions interact with the gold to form gold chloride ions which will go into solution. This reaction may not go as fast as the aqua regia procedure, as it does not have the additional strong oxidizing effect of the nitric acid.
Non-acidic methods
There are non-acidic compounds that also have a strong affinity for gold and can dissolve it. These are hazardous materials and should not be used if one is not trained in their use. Sodium cyanide or potassium cyanide readily disassociate and form ions with gold molecules to dissolve the gold. Liquid mercury is a gold solvent and can be used to separate gold from surrounding rock.