Sorry I didn’t take up the gauntlet and explain the reactions taking
place when gold dissolves in aqua regia. I’ve been criticised for
being too specific re chemistry in the past so didn’t go into it.
As you all know, aqua regia is a 3:1 mixture of concentrated
hydrochloric acid and concentrated nitric acid used for etching gold.
Neither gold nor platinum are very reactive and form only a few known
compounds and as such require a highly corrosive and therefore
reactive mixture like aqua regia to enable it to be etched.
Hydrochloric acid on it’s own will not dissolve gold (although as
this thread has shown, if it’s in contact with chlorine for long
enough or regularly enough, it would react - but who’s got years to
spare in order to etch a piece of gold?!). Nitric acid on its own
won’t dissolve gold readily either, but is a powerful oxidiser and
will dissolve a virtually undetectable amount of gold, forming gold
ions (Au3+). Because hydrochloric acid is also present in aqua regia,
there is a ready supply of chloride ions (Cl-). These chloride anions
(negative ions) react with the gold cations (positive ions, Au3+) to
produce chloraurate anions (AuCl4-). This reaction is an equilibrium
reaction (reversible reaction but in this case the forward reaction
(reactants to products) is favoured. This means that the gold ions
are removed from the solution which allows further oxidation of the
gold metal by nitric acid to occur. Thus the reaction is driven
forward resulting in the dissolution of gold which is why etching
with aqua regia is possible.
I hope that is understandable. I always think if I can understand it,
then everybody else must be able to but it’s not always the case,
however, you lot on here are very intelligent so it should suffice.
If still unsure, ask again.