Aqua Regia dissolves Gold - Periodic Table of Videos


Uploaded by periodicvideos on 11.01.2010

Transcript:
Aqua Regia, Royal Water, is a mixture of two acids,
hydrochloric acid and nitric acid. The reason it’s called
‘royal water’ is because it dissolves gold. Gold has
fascinated people for thousands of years because it
doesn’t dissolve in anything, doesn’t tarnish, always
looks nice and shiny, and this was a material that could
dissolve it. So it was really something, the lion of the
acids, like the lion of an animal, king of the animals!
So Aqua Regia is a mixture of hydrochloric acid and
nitric acid, but each of those is a mixture of something
in water, a solution of something in water. So
hydrochloric acid is hydrogen chloride, which is an atom
of chlorine bonded to an atom of hydrogen, now the
importance of this is that when the HCl dissolves in
water the bond between them breaks and you get
chloride ions, Cl-, and H+, hydrogen ions, and it is the
formation of hydrogen ions which makes something an
acid. So when you have the mixture of all these ions you
get quite a soup. Now the exciting thing about Aqua
Regia is that neither of the two acids by themselves can
dissolve gold. Nitric acid doesn’t really react with gold
very much at all, and HCl, hydrogen chloric acid can’t
get a hold of the gold to get the reaction going. But if
you can gold into solution then it can react. So what
happens is as follows: the nitric acid can attack the gold
and a very small proportion of the gold, relatively few
atoms, are persuaded into going into solution and there
is an equilibrium, some are coming off and an equal
number are going back on to the surface. So if you just
have the gold in there it appears to be unaffected. If you
were on an atomic scale you would see atoms coming
off and others going back, but nothing changes. But
when you have the hydrogen chloride there, the chloride
ions can attack the gold and turn them into a different
compound so they can’t go back. So more and more and
more gold atoms come off, until eventually all of it is
converted to another acid, which is an acid of gold with
four chlorine atoms around it. This material is then quite
soluble in water, so provided you don’t have too much
gold it will all dissolve. And once you have the solution,
you will get a brownish solution or greenish solution
depending how much gold is in there.
I’ve never seen Aqua Regia because it’s quite a nasty
material, you have to use it in the fume cupboard, because otherwise, because of these fumes
of NO2 and unless you want to dissolve gold, you don’t
usually need this. It is a pretty good cleaning material
if you have glassware, beakers things like that that you
can’t clean any other way. The sort of last resort is
‘we’ll try Aqua Regia to see if that’ll do it?’ you know
really bang the dirt. And it sometimes works, not always,
depends what the dirt is.
Aqua Regia is Latin, for Aqua is the Latin for ‘water’ as
in ‘aqueduct’ and also we use it in English and the word
‘aqueous’ and things like that. There is…and Regia is the
Latin for ‘royal’ as in ‘regal’, and comes from the word
Rex which means the word ‘king’ in Latin.
Well there is quite a famous story about Aqua Regia and
the institute of Niels Bohr, the Nobel Prize winner, who
had an institute in Copenhagen and at the beginning of
the Second World War when Denmark was still neutral,
two German Nobel Prize winners, Max von Laue and
Frank who had won the Nobel Prize before the Second
World War sent their gold medals for safe-keeping to
Denmark and someone in the lab at, in Denmark, dissolved them in Aqua Regia and just put
the bottle on the shelf where there were lots of other bottles
of chemicals and they stayed there the whole
war because nobody realised that they were gold and after
the war the gold was recovered, the medals were re-struck
and given back to the Nobel Prize winners.