Aqua Regia Dissolves Gold - Periodic table of videos

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 *** 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.