It’s April 9th, 1940, Germany has invaded Denmark and Copenhagen is now occupied. You are a world-renowned physicist whose institute has helped Jewish refugees for years. The Nazis know this and you know that makes you a target.
To complicate matters, you are also in possession of two Nobel prizes that were sent to you for safekeeping—one belonging to Max von Laue, a German physicist who was highly critical of the Nazi regime, and the other belonging to James Franck, a German physicist of Jewish descent. Each medal is 200g of 23-carat solid gold. The Nazis have declared it illegal for any gold to leave Germany so by sending their medals to you, they may have just signed their own death warrants.
What do you do?
This was the problem facing Niels Bohr, a Danish Physicist and founder of the Institute of Theoretical Physics at the University of Copenhagen, (now known as the Niels Bohr Institute). With German soldiers swarming throughout the city, he knew he had hours at best to make sure no one would find the medals. Hiding or burying them was out of the question since Bohr knew that the Nazi forces were likely to dig up the gardens in search of any hidden contraband. Luckily, a Hungarian chemist (and future Nobel prizewinner himself) named Georgy de Hevesy was also at the institute that day and he suggested something... a little more drastic.
Instead of simply hiding the medals, he proposed they dissolve them.
The Science-y Bit
Aqua regia meaning “royal” or “king’s water” is a mixture of 3 parts hydrochloric acid to 1 part nitric acid and was named for its unique ability to dissolve the noble metals of gold and platinum. It works like this: the nitric acid oxidises a tiny amount of gold to form gold ions. The hydrochloric acid provides chloride ions which react with the gold ions, removing them from the solution. Eventually, the solid metal disappears and an orange-coloured solution of chloroauric acid is produced.*
The process of dissolving gold in aqua regia is time-consuming and although there is no record of how long it took de Hevesy to dissolve all 400g of gold, we do know that he did manage to complete the task on time. He stashed the bottle full of liquid-Nobel-prize on top of a high shelf before the Nazi soldiers arrived.
According to Sam Kean in his book, The Disappearing Spoon:
“...When the Nazis ransacked Bohr's institute, they scoured the building for loot or evidence of wrongdoing but left the beaker of orange aqua regia untouched.”
But the story doesn’t end there.
The Surprisingly Happy Ending for a Story Involving Nazis
De Hevesy was forced to flee to Stockholm in 1943 but when he returned to Copenhagen after the war, he found the bottle of aqua regia right where he had left it, completely undisturbed. He then used sodium bisulphate (or a similar reagent) to reverse the entire process, precipitating out the metal into a lump of raw gold which he sent back to the Nobel Foundation in Sweden in January of 1950.
The Nobel Foundation used this gold to recast both James Franck and Max von Laue’s medals and, in 1952 both physicists were reunited with their prize.
So there you have it, a real-life magic trick. Who says chemistry is boring?
*If you’d like a more thorough explanation and demonstration of Aqua Regia at work, check out this video by Periodic Videos (it’s always fun to watch this motley crew of scientists at work so check out a few of their other videos while you’re there!).