Next week, the winners of this year’s batch of Nobel Prizes will be announced. Every winner receives a Nobel Prize medal, featuring a portrait of the founder of the prizes, Albert Nobel. This graphic takes a look at the composition of these medals – and how chemistry was once used to make them disappear!

You might think that the Nobel Prize medals are made from solid gold, but much like the Olympic gold medals their appearance is somewhat deceptive when it comes to their composition. In fact, the Nobel medals have never been made from 24 carat gold; since the inception of the prizes the medals were made from 23 carat gold, which is around 96% pure. However, since 1980 the prizes have been made from only 18 carat gold (green gold), with a 24 carat gold coating to preserve their golden external appearance. They’ve also seen a decrease in weight, down from roughly 200 grams per medal to 175 grams per medal.

Every Nobel Prize medal has an interesting story behind it, if only for the reason behind its award. The stories of some, though, are more interesting than others. Perhaps the most famous is that of the medals of German physicists Max von Laue and James Franck – and though it’s well-known, it bears repeat tellings!

Back in the Second World War, in April 1940, Denmark was being invaded by Germany. At famous physicist Niels Bohr’s laboratory, based in Copenhagen, a group of scientists were trying to find a way to hide von Laue and Franck’s Nobel Prize medals, which had been smuggled out of Germany to the laboratory. The act of smuggling them out of Germany was itself a criminal one at the time, as no gold was permitted to leave the country. The Nazis suspected what had happened, but needed to find the medals to have solid evidence.

This is where Hungarian chemist George de Hevesy comes in. His initial suggestion to bury the medals was discounted as being unlikely to succeed, but his second, to dissolve the medals, had promise. The only slight problem was that gold is extremely unreactive, and it’s hard to get it to dissolve in anything, with one exception: aqua regia.

Aqua regia is a mix of two different acids, hydrochloric acid and nitric acid, usually in a 3:1 molar ratio. Individually, neither of the acids are capable of dissolving gold, but combined they can. This is because the nitric acid oxidises a minuscule amount of gold to form gold ions, and the hydrochloric acid then provides chloride ions which react with the gold ions. This removes gold ions from the solution, allowing the nitric acid to oxidise a little more. Eventually, an orange solution of chloroauric acid is produced.

Even though aqua regia does dissolve gold, it does so very slowly. Just a small amount of gold can take some time, and de Hevesy had two 200 gram medals to dissolve. There’s no record of exactly how long it took, but eventually the medals were rendered essentially invisible, dissolved in an orange solution, hidden amongst an array of other chemical solutions.

The plan was a great success – though the Nazis combed the lab from top to bottom searching for the medals, they never suspected de Hevesy’s chemical trick, and the medals survived the war undiscovered. Many years later, the gold was recovered from the solution, by use of sodium bisulfite or a similar reagent which precipitates out the elemental gold. It was then delivered back to the Nobel Prize committee, who were able to recast the medals.

De Hevesy received a Nobel Prize medal himself just a few short years after his efforts in dissolving two of them. Of course, it wasn’t awarded for his gold-dissolving escapades in Copenhagen, but for his work on using isotopes as tracers.

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