How the gold Nobel Prize medals were hidden from the Nazis.

The gold in a Nobel Prize medal is dense enough to make a big impression when you try to take it through an airport X-ray scanner. It's also very resistant to being dissolved—but that didn't stop one chemist who needed to hide two medals from the Nazis, as Dr Karl Kruszelnicki explains.A few years ago, a colleague of mine was leaving the town of Fargo in North Dakota after visiting his grandmother. He was at the airport and his computer bag was passing through the X-ray scanner.Professor Brian Schmidt had just won a Nobel Prize. The Nobel Prize medal is made of gold, which is very dense. The X-ray scanner at the airport was showing a completely black circle inside his computer bag, which is very unusual. The dense gold was completely stopping the X-rays.The Transport Security Administration (or TSA) officer said: "There is something in your bag."Professor Schmidt opened his computer bag and pointed at the little carry case that contained the Nobel Prize medal and said: "I think it's probably this."The TSA officer said: "What is this?" And Brian replied: "A large gold medallion."Like me and most people on Earth, the TSA officer had never actually seen a physical gold Nobel Prize medal. So he asked: "What is it made of?""Gold.""Who gave this to you?""King Gustav of Sweden," was the reply.The TSA officer asked: "Why did he give it to you?"And Brian replied: "Because I was the leader of a team that discovered that the expansion rate of the universe was accelerating."The security officer replied: "It's a Nobel Prize? You're a Nobel Prize winner?" And Professor Schmidt replied: "Yes."And once the TSA officer had worked out that he was carrying a gold Nobel Prize medallion to show to his grandmother, everything was fine.But sometimes, having a Nobel Prize medal with your name on it can be a death sentence.This was the case in April 1940, when the Nazis invaded Denmark.The trouble with gold medalsSome years earlier, two German Nobel Prize winners—one a Jew, the other a Jewish sympathiser—had sent their gold medals to legendary physicist Niels Bohr in Copenhagen for safekeeping, so they wouldn't get confiscated.Sending gold out of Hitler's Germany was illegal. If the invading Nazis found the gold Nobel Prize medals engraved with the names of the German scientists Max von Laue and James Frank, it could have led to their executions.So one member of Bohr's team, George de Hevesy, decided to dissolve the gold.Like all precious metals, gold is pretty inert. But even gold won't stand up to aqua regia—a 3-to-1 mixture of hydrochloric acid and nitric acid.The two acids work together like a super-destructive tag team. The nitric acid gets in first, ripping electrons off a few gold atoms at the surface, turning them into charged gold ions. Then it's the hydrochloric acid's turn. Chloride ions—the Cl in HCl—react with the charged gold ions, dissolving them.Once all the gold ions have been mopped up this way, the nitric acid gets back in there and makes some more ions. Repeat indefinitely, and you can—very slowly—dissolve gold.