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To keep their solid gold Nobel Prizes away from the Nazis, James Franck and Max von Laue sent their medals to trusted colleague Niels Bohr. But when Germany invaded Denmark in 1940, the medals were no longer safe - so chemist George de Hevesy used his knowledge of chemistry to dissolve the gold into solution, hiding the medals in plain sight for 10 years.

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[♪ INTRO].

It's Germany in the 1930s. The Nazi party is gaining power, and two Nobel prize-winning physicists are getting very worried.

One is Max von Laue, an outspoken opponent of the Nazis. The other is James Franck, who was a target because of his Jewish heritage. The Nazis are seizing items of value from anyone who isn't also a Nazi.

And Nobel medals are made of solid gold. So both men sent their medals out of the country to a trusted colleague: a fellow prize winner Niels Bohr in Copenhagen, Denmark. The Nazis had made sending gold outside of Germany a crime, so this was a risky move.

And these medals had their owner's names prominently engraved on them, so it wasn't like the bad guys weren't gonna catch on. So, when Germany invaded Denmark in 1940, Bohr was desperate to hide them. Enter chemist George de Hevesy, who would win his own Nobel a few years later.

He was working with Bohr at the time. He turned to his knowledge of chemistry and decided to dissolve the gold. This was easier said than done, because Nobel medals are made from... a noble metal.

A metal is considered noble when it doesn't react much with other chemicals. So it's resistant to things like rusting or being dissolved in acid. Which elements are included among the noble metals depends on the context you're talking about.

Chemists define about eight to ten noble metals based on their lack of reactivity. Physicists, however, pay more attention to what's going on inside the atoms. They use a stricter definition: metals with full d-subshells.

That refers to the configuration of their electrons. The classic cartoon of an atom has electrons orbiting the nucleus like planets in the solar system, but things are a bit more complex than that. Electrons exist at different energy levels, and in differently shaped orbitals, around an atom's nucleus.

These are given different numbers and letters depending on their shape and how much energy they have. Electrons at the highest energy level tend to be the ones participating in reactions, and for metals, these are in their d-subshell. If that subshell is full, the metal will be particularly stable.

Chemical reactions are all about swapping electrons, but that full subshell means it has a complete set already. The three metals that meet this definition are copper, silver, and gold. And even among noble metals, gold is king.

Along with platinum, it's usually considered the least reactive of all the metals. So de Hevesy had a challenge ahead of him: dissolving something that barely reacts with anything. He turned to a particularly potent mix of acids: one part nitric to three parts hydrochloric.

For centuries, this concoction has been known as aqua regia -- meaning “royal water”. And it can dissolve gold because each acid has a particular role in shifting the equilibrium of the reaction. See, a lot of chemical reactions don't just… react to form their products, and then stop.

Some of those products go the other way to form the reactants again -- so there's a balance between reactants and products in your flask. In fact, the forward reaction doesn't always “go” very far at all. These reactions are called reversible reactions, and they will eventually reach an equilibrium point when the rate of the forward reaction is the same as the rate of the reverse.

But that equilibrium point can shift. If you use a second reaction to remove the products of the first, you can shift this equilibrium point and drive the reaction further than it would otherwise go. When the Nobel medals hit the aqua regia, the nitric acid was able to pry off just a few gold atoms in the metal to make gold ions in the solution.

Now, if it were just nitric acid, things would have stopped here. But the hydrochloric acid stepped in to provide chloride ions, which reacted with the gold ions to form a molecule called tetrachloroaurate. This shifted the equilibrium of the first reaction by removing gold ions, and allowed the nitric acid to dissolve a little bit more of the gold metal.

This continuous removal of gold ions allowed the reaction to continue until the solid gold vanished into the solution. de Hevesy wrote that this was going down even as occupying soldiers were marching in the streets of Copenhagen. But by then, the only thing left for them to find was a glass container of the gold solution sitting in his lab. And that glass was perfectly safe -- from the acid.

Just because aqua regia can dissolve gold, doesn't mean it can dissolve anything, and glass doesn't have the same vulnerability that gold does to aqua regia's two-pronged attack. The gold hid in plain sight for nearly 10 years. But it wasn't gone, just in a different form: chloroauric acid.

After the war, de Hevesy was able to get the gold back out of the solution. He didn't record exactly what he used, but some chemicals can shift the equilibrium so far in the other direction, it drives the reaction in reverse -- and causes the gold to precipitate out. de Hevesy delivered the gold to the Nobel Foundation, who remade the medals using the same material and gave them back to their rightful owners. He was presented with a problem, and you could say... he found a solution.

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