K: I’m Kate Powers from NIU STEM Outreach here in the studio with Sam Watt and this is the Sound of Science.
S: Kate, I have a question from Colton today. He is wondering why you have to polish silver but not gold?
K: Yeah gold is pretty amazing. Colton is right, gold doesn’t tarnish like other metals. Before we talk about gold, though, we should talk about what is happening when a metal tarnishes.
S: Well, I know silver turns black when it tarnishes. What other metals tarnish?
K: Most metals tarnish to some level. When iron tarnishes we call it rust, and the metal turns red. All tarnishing is the result of the metal interacting with chemicals in the atmosphere.
S: You mean a chemical reaction is happening between the metal and the air?
K: Exactly. Usually the oxygen in the atmosphere slowly reacts with the metallic surface, forming a metal-oxide. Even metals that don’t change appearance react with the atmosphere. Aluminum looks like it never tarnishes, but that’s because the resulting tarnish is the same color as the original aluminum.
S: So if most other metals react with the atmosphere, why doesn’t gold?
K: The answer is in the periodic table! All elements are made up of protons, neutrons and electrons. Depending on how these particles are ordered determines the reactivity of an element. The organization of gold’s subatomic particles makes the compound very unreactive to air and even corrosive substances like sea water.
S: Oh, is that why gold is one of the few things divers can bring up from shipwrecks?
K: Exactly! Just clean off the sea junk and those Spanish doubloons are as good as new. But there is one thing that does dissolve gold: Aqua regia.
S: Aqua regia? That sounds more like magic than science.
K: It seems like magic! It’s a strong acid… and can fool Nazis.
S: Wait, what? Nazis?
K: During World War II two Nobel Laureates hid their medals from the Nazis using chemistry. Nazis would sweep through buildings, stealing anything of value. The solid gold Nobel medals would have been very tempting to the Nazis. So the medals were dissolved in aqua regia and left in a glass beaker until the end of the war. Then the gold was precipitated out of the solution and sent back to the Nobel committee to reform the medals!
S: Whoa, that’s quite a chemistry tale. I’m starting to understand why gold had such a long grip on human kind! Thanks for telling us that story Kate! Make sure to keep your questions coming to email@example.com. This is WNIJ.
K: Where you learn something new every day.