Brass Pennies

Brass is an alloy of copper and zinc. Pennies, fortunately, have both, so you can actually use them to make some brass.

To get at the zinc (which makes up the creamy center of a penny), use a file to make some ridges in the edge of your penny. You can use electricity to oxidize the Zn metal on the inside to Zn (II), but it's easier to just use the reaction:

Zn + 2H⁺ -> Zn²⁺ + H₂

That is, the zinc will reduce protons to hydrogen, becoming oxidized to zinc (II) cation on the way there. This is really easy. Aqueous acid is a source of protons, and any would work in theory, but I had the best luck with strong acids (I used HCl - acetic acid/vinegar didn't really seem to work). After the solution bubbles hydrogen gas for about 15 minutes (not enough to do much, but don't lean over your cup of zinc/acid/hydrogen with a cigarette dangling out of the corner of your mouth), you'll have oxidized enough zinc to plate a penny. That is, you'll have a solution of zinc (II) chloride, which won't look too different from before. (Note that all the zinc in the penny won't be gone, which is fine. You can dissolve all the zinc if you let it go long enough).

Now, you can deposit the zinc on your penny (and reduce it back to zinc metal). You'll remember that batteries are a source of electrons, so we can use the reaction:

Zn²⁺ + 2e^- -> Zn (0)

To plate your penny, you'll need to make the solution conductive. The acid might be enough, but I added a little table salt in case. The amount doesn't matter that much, the conductivity of water shoots up when you have any electrolyte in there at all. I added 5g/1tsp of salt to 500mL/1pint of electroplating solution, which is more than enough.

Now, you hook up your battery (I used a 9V, if you use AA/C/D batteries, you may need two in a series) to some alligator clips, or tin foil, or jerry-rig it however works. You need the positive and negative terminals in solution, not touching, and the penny connected to the negative terminal (cathode). Also remember only the uncovered surface of the penny will plate (so only clip/attach at the edge). Over about 10 minutes, you should get a decent looking layer of zinc on the penny. It should look a lot like the steel pennies from WWII:

Public domain image from wikipedia article

Make sure you use a clean penny so the metal deposition goes cleanly. If you use a dirty one, you won't grow a clean metal surface, and it will look like this:

Once you have your zinc-plated penny, hit it with a blowtorch for 5 or 10 seconds (putting it on the stove burner might work, but I used a torch). It should suddenly go from silver colored to gold colored. This is because you've successfully alloyed the copper and zinc surfaces, making brass. It will look like this if everything went right:

See you tomorrow.