This old post serves as the Video Companion to this video.
This is an extremely cool demonstration of electrochemistry involving one of my favorite elements - mercury. It's a shame that it's so toxic, otherwise it would be incredibly fun to play with. The setup for this experiment is simple, but very difficult to get it to work in practice. A droplet of elemental mercury is placed in a solution of an electrolyte and a strong oxidizer. Next a piece of iron is brought near the drop, which then starts to oscillate and looks like a beating heart.
I used sulfuric acid as my electrolyte and potassium permanganate for the oxidizer. First, I immersed the mercury completely in sulfuric acid, and added a small amount (~10 drops) of a potassium permanganate solution. Then the tricky part was positioning the iron wire correctly. I tried this many times, but only got it to work once. When everything comes together right, the drop starts to beat rhythmically like a heart. This will continue until the oxidizer is used up, and mine lasted for around 20 seconds. It's hard to appreciate this reaction with pictures alone, so please go watch the video linked at the top of the post. I'll try to improve my experiment technique and camera work in the future, to make better and more easily reproducible results.
The explanation: The permanganate oxidizes the mercury to mercury (I) ions, which combine with sulfate ions from the sulfuric acid to form a thin layer of mercury (I) sulfate on the surface of the drop. The film reduces the surface tension of the drop, causing it to flatten out slightly. When the drop touches the iron wire because of this effect, electrons can flow from the iron to the mercury. This reduces the mercury (I) back to mercury metal, causing the drop to regain its normal surface tension and pull away from the wire. When the contact is broken, the film forms again and the cycle repeats.
Materials: sulfuric acid (battery acid, in the plastic container), potassium permanganate in solution (purple liquid), some iron wire, and a bead of mercury in a plastic weigh boat.