Click the play button on the video above, and you'll see a piece of sodium leaping over and over to get its (metaphoric) toes out of the water. It looks like a cute little experiment — but it's not quite as safe as it looks. Find out why!
You'd think that sodium would be right at home in the water. If it weren't, why would the oceans around the world be so full of salt? The problem is, sodium tends to have company whenever it gets its feet wet. Sodium tends to have company all the time, because it will react with pretty much anything. But when, all on its own, it makes contact with water, its reaction gets dramatic.
Very dramatic. The sodium atom (NA) and the water (H2O), combine to form NaOH, with a lot of hydrogen gas left over. The reaction releases a lot of heat, and the heat ignites the hydrogen gas. The rest is a boom. The reaction is so immediate and strong that pure sodium, which has a metallic luster when it's freshly cut, will often react with water vapor in the air. In labs, it's carefully stored in gases with no water vapor so it doesn't ignite all on its own and burn the place down.
So what's happening here? Gasoline is one of the few things that sodium doesn't react with. It provides a little support for the lump of sodium, but the sodium still sinks down towards the water. Just as soon as the first edge touches the water below, the sodium reactions, producing heat and gas. This heat and gas wouldn't be enough to push it upwards under usual circumstances, but with the gasoline there, it can float up like it burned itself, only to inevitably sink back down again towards the water.
When something catches fire it oxidizes, which requires oxygen. Since the gasoline supports the sodium enough to keep it from sinking into the water and setting off an even hotter reaction, and since it also keeps oxygen from getting to the reaction, the entire thing doesn't burst into flame. Still, putting heat, gasoline, sodium, and water into the mix - with the necessary oxygen sitting right there on top of the whole thing, isn't the safest of experiments. The lesson is: be careful even when an experiment looks whimsical.