Adding sodium to water is a classic thing on high-school chemistry classrooms. If you’ve seen it before, it’s usually done with tiny nuggets of sodium and a puny water bath, for fairly sensible reasons. Well, this video by EatsTooMuchJam shows what happens when you throw a 0.45 kilograms (1 pound) of sodium into a river.
As you can imagine, this isn’t one you should do yourself, kids. That violent chemical reaction is releasing a fair amount of hydrogen gas and sodium hydroxide, which definitely won’t do any good to the river itself, nor to its inhabitants.
So, the theoretical explanation for it would be that a fuel-air explosion caused by mixing of the hydrogen gas with air, ignited a second or two later (as you can see in the video) by the heat that builds up in the sodium. The heating of the sodium acts as the time fuse needed to make any fuel air bomb work. This theory would imply that only a minimal shock wave should be transmitted into the water, since the explosion would be happening well above the surface, as the picture seems to show. Unfortunately that theory is not supported by the fact that the metal bucket was split at the seams, even though less than an inch of rim extended over the level of the water.
Which brings us to a safety warning: Sodium is really rather dangerous. If they had been anywhere within 15 feet of this explosion, it would have sprayed us with molten sodium and sodium hydroxide. Even a tiny amount in the eyes would have been a serious medical emergency. That’s why I built a device that let me release it in a very controlled way from a great distance: If you want to do anything even remotely like this, you should take similar precautions.
While it’s safe to drop a tiny piece, maybe a few millimeters on edge, into a bowl of water, if you are wearing safety glasses, the force of the explosion goes up non-linearly with size. A lot of people have hurt themselves by going to bigger and bigger pieces thinking it’s just going to do more of the same. It doesn’t: At some point it turns from a fizzle and flame into a real explosion, like a shotgun.