Watch Sodium Explode In Water At 10,000 Frames Per Second

Using high-speed video, chemists figure out the mechanics behind the famous chemistry class demonstration

photo of a large orange flame over a grayish block of sodium

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A photograph taken the instant that hydrogen gas, generated by the reaction of sodium and water, ignites.Theodor Gray

Drop pure sodium into water, and you'll get some explosive results. You might have even seen this yourself. Sodium-in-water is a common chemistry class demonstration and scientists have a solid grasp of what's happening during the reaction. Yet one team recently discovered an additional step to the process. Just before the explosion occurs, spikes of sodium metal shoot out into the water, a team of chemists from the Czech Republic and Germany discovered.

The chemists made the discovery by filming the reaction with a high-speed camera that captured 10,000 frames a second. Here's a taste:

The sodium spikes help swiftly mix the sodium and water, the team members wrote in a paper published in the journal Nature Chemistry. The spikes create more surface area where the sodium and water touch. Once the sodium and water are in good contact with one another…

...electrons move from the metal into the aqueous solution where they react to form hydroxide and hydrogen. The related heat release can be sufficient for melting of the metal, massive steam formation and ignition of the hydrogen gas, which leads to an explosive behavior.

So what makes the spikes? Using a computer model, the team found that when the sodium's electrons head out into the water, that leaves behind positively charged cations of sodium that are strongly repulsive to each other, the way magnets of the same charge repulse one another. In their hurry to get away from one another, the sodium cations shoot outward in spikes, in a reaction called a coulomb explosion.

Without the spikes, the team hypothesizes, the explosion might not happen at all. Instead, the initially-formed steam and hydrogen gas might sit in a layer around the sodium, keeping the water and sodium from reacting any further. That's why sometimes school demos don't work, the study's lead scientist, Pavel Jungwirth of the Academy of Sciences of the Czech Republic, told Chemistry World. Impurities in classroom water or sodium samples can interfere with spike formation.