High-Speed Cameras Reveal Why Sodium Explodes In Water

You remember the sodium demo from chemistry class: Just a small chunk of the pure metal dropped into water causes a great big flash and bang. You might even remember your chemistry teacher’s explanation for why. But there’s always been a missing piece of the puzzle, which scientists have finally figured out using high-speed video cameras.

Sodium is one of the highly reactive alkali metals. In its pure form, the soft, shiny metal reacts in water to form sodium hydroxide and hydrogen gas. Oh, and lots of heat — enough that the hydrogen gas ignites. That’s the flash and bang for you.

That’s probably what your chemistry teacher told you, but Pavel Jungwirth of the Academy of Sciences of the Czech Republic was puzzled by a couple more things. Only the surface of the sodium reacts with water and wouldn’t the hydrogen gas envelope the sodium chunk and slow the reaction? So of course, he pulled out a high-speed camera. Emily Conover of Science explains what happened next:

The cameras captured a never-before-seen effect. Less than a millisecond after the reaction begins, tens to hundreds of spiky metal protrusions pierce the water… The spikes appear, the researchers deduced, because when electrons flee the metal for water, an intense positive charge builds up. The mutual repulsion of those positive charges rips the metal apart, and it blasts outward in tiny needles. This increases the surface area of the metal in contact with water, generating a vigorous reaction. Computer simulations performed by the researchers confirmed this effect, although for much smaller quantities of sodium due to the limits of computing power.

So there you have it — a classic high school chemistry experiment finally explained with the help of 21st-century technology. [Nature Chemistry via Science]