The initial mechanism of the explosion that occurs when alkali metals, such as sodium or potassium, are dissolved in water is described in Nature Chemistry this week.
Dropping alkali metals into water is frequently used to wow high-school chemistry classes. Students are taught that, once triggered, the vigorous explosion is caused by heat release, steam formation and the ignition of the hydrogen gas that is produced. Whilst true, the trigger, and why the initial formation of steam and hydrogen gas does not simply deactivate the surface reaction before an explosion can occur, has not previously been well understood.
Pavel Jungwirth and colleagues used high-speed cameras and simulations to study the explosive reaction. Their data suggests that the explosive behaviour is triggered by an almost immediate release of electrons from the metal, which leaves behind positively charged metal atoms that strongly repel each other. This leads to the rapid protrusion of metal spikes from the surface, which increases the surface area over which the subsequent metal-water reaction can occur, explaining the propagation of the reaction.