Throwing a piece of sodium into water is a dangerous
but popular experiment demonstrating an explosive chemical
reaction. Every chemistry teacher knows that, during
the explosion, heat is released while electrons move from
the metal to water generating water vapor, hydroxide, and
hydrogen which can ignite. Gases generated at the interface
between the metal and water should, however, separate
the reactants and thus quench the explosion. How is it
possible that the explosion can nevertheless take place?
Thanks to pictures from an ultrafast camera and molecular
simulations we found a hitherto unknown primary mechanism
of the explosive reaction of alkali metals in water.
When the electrons move from the metal to water, the former
acquires a large positive charge. Repulsion between
these charges leads to a Coulomb explosion with metal
spikes shooting into water. This leads to effective mixing
of reactants enabling thus the explosion.