Sodium’s Explosive Relationship With Water Explained

by Michael Keller

Remember that time in chemistry class when the professor dropped a bit of sodium into a tub of water? You probably recall it if you were there because the demonstration, meant to show the reactivity of certain metals with water, isn’t soon forgotten.

Sodium (Na) is part of a group called the alkali metals, which take up the far left column of the periodic table of elements. The lower down in the column you go, the increasingly powerful exothermic reaction the element produces when it is exposed to water.

Scientists have long known that the fizzling, explosions and flames these elements create when wet is due to the metal rapidly shedding electrons into the water, which transforms the surrounding liquid into steam while also breaking some of the water molecules into hydroxide and flammable hydrogen gas. What had become a bit murky in this process was how the quick production of steam and gas doesn’t form a vapor layer that separates the element from the liquid, which should quench the reaction on the metal’s surface and stop it before it runs away into an explosion.

The question goes beyond just one of curiousity for chemistry professors. Liquid sodium and Na-K alloys are used as a primary coolant in a type of nuclear power generation system called a fast-neutron reactor. For obvious reasons, engineers and scientists need to know all they can about what happens when these alkali metals inadvertently touch water.

Now a team of chemists at the Academy of Sciences of the Czech Republic and Germany’s Braunschweig University of Technology say they have an answer. By training a very high-speed camera on a drop of sodium and potassium (K) as it falls and then running simulations of moving molecules, they have discovered that the metal’s surface rapidly deforms when it makes contact with water.

This deformation happens when the electrons bolt into the water and what is left is positively charged metal ions, which strongly repel each other. Repulsion causes metal spikes to shoot out from the element’s surface in a miniscule fraction of a second. This dramatically increases the surface area of the metal available to feed into the propagating reaction.

“High-speed camera imaging of liquid drops of a sodium/potassium alloy in water reveals submillisecond formation of metal spikes that protrude from the surface of the drop,” the authors write in a paper published today in the journal Nature Chemistry. “Consequently, a new metal surface in contact with water is formed, which explains why the reaction does not become self-quenched by its products, but can rather lead to explosive behaviour.”

A high-speed camera shows the explosive reaction that happens when a drop of sodium/potassium alloy makes contact with water. Courtesy Mason et al.