Even if you slept your way through high school chemistry, there was typically one day that ensured you were awake. That's when, after an extensive safety lecture, the teacher brought out the alkali metals and dropped them into some water. Typically, what followed was a hissing, spitting explosion as the metal danced on the surface of the water, often sending out flaming chunks of metal that repeated the process.

(I'm old enough where we were actually given small chunks of sodium or potassium to set off our own explosions. I suspect that's no longer common in our lawsuit-happy society.)

The chemical reaction that powers this display is a simple one: metals like sodium and potassium like little more than to give up a single electron. Place them in water, and that electron will split the otherwise stable water molecule, creating a negatively charged hydroxide ion and freeing hydrogen. It's the free hydrogen that creates the explosion that wows chemistry classes.

There's a small problem with this version of the story. Because reaction is so energetic, it quickly builds up a barrier of steam and hydrogen around the metal. This should act as a protective shield between the metal and water, limiting the rate of the reaction, and stifling its explosive oomf. So, why does this go boom?

To answer that question, researchers turned to high-speed cameras, using them to capture the moment the water hits the metal. This is hardly the first time the reaction's been filmed at high-speed, but the events are very variable, and depend on things like the metal chosen, its shape and velocity, as well as how clean its surface is. So one of the big innovations this time around was simply finding a way to get a consistent explosion.

The authors used a mix of sodium and potassium that ended up liquid at room temperature. In this form, a precise volume could be dispensed with a syringe. The syringe was kept at a distance from the water such that the drops, at the time of their impact, were roughly spherical in shape. The result was what the authors term "fully reproducible explosions" that were filled at one frame every 100 microseconds.

That allowed them to capture an unusual phenomenon as the drops of metal hit the water: spikes that shot out of the surface. A close examination of the images indicate that these were accelerating from the drop at the astonishing rate of 10,000m/s2. These spikes would often branch as they extended, creating a dendritic pattern as they spread into the water.

This explains the continued robust reaction: the spikes kept placing fresh sodium in contact with the water, preventing the formation of a steam/hydrogen barrier that would choke off the reaction. So, enough hydrogen could build up to feed an extended explosion. But what explains the spikes themselves?

To understand that, you have to dive into the details of the reaction, which doesn't go instantly from metal and water to the end products. Instead, it proceeds through steps, the first of which is simply the metal giving up an electron to the water, which essentially dissolves a cloud of electrons as if they were any other ion. This takes a little time before it proceeds to create a hydroxide ion and hydrogen; in fact, by replacing the water with liquid ammonia, the authors slowed down the reaction enough so that electrons stayed in solution longer. Hydrogen was produced at a slower rate, and the mixture never exploded.

But the metal still formed spikes in ammonia, suggesting the explosion wasn't needed for them to emerge. So the authors focused on what happens after the electrons leave: the metal that remains is a collection of charged ions. Computerized simulations of this reaction showed the surface of the metal rapidly forms a large positive charge, and this charge repulsion leads to a rapid expansion and disintegration of the surface. Thus, charge repulsion causes the spikes.

The authors call this process a "Coulomb explosion," and suggest that it's necessary for all of the explosive reactions between water and the elements in the first column of the periodic table. It also may explain why the explosions can be finicky, and vary depending on the state of the metal and any contaminants present.

Nature Chemistry, 2014. DOI: 10.1038/NCHEM.2161 (About DOIs).

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