Why does sodium react so explosively with water?

In January 1947, the American War Assets Administration dumped drums of sodium left over after the end of World War II into Lake Lenore in eastern Washington state. A video of the event – it really was an event – is available from the Internet Archive.

Sodium’s reaction with water – or most other substances in general – is so violent because of the number of electrons in its atoms. Specifically, each sodium atom has one electron more than the atom needs to be in a highly stable state. That one electron keeps the atom highly unstable, and is given away at the first available chemical opportunity. So, when sodium (Na) meets water (HO–H), it rapidly forms sodium hydroxide (Na–OH) and releases hydrogen (H). Simultaneously, the sodium atoms release their extra electrons to form the molecules, and from go being highly unstable to highly stable. As a result, they produce such heat that the hydrogen is ignited, which burns with a bright flame, even as some of the water boils off as steam.

Even if all of this makes sense – and is true – could there be more to this reaction than meets the eye?

That’s the question a bunch of chemists, from the Academy of Sciences of the Czech Republic and the Technical University of Braunschweig, Germany, chose to ask. They figured the explosive nature of the reaction wasn’t solely due to sodium’s eagerness to react with water but also had to do with how its surface changed shape when in contact with water. Using high-speed cameras, they studied how drops of an alloy of sodium and potassium, another explosively reactive metal, responded when they were dropped into water. Watch this closely.

Credit: Mason et al.

Toward the end of the video (as Thunderf00t explains in the 17th minute), you can see how almost as soon as it is dropped, the alloy rapidly develops spike-like protrusions on its surface, on the underside. These spikes form within a few thousandths of a second, and increase the surface area of the metal that is available to react with water. The scientists calculated based on their video footage that the spikes start and finish extending out of the surface at an acceleration of 10,000 ms^-2. That’s almost the same acceleration at which you could be shot out of a space gun.

When they performed the same experiment with a drop of liquid aluminium, they couldn’t see any spikes forming on the metal droplet. Apparently, it happened only with the sodium-potassium alloy. Was it because of the explosion that happens? Nope, because sodium-potassium reacts non-explosively with ammonia, but the spikes formed there again. So it definitely happens only with sodium-potassium.

So, to get their answer, the scientists used a computer simulation, which revealed an all too familiar devil in the details.

As soon as the sodium-potassium drop meets with the surface of water, its outermost layer of atoms loses electrons rapidly to the water – so fast that the transfer happens in a few trillionths of a second. The water molecules accept the electrons and subsequently break down into hydroxyl (HO) and hydrogen (H) ions. As a result, at the interface of the drop and the water, there are now four layers: the remaining drop of sodium-potassium atoms, next a layer of sodium-potassium ions (positively charged because they’ve lost electrons), then a layer of hydrogen and hydroxyl ions (positively and negatively charged, respectively), and finally the rest of the water.

As you can see, there is a layer of positively charged sodium and potassium ions, and like charges repel each other. Because the sodium-potassium alloy is in liquid form, the repulsion manifests as highly distended droplets, or spikes. In technical parlance, this phenomenon is called coulomb fission. The resultant increase in surface area prevents the reaction from stalling, which might have happened if the first layer of sodium hydroxide to form was let to act like a blanket protecting the rest of the alloy.

The English physicist John William Strutt (better known as Lord Rayleigh) first predicted this for liquids in 1882. He reasoned that an electrically charged drop could only contain so much charge before its surface tension gave way and let the drop break up into droplets by ejecting jets – called Rayleigh jets – out of their sides. The Czech and German scientists used high-school math to figure that this breakdown happens as soon as the distance between the sodium-potassium ions and the electrons and hydroxyl ions becomes more than 5 angstrom (one angstrom is a ten-billionth of a meter).

So, that’s one high-school chemistry lesson fully unraveled. How about going after the vinegar volcano next?

The scientists’ paper was published in Nature Chemistry on January 26 .