Explained At Last: Why Alkali Metals Explode in Water
/Benjamin Yin
Originally published April 14, 2015
Photo by: Kristen thomas and Jadiel wasson
In the pilot episode of the iconic 80s TV show, MacGyver, the titular character made his debut as a resourceful secret agent by making a sodium bomb to take down a wall, rescuing a couple of scientists. For MacGyver, with his extensive knowledge of the physical sciences, the process was simple: he immerses pure sodium metal inside a bottle of water and the explosive reaction between sodium and water is great entertainment for viewers of all ages.
Today, this little display of pyrotechnic shenanigans is often seen in high school chemistry demos. Alternatively, one can find many dozens of internet videos documenting this violent reaction between alkali metals like sodium or potassium and water, often accompanied by exclamations and whistles of joy. It’s no surprise that some of these videos have also gone viral. This amusing diversion of chucking alkali metals into water to watch it explode has been around since the 19th century and scientists have had a solid description of the nature of this reaction for about as long. Or so we thought.
The classic explanation of elemental sodium’s volatile reaction with water involves the simple reduction-oxidation chemistry of sodium and water: electrons flow from sodium metal into the surrounding water, forming sodium hydroxide and hydrogen gas. This is a very fast reaction that produces a lot of heat. Hydrogen gas is extremely flammable in air, and in the presence of a heat source, this mixture can lead to a hydrogen explosion, not unlike the infamous incident that allegedly set the Hindenburg zeppelin aflame. The release of the large amount of energy in these reactions results in rapid expansion of the surrounding gas, which is what causes chemical explosions.
Generations of chemists have accepted this seemingly obvious explanation without much deliberation. It is perhaps surprising then, that one curious soul decided to look at this century-old reaction more in-depth.
Philip Mason earned his PhD in chemistry and has co-authored more than 30 scientific papers, but is probably better known for his YouTube channel, where he regularly posts videos, often in vlog format, under the pseudonym “Thunderf00t” (yes, that’s two zeros substituting for the letters “O”). His favorite post topics are often pieces of popular science he encounters, and Mason has earned the support of a huge public following with his YouTube channel. In 2011, using donations from some of his more than 300,000 YouTube subscribers, Mason purchased the materials and consumer grade high-speed cameras necessary to look at what he thought would be “home chemistry.”
The YouTube project, it turns out, raised many questions, for which Mason found traditional answers unsatisfactory, namely the explosive nature of alkali metals in water. Compelling footage also showed a secondary gas explosion above the water surface that resembles a hydrogen explosion, demonstrating that the initial stronger and faster explosion can’t be explained with our traditional understandings of this reaction. Some scientists have suggested, instead, that the explosion is caused by the sheer amount of heat released during the reaction. If this were the case, the heat would boil the water and a rapid generation of steam leads to explosion. Mason remained unconvinced. A key insight by Mason and his colleagues was that as hydrogen and steam are generated when the alkali metal comes into contact with water, the interface between the metal and water should be blocked off by the products and therefore inhibit further reaction. This would result in the exact opposite of the explosive reactions being observed. Crucially, immersing solid chunks of sodium and potassium under water still results in rapid explosions, so this too could not be the explanation for the initiation of the explosion. These enigmas led Mason to bring his YouTube project into the lab.
To get a better look at the reaction, Mason and his colleagues turns to research grade high-speed cameras. Filming at around 10,000 frames per second, they were able to capture the beginning of the reaction between alkali metals and water in astounding detail. What they captured is striking: the reaction is immediate, and the metal shatters on contact with the water surface. Within two-ten thousandths of a second, spikes of metal are flying apart from anywhere the surface touches water. As the sheer force of the rupturing metal bursts forth, a brilliant blue wash appears to stain the blast of water in the very next frame. This stunning blue color is due to solvated electrons in water, which is usually far too short-lived for people to see.
What isn’t so easy to interpret are the metal spikes flying apart, piercing the water in the process. However, with some chemical intuition and computing time on supercomputers, Mason and his colleagues came up with an explanation for this observation that ultimately describes the explosive nature of alkali metals in water.
When large numbers of electrons escape from the alkali metals into the surrounding water, the metal itself becomes extremely positively charged. Like the static charges that can make our hair spike up for that mad scientist look, the positive metal atoms now repel each other, except with much more violent force. Atoms that were previously bonded together as a solid now suddenly fly apart at extraordinary speed. This, in turn, exposes fresh metallic surfaces to water for the explosive reaction to take place. This little-known phenomenon is called Coulomb explosion.
The immediate application of this knowledge for preventing explosions in industrial use of alkali metals will be useful. Just as important, the discovery of this mechanism of explosion in a chemical reaction over a century old reminds us not only of how little we know, but also how much we simply fail to even consider. In the face of public apathy for science, it is encouraging that such a significant scientific discovery should come from a YouTuber, funded partially by the YouTube community, and documented in vlog format throughout the research process. It leaves us wondering what other remarkable discoveries such public engagement could lead to.
Mason and his colleagues published their research in the February issue of Nature Chemistry, they acknowledged the support of his YouTube followers.
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