How strong is a sodium water explosion

Spectacular school chemistry: the starting point of an explosive sodium-water reaction deciphered

Prague (Czech Republic) / Braunschweig - The explosive reaction of elemental sodium with water is one of the spectacular highlights of chemistry class. In the highly exothermic process, heat is released, water evaporates suddenly, and hydrogen and caustic soda are formed. German and Czech scientists observed this explosion with a high-speed camera and discovered a previously unknown phenomenon. As they report in the journal "Nature Chemistry", the alkali metal formed star-shaped spikes immediately after contact with water.

"If we understand exactly how alkali metal explosions start in water, we can reduce the risk of explosion in industry, for example when cooling nuclear reactors," says Pavel Jungwirth from the Czech Academy of Sciences in Prague. To do this, he and colleagues from the Technical University of Braunschweig analyzed the first moments of the explosive reaction with a high-speed camera. The researchers dripped about 100 milligrams of a potassium-sodium alloy that is liquid at room temperature into water. After only half a millisecond, the explosion had developed completely and hydrogen was released.

But the extremely short period before that was all the more revealing in the camera recordings. The alkali metal drop displaced some water after 0.2 milliseconds, comparable to dipping a stone. But just 0.1 milliseconds later, the researchers were able to see numerous star-shaped spikes that spread rapidly around the drop. These spikes pushed into the surrounding water with an extreme acceleration of about a thousand times the acceleration of gravity. The metallic prongs expanded and thus enlarged the contact area between the water and the metal alloy many times over. According to Jungwirth, this jagged formation can explain why a protective, reaction-inhibiting oxide layer does not quickly form around alkali metals when they come into contact with water.

In addition to the mere observation, the researchers got to the bottom of the details of the jagged formation. They simulated the very first moments of a sodium-water explosion using molecular dynamic models. It turned out that after a trillionth of a fraction of a second (picosecond) the sodium atoms gave up their outermost electron to the water. The remaining positively charged sodium ions repelled one another so strongly that a so-called “Coulomb explosion” occurred. The researchers see this as the cause of the spike formation observed.

Camera recordings and simulation are in very good harmony with each other. Jungwirth and colleagues were now able to explain a chemical reaction that was actually well researched in a significantly better way. "I would be very happy if these results could now also get through to the teachers in the schools," says Jungwirth.

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