There are lots of videos on YouTube showing sodium, potassium, etc. exploding when dropped into water (this, for example).
I understand that when an alkali metal is exposed to water, a violent exothermic reaction occurs where a hydroxide and hydrogen gas are produced, but why and how does the sample of metal end up detonating and fragmenting? Physically speaking, how can a block of metal seemingly blast apart from the inside when the reaction occurs on the surface of the sample?
The Wikipedia article on alkali metals explains this, but I still don't seem to understand how this would result in an explosion. Diagrams would be very helpful.
The key of the question is how does the release of hydrogen gas and energy result in an explosion? Is there thermal runaway, and is the metal physically destabilized in some way during the reaction?
Answer
Until recently the answer was unknown, but a short time ago it was discovered that the reaction is in fact a Coulombic explosion. The rapid exchange of electrons between the sodium and the water causes the surface of the sodium droplet to become positively charged, and the ions repel each other. This behaves very like a negative surface tension, and the surface of the droplet increases in area rapidly forming a spiny, porcupine like shape as fingers of the molten metal are shot into the liquid at astonishing speed. The larger the surface area gets the faster the reaction occurs, leading to a runaway effect. The study was published in Nature chem.
http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2161.html
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