Chemistry - What exactly is happening when sodium, potassium, or other alkali metal explodes in water?
Solution 1:
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. (Ref.1).
References:
- Philip E. Mason, Frank Uhlig, Václav Vaněk, Tillmann Buttersack, Sigurd Bauerecker, Pavel Jungwirth, "Coulomb explosion during the early stages of the reaction of alkali metals with water," Nature Chemistry 2015, 7, 250–254 (https://doi.org/10.1038/nchem.2161).
Solution 2:
EDIT/Retraction: This answer was written prior to the publication of the research described in the accepted answer, and is now known to be incorrect. Science!
I assume this is due to mesoscopic defects -- if you think about a piece of metal on a molecular scale, it's not going to be presenting a totally flat sheet of atoms at its surface. Hydrogen gas forming within cracks in the metal can force those cracks to open wider, and this then exposes more surface to react, and produces more cracks.
This process will result in the force of the generated hydrogen ripping the bulk metal apart if it happens quickly enough. If the heat generated from the reactions then also starts boiling water in those same cracks and defects, that'll only add to the effect.
Solution 3:
I don't think any clever effects need to be invoked to explain this phenomenon. The simple fact that the heat of the reaction is always enough to melt the blobs of metal involved is enough. Once you have a mobile molten drop of metal (which is often on fire), very little energy is required to cause the drop to split up so the violence of the continuing reaction is often quite sufficient to cause the drop to split and disperse.
You can easily verify that drops of liquids don't need much energy to cause them to disperse and, if you were still allowed to play with mercury, you could also prove this applies to liquid metals. Drops of mercury will easily split into smaller drops when small amounts of energy are added (e.g. letting a drop fall a short distance onto a surface). These smaller drops are a lot harder to contain than their parent (which is one reason why mercury contamination is so prevalent and hard to remove in old labs).
So in short, exothermal reaction + liquid metal = readily dispersed drops of burning metal.
PS While what I say in this answer is broadly correct, the recent research (see accepted answer) add additional explanatory power to the detail of the reaction. The reaction is more violent that my simple mechanism would explain and the high-speed camera footage demonstrates that more factors were at wok than most people expected.