Why exactly does molten NaCl explode, when it is poured into water?

Why does molten sodium chloride explode, when it is poured into water?

$\mathrm{NaCl}$ has high melting point, $1074\,\rm K$ ($801\,\rm^\circ C$). $\mathrm{NaCl}$ has molar mass $58.44\,\rm g/mol$, it has specific heat capacity $36.79\,\rm J/(K\,mol)$ ($629.53\,\rm J/(K\,kg)$), therefore the $\mathrm{NaCl}$ in melting point has $491,033\,\rm kJ/kg$ thermal energy than $\mathrm{NaCl}$ at STP conditions ($1\,\rm atm$, $20\,\rm^\circ C$, temperature difference is $780\,\rm K$).

According to one amateur video in youtube, the molten salt explodes when it is poured into water. The author of the video reasons that the phenomenon is purely physical, and it is caused by that water heats up, vaporises and expands as a gas inside the glimp of very hot $\mathrm{NaCl}$ salt. But is it really everything that happens there?

Other possible processes present in such occasion are (this is just a list what comes into my mind):

1. Rapid crystallisation of the $\mathrm{NaCl}$.
2. Chemical reaction between sodium and water: $\mathrm{2 Na (s) + 2 H_2O (l) \to 2 NaOH (aq) + H_2 (g)}$ (this causes an explosion if $\mathrm{Na}$ is put into $\mathrm{H_2O}$).
3. Reaction between chlorine and water: $\mathrm{Cl_2 + H_2O \to HOCl + HCl}$ (?).
4. The rapid solubility of hot $\mathrm{NaCl}$ into water.
5. Thermal decomposition (thermolysis) of $\mathrm{H_2O}$ into either monoatomic or diatomic hydrogen and oxygen, and reactions that follow this.
• Put anything really hot in water in a confined space and there's an explosion by the exact same mechanism - rapid evaporation of water. – OrangeDog Jun 14 '16 at 8:49
• Almost certainly the salt has trapped a significant body of water in a cavity as it froze. This water then absorbs heat from the cooling, now-solid salt, and its pressure rises very rapidly, until the salt surrounding the water fractures and you get a big bang. You have to do it lots of times because you don't always get these trapped bodies of water. You get this sort of thing if you pour molten metals into water sometimes as well, where it can be significantly unpleasant. – tfb Jun 14 '16 at 9:24
• I recommend you re-post your question to the chemistry stack exchange. – docscience Jun 14 '16 at 13:58
• I agree with you that there is more going on than the heating of the water. In addition, I think it is also possible that chlorine separates from the sodium and instantly becomes gaseous , adding to the "explosive" expansion. – Guill Jun 15 '16 at 6:04
• I would say it's a chemical reaction as molten aluminum has been poured into water with much less fanfare. And the melting point of aluminum isn't too far from NaCl – Triatticus Jun 15 '16 at 20:14

Taken from this answer by user Jan on chemistry SE for the repost of this question there.

Understand that sodium chloride is not made up of sodium metal and chlorine gas but of sodium ions and chloride ions, held together by ionic interactions. Under these thermal conditions (liquefication), the compound will not decompose into its elements and therefore all reactions you suggested which include elemental chlorine or sodium cannot occur.

Dissolution of sodium chloride in water is neither strongly exothermic nor strongly endothermic, so any effects stemming from the dissolution are neglegible.

Also note that you neglected an important variable in your calculations. As far as I can tell, you only calculate how to arrive at melting-point hot solid sodium chloride. To liquefy, additional melting enthalpy has to be applied, a further reservoir of energy to draw from. This melting enthalpy is of course released upon rapid crystallisation, but you should really subsume it into the heat energy difference altogether.

Finally, thermal decomposition of water is not exactly a process that will happen quickly, as exemplified by (water) steam temperatures that can be huge. Check out this unrelated answer of mine about how much energy is required to heat water to $$100~\mathrm{^\circ C}$$ and boil it.

So all things considered: The principle contribution to the explosion is the rapid heating of water including boiling, leading to tightly compressed gaseous $$H_2O$$ which expands explosively. This concurs with the recrystallisation of $$NaCl$$ which accounts for a non-neglegible portion of the energy added, but which is technically already present in molten $$NaCl$$. All other processes are minor or impossible.

I don't know the answer with certainty. My theory is that some water pockets get into the molten salt and then the molten salt freezes around it and then the water trapped inside heats up and starts exerting a lot of pressure on the solidified salt around it until the pressure of the trapped water is high enough to push the solid salt apart at high speed causing an explosion.