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.
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    $\begingroup$ Put anything really hot in water in a confined space and there's an explosion by the exact same mechanism - rapid evaporation of water. $\endgroup$ – OrangeDog Jun 14 '16 at 8:49
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    $\begingroup$ 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. $\endgroup$ – tfb Jun 14 '16 at 9:24
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    $\begingroup$ I recommend you re-post your question to the chemistry stack exchange. $\endgroup$ – docscience Jun 14 '16 at 13:58
  • $\begingroup$ 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. $\endgroup$ – Guill Jun 15 '16 at 6:04
  • $\begingroup$ 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 $\endgroup$ – Triatticus Jun 15 '16 at 20:14

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.


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