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Why does molten $\ce{NaCl}$ explode, when it is poured into water?

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

According to one amateur video in youtube, the molten salt explodes when it is poured into water (c. 2:15)

What is the exact cause of the explosion?

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 $\ce{NaCl}$ 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):

  • rapid crystallisation of the $\ce{NaCl}$
  • chemical reaction between $\ce{Na}$ and water: $$\ce{2 Na (s) + 2 H2O (l) -> 2 NaOH (aq) + H2 (g)}$$ (this causes explosion if $\ce{Na}$ is inserted into $\ce{H2O}$)
  • reaction between $\ce{Cl}$ and water: $$\ce{Cl2 + H2O -> HOCl + HCl}$$
  • the rapid solubility of hot $\ce{NaCl}$ into water
  • thermal decomposition (thermolysis) of $\ce{H2O}$ into either monoatomic or diatomic hydrogen and oxygen, and reactions that follow this.

Also, the author fails to explain why no explosion occurs with sodium tetraborate nor sodium carbonate when each of them, in molten state, was poured into the water.

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    $\begingroup$ The author is completely right, and all other processes you mention are either totally negligible or downright impossible. $\endgroup$ – Ivan Neretin Jun 17 '16 at 4:36
  • $\begingroup$ Also, Water is not $\ce{H20}$, but $\ce{H2O}$ (‘H twenty’ versus ‘H two Oh’). $\endgroup$ – Jan Jun 17 '16 at 10:36
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    $\begingroup$ @IvanNeretin That comment is not very helpful. Cool that you know the answer, but it doesn't really help OP. The most interesting part is why this doesn't occur for sodium tetraborate and sodium carbonate. Perhaps this is why OP is sceptical of the explanation given by the author. $\endgroup$ – Brian Jun 17 '16 at 10:53
  • $\begingroup$ @Brian You comment sounds quite puzzling to me, as OP never said a thing about sodium tetraborate or sodium carbonate. $\endgroup$ – Ivan Neretin Jun 17 '16 at 11:23
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    $\begingroup$ @Brian I'll modify the question to ask about why the same behavior is not observed with sodium tetraborate and sodium carbonate. $\endgroup$ – shre_sudh_97 Jun 18 '16 at 4:24
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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 $\ce{H2O}$ which expands explosively. This concurs with the recrystallisation of $\ce{NaCl}$ which accounts for a non-neglegible portion of the energy added, but which is technically already present in molten $\ce{NaCl}$. All other processes are minor or impossible.

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    $\begingroup$ Yes. Your answer makes sense. But when the experimenter tried with another salt the same didn't happen. What is unique about NaCl for this to happen? $\endgroup$ – shre_sudh_97 Jun 17 '16 at 15:48
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    $\begingroup$ @shre_sudh_97 That is a different question that you didn’t ask. $\endgroup$ – Jan Jun 17 '16 at 15:54
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    $\begingroup$ The question has been modified to include this question as well. $\endgroup$ – shre_sudh_97 Jun 18 '16 at 4:28
  • $\begingroup$ Also really interested why it does not work with other types of salt $\endgroup$ – kirhgoff Nov 23 '18 at 3:17
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$\ce{NaCl}$:

  • Molar mass: $58.44~\mathrm{g~mol^{−1}}$
  • Melting point: $801\mathrm{°C}$
  • Specific heat capacity: $36.79~\mathrm{J~K^{-1}~mol^{−1}}$
  • Std enthalpy change of fusion: $27.95~\mathrm{kJ~mol^{−1}}$

The energy required to melt $58~\mathrm{g}$ $\ce{NaCl}$ ($20\mathrm{°C} - 801\mathrm{°C}$) is roughly $56.6~\mathrm{kJ}$.

$\ce{H2O}$:

  • Molar mass: $18.02~\mathrm{g~mol^{−1}}$
  • Specific heat capacity: $75.375~\mathrm{J~K^{-1}~mol^{−1}}$
  • Heat of vaporization: $40.657~\mathrm{kJ~mol^{−1}}$

The energy required to vaporize $18~\mathrm{g}$ $\ce{H2O}$ ($20\mathrm{°C} - 100\mathrm{°C}$) is roughly $46.7~\mathrm{kJ}$.

So $58~\mathrm{g}$ $\ce{NaCl}$ of $801\mathrm{°C}$ could vaporize $21.8~\mathrm{g}$ water of $20\mathrm{°C}$ giving roughly $27$ liters of vapor. This suffices to make some kind of explosion. However, if a explosion really occurs depends also on how fast the heat is transferred from the molten salt to water. A high viscosity of the molten salt e.g. will prevent fast heat transfer.

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  • $\begingroup$ Nice quantitive answer. As you say its a sort of explosion, a physical one due to pressure not due to chemical reaction. Incidentally I don't see why a viscous solid will prevent heat transfer since there has to be lots of interaction between molecules; if anything would have thought that the opposite would be true. $\endgroup$ – porphyrin Jun 18 '16 at 6:25
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    $\begingroup$ When the molten salt has low viscosity it is more likely to form smaller parts (drops) when poured into water. The greater surface would then enable a faster heat transfer and thus a more vigorous reaction. A liquid with high viscosity, such as a molten glass, has less tnendeny to do so. This can be seen in the video where the borate retains the shape of a single drop that slowly cools down from the surface to the middle. $\endgroup$ – aventurin Jun 18 '16 at 8:02
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Having looked at the video, it is clear that in the last experiment a small region of water around the very hot molten NaCl boule is turned to steam that rapidly expands and the shockwave causes the fish tank to explode. This must occur as there was sufficient energy left in the molten salt to completely vaporise a layer water next to it even though thermal diffusion (thermal conduction) in the water is carrying heat away and any bubbles of steam will try to rise out of the water. The steam produced is probably also super heated by the > 800 C molten salt and will continue to expand after the water boils. The reason that the cooler but still liquid salt did not vaporise enough water to crack the tank is probably that thermal conduction cools the melt and also removes heat from the water surrounding the boule and this wins out over heating enough of the water to vaporisation do do any damage. Incidentally the molten salt is probably molecular in nature; there is no water in the melt to make ions. Molecular NaCl (as a diatomic molecule) is well known in the vapour phase and its spectroscopy has been well studied as has NaI the iodine equivalent.

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  • $\begingroup$ Yes, I think you nailed it. What I saw: As the molten salt entered the water, it was sheathed by a bed of superheated steam that keeps water away. Perhaps because of splashing, a small amount of superheated steam near the surface exploded. This caused a) a venting of the steam near the surface upwards and b) a sudden unsheathing of the molten salt from its steam bed as the surface shock propagated downwards and disrupted the hitherto organized molten salt flow. Once it became turbulent, vastly more energy transferred to the water, water instantaneously superheated and an explosion resulted. $\endgroup$ – Iwillnotexist Idonotexist Dec 15 '16 at 23:39

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