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

Question

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.

Answer 1

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.

Answer 2

$\ce{NaCl}$:

• 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.

Answer 3

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.

Answer 4

I suspect rather strongly that this is a coulombic explosion.

This phenomenon was only recently described to explain the violent explosion of alkali metals in water. The explosion of alkali metals in water share some properties with molten salt explosions, specifically:

• it is extremely fast and violent - much faster than expected
• a similar set of alternate explanations (steam pressure, hydrogen combustion) have been attempted but have not been shown to work

Since molten salt is also conductive, a similar thing could be happening here.

One important observation that supports this is that the molten salt explosion only occurs when the salt is heated above a certain temperature. It is not enough for the salt to be simply molten. If the salt is below this temperature, there is almost no reaction (apart from steam). But when above this temperature, there is a completely different and explosive reaction. This can readily be seen in the different attempts in the OP's video. The third attempt (in which he made the salt 'really hot') is the only one that explodes.

An explosion will occur if the repulsion forces of the built-up charge within the salt exceeds the surface tension of the liquid salt. And it seems likely to me that both the electron affinity of the salt and its surface tension depend on temperature. If the force is below the surface tension, the salt will hold together while eventually cooling. If the force is above the surface tension, the salt will explode. This is consistent with observation.

This also provides a potential explanation for why the same explosion does not occur for sodium tetraborate or sodium carbonate. The electron affinity and surface tension are both likely to be different from those of NaCl.

The key to achieving an explosive effect is to bring out the energy within the salt (not just the energy at its surface). A coulombic explosion explains this because it predicts that the salt itself blasts apart, instantly exposing every internal portion in a violent chain reaction. This makes that internal thermal energy available almost instantly.