# Why does the Leidenfrost effect (seemingly) not apply to the case of molten NaCl when it is poured into water?

On pouring molten sodium chloride into water you can see that when molten $\ce{NaCl}$ (table salt, i.e. sodium chloride) is poured into water ($\ce{H2O}$), the Leidenfrost effect appears to be nonexistent, whereas it clearly manifests itself in instances where other molten salts such as sodium tetraborate (1) (borax) and sodium carbonate (2) are poured into water.

Why does the Leidenfrost effect in molten NaCl last much shorter than that of the other salts, appearing to be nonexistent?

Input from paracetamol:

"The Leidenfrost effect (physical phenomenon) does show up. See the "cushion" of water vapour around the blob of molten salt in the video? Though short-lived, it's pretty apparent. Now why molten table salt would react with water faster than molten borax or soda (as implied in your post) is an interesting question."

• The Leidenfrost effect (physical phenomenon) does show up. See the "cushion" of water vapour around the blob of molten salt in the video? Though short-lived, it's pretty apparent. Now why molten table salt would react with water faster than molten borax or soda (as implied in your post) is an interesting question. My fingers are crossed for the answers! O:) – paracetamol Jun 1 '18 at 5:02
• – aventurin Sep 1 '18 at 20:08

In the video, TheBackyardScientist offered an explanation for the huge explosive effect when 50 grams of molten NaCl is poured into water. He said that some water penetrated the blob of molten NaCl and blew it apart from the inside. A Leidenfrost effect on the outside of the blob is visible, but not the major effect.

The borax blob did not explode, so it must not have been penetrated by a significant amount of water. Borax is a tetrahydrate; when it melts and then solidifies, it goes thru a glassy stage which might be expected to be somewhat cohesive, perhaps resembling Silly Putty. In addition, it loses a lot of water on melting: an initial amount of 50 grams melts to only 26 grams. (Boric acid solidifies similarly.)

NaCl does not exhibit a glassy stage on cooling (note the crystalline surface on the patty from solidified NaCl). Perhaps 26 grams of NaCl would also not explode but merely show the expected Leidenfrost effect. As a 50-gram blob of NaCl cools, solid (strong) regions form over weak molten interior; the weak part of the blob splits open, allowing water into an interior hot enough to boil the water quickly and blast the blob skins apart. The hardened skins of the blob serve as a partial container - like a pipe bomb.

50 grams of washing soda (sodium carbonate decahydrate) will melt down to 18 grams; again, perhaps this is too little to give an explosion.

The Leidenfrost effect should be expected for small blobs of anything. The explosive effect might be expected only for larger quantities, but including even materials cooling thru a glassy stage, if the amount were large enough.

• Thanks for the answer, I'll do a bit more research with your answer in mind. – Peter Johnmeyer Aug 31 '18 at 3:48
• +1 for the mass analysis and mentioning the possibly glassy state of borax. The rest is a little bit speculative, imho. – aventurin Sep 1 '18 at 19:28

The Leidenfrost effect breaks down when stable film boiling stops and transition boiling starts. The temperature when this occurs is called the Leidenfrost point and its value is about $\pu{100°C}$ above the boiling point of water. This means that when the surface temperature of the salt drop is below $\approx \pu{200°C}$ the Leidenfrost effect breaks down.

Now consider liquid sodium chloride at its melting point temperature of $\pu{801°C}$. The thermal energy it contains compared to $\pu{100°C}$ is $\pu{28 kJ mol^-1}$ heat of fusion and $\pu{26 kJ mol^-1}$ from heating the solid from $\pu{100°C}$ to $\pu{801°C}$.

From this we can calculate that the sodium chloride must have released about

$$\frac{601°C}{701°C} \cdot \pu{26 kJ mol^-1} + \pu{28 kJ mol^-1} = \pu{50.3 kJ mol^-1}$$

or 93% of its effective thermal energy before the Leidenfrost effect breaks down.

Given this value it does not seem plausible to me that the vapor explosion observed when pouring molten sodium chloride into water is related to a global breakdown of the Leidenfrost effect.

Instead it seems to be more plausible that the high temperature difference between the water and the molten sodium chloride allows for sufficient heat transfer, even in the film boiling regime, if the drop of liquid sodium chloride is fragmented into small droplets by dynamic processes when the hot liquid enters water. This assumption is backed by the observation of Matsumura and Naria that the occurrence of vapor explosions when dropping liquid tin into water depends (besides other factors) on the dropping distance.

Parameters that might affect the fragmentation process might include

• temperature of the hot liquid
• temperature of the cold liquid
• dropping distance
• thermal conductivity
• viscosity
• surface tension
• density
• mass of the drop
• shape of the drop
• energy content

and other factors.

Viscosity probably explains why the glassy sodium borate melt does not show a vapor explosion.

I want to acknowledge that I think the answer by James Gaidis is close and identifies key details but makes a wrong analysis. The Idea that Lidenforsting water could penetrate a small crack is an inconsistent explanation. If you watch the video, there is a clear "detonation" of some sort; An instantaneous point in time where the hot sodium chloride violently expands. The final size of the blob might explain the difference in results, but I am unconvinced.

1. The first thing to note is that sodium chloride will always solidify as a crystalline material even when rapidly cooled, but borax and boric acid will not. This means that on cooling sodium chloride undergoes a first-order phase transition releasing energy from the latent heat of fusion and experiencing discontinuous viscosity that borax and boric acid do not.

2. The second thing to realize is that sodium chloride shrinks upon solidification from about $\pu{1.54g {cm}^{-3}}$ as a liquid to $\pu{2.17g {cm}^{-3}}$ as a solid. This creates a phenomenon similar to producing Prince Rupert's drops that creates a very tempered glass.

With these concepts in mind, we can now know that upon pouring sodium chloride into water it will cool rapidly starting at the leading edge entering the water first. This rapid cooling causes the sodium to solidify on the outside at net shape and will not shrink appreciably further since it is in a crystalline, not a glassy state. As the inside cools and solidifies it shrinks and pulls the outer hardened shell of the salt inward causing it to be loaded under a great deal of pressure similar to the creation of a Prince Rupert's drop.

Salt does not have the covalent bonding needed to tolerate this tensile force well and eventually the inner part will fracture off the outer part. This fracture causes other fractures and the outer layer to decompress and spring outward similar to when a Prince Rupert's drop is scratched or fractured (usually at the tail). As the outer layer of salt springs outward, it further fractures and the pieces come closer to the water which rapidly increases the rate of vaporization causing the observed explosion.

Because, physically speaking, the salt water should never come in contact with the water, thanks to something called the Leidenfrost effect. This effect occurs when a liquid comes into contact with a substance significantly hotter than its boiling point and forms an insulating layer of vapour

• @PeterJohnmeyer-Here if you notice my friend,the salt water comes in contact with the water as soon as you pour molten $\color{aqua}{NaCl}$ into it – user67369 Aug 29 '18 at 15:00