Reader Questions

Reader Question: When Mercury Meets Lava

Reader lucondri asks:

What happens when mercury touches lava?

That’s an interesting thought experiment, but hopefully no one tries it any time soon given mercury’s toxicity. So, what might happen? Mercury has a boiling point just under 630 Kelvin, and, although the temperature of molten lava varies, it’s between 970 and 1470 Kelvin when it first erupts. So mercury would definitely vaporize (i.e. boil) on contact with lava. (Again, this is very bad for anyone nearby.) If you’re curious what boiling liquid mercury looks like, wonder no further.

Molten lava is much, much hotter than the boiling point of mercury, though, so there’s a possibility that the mercury won’t boil away instantly. This is because of the Leidenfrost effect, where a thin layer of vapor forms between a liquid and an extremely hot surface. The vapor has such low friction that the liquid can essentially skate across a surface, and it doesn’t boil away instantly because the vapor insulates it from the extreme heat. After some digging, I found a paper that placed the Leidenfrost temperature of mercury between about 850 and 950 Kelvin, meaning that fresh lava is probably hot enough to generate mercury Leidenfrost drops.

So pouring a lot of mercury on lava will probably result in some boiling, but there’s also a good chance that it will form a bunch of skittering mercury droplets that will stick around awhile before they evaporate into toxic mercury gas. That said, it’s a lot easier and safer to watch awesome Leidenfrost drop videos with other liquids. (Collage credit: N.Sharp; images sources: Z. T. Jackson, and A.Biance)

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