If you heat a metal plate (think saucepan) up to around 200 degrees Celcius, then a drop of water dripped onto the plate will fizzle violently and last about 10 seconds before evaporating completely. However, if the plate is at a temperature of around 250 degrees celcius, the water drop will sit on the plate quite happily and last for about 70 seconds. Why?

This fact was first reported by Herman Boerhaave in 1732. It was not investigated until 1756 when Johann Gottlob Leidenfrost published "A Tract About Some Qualities of Common Water." This was not translated from Latin until 1965. A quick explanation:

When the temperature of the plate is less than the "Leidenfrost point" (about 220 degrees Celcius for water), the water spreads over the plate and rapidly conducts heat from it, resulting in complete vapourization within seconds. However, when the temperature is at or above the Leidenfrost point, the bottom surface of the drop almost immediately vapourizes. The pressure of the gas in the vapour layer prevents the rest of the drop from actually touching the plate. The layer thus protects the droplet from conduction and radiation of heat for a minute or so. Although the vapour layer is only about 0.1 mm thick, it still floats the droplet there and dramatically slows its vapourization.

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