Water lasts longer on a very hot pan than on a warm one.
Flick water onto a pan just above boiling and it hisses away at once. Make the pan much hotter, around 193°C, and the drops bead up and skate about for far longer. They are hovering on a cushion of their own steam, which insulates them from the heat.
The Leidenfrost effect is named after the German doctor Johann Gottlob Leidenfrost, who described it in 1756. You can watch it happen by sprinkling water onto a pan as it heats. Below 100°C the water spreads out and slowly evaporates. Just above boiling, droplets hiss and vanish quickly. But once the pan passes the Leidenfrost point, roughly 193°C for water, drops curl into little balls that glide across the metal and survive surprisingly long.
The reason is that the bottom of each droplet flashes into vapour the moment it touches the hot surface. That thin layer of steam holds the rest of the drop up, so the liquid no longer touches the metal. Steam conducts heat far worse than metal, so heat reaches the drop much more slowly. The vapour also acts as a near-frictionless bearing, which is why the drops skitter about.
The exact temperature at which this starts is hard to predict: it depends on the surface, on impurities in the liquid and more. Victorian boiler engineers found it mattered. The designer William Fairbairn cited experiments in which a drop that vanished almost at once at 168°C lasted 152 seconds at 202°C, meaning an overly hot boiler surface could actually transfer heat to water less effectively.
The same physics works in reverse with very cold liquids. Liquid nitrogen is so far below room temperature that droplets can roll harmlessly off skin, cushioned by their own vapour. Researchers have also built a prototype Leidenfrost heat engine, which benefits from the extremely low friction, and used levitating droplets to concentrate molecules for sensitive chemical analysis.
Source: Wikipedia — Leidenfrost effect · Text summarised from Wikipedia (CC BY-SA 4.0)
