Two identical pans are maintained at 300°F and 400°F on a burner. A half ounce of water is added to each pan. How much faster does the water in the 400°F evaporate than the 300°F pan?

The principal was first observed by Johann Gottlob Leidenfrost, an 18th century German doctor. Turns out that if you give a drop of water on a pan enough energy, the steam or gas that it produces will actually suspend or lift the water droplet off the surface of the pan. No longer in direct contact with the pan and insulated by this layer of steam, the transfer of energy between the pan and the water becomes quite inefficient, thus the water takes a long time to evaporate.

The Leidenfrost effect is a physical phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid's boiling point, produces an insulating vapor layer keeping that liquid from boiling rapidly. Due to this ‘repulsive force’, a droplet hovers over the surface rather than making physical contact with it.The effect can be seen as drops of water are sprinkled onto a pan at various times as it heats up. Initially, as the temperature of the pan is just below 100°C (212°F), the water flattens out and slowly evaporates, or if the temperature of the pan is well below 100°C (212°F), the water stays liquid. As it goes above 100°C (212°F), the water droplets hiss when touching the pan and these droplets evaporate quickly. Later, as the temperature exceeds the Leidenfrost point, the Leidenfrost effect comes into play. On contact with the pan, the water droplets bunch up into small balls of water and skitter around, lasting much longer than when the temperature of the pan was lower.