If you’ve ever dropped water onto a hot surface, you’ve probably noticed that it skitters around rather than just staying in place. It also doesn’t immediately evaporate into water vapor. This occurs because of a physical phenomenon known as the Leidenfrost Effect.
When a liquid comes in contact with a hot surface that is significantly above its boiling point, the bottom portion of the droplet vaporizes immediately. The resulting layer of vapor acts as a cushion for the remainder of the liquid water and prevents direct contact with the hot surface. Because steam is a poor thermal conductor, further heat transfer between the hot surface and liquid water is slowed dramatically. This prevents the remainder of the liquid water from immediately evaporating. Since the droplet is resting on a cushion of vapor, it can easily slide around the surface of the pan.
The temperature necessary for this effect to occur (Leidenfrost Point) with water is 193°C or 379°F.
Source: https://youtu.be/mZenCYF1IpM
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