Earlier this week, I noticed some folks over on Facebook sharing a video of a red hot ball of nickel being dumped in a bucket of water, so out of sheer curiosity, I decided to take a look.
It doesn’t do what you might think it does.
Of course, this made me even more curious, so I did some Googling, and found this explanation:
For those of you who don’t know, this is an example of the leidenfrost effect. The ball is so hot that the water touching it immediately evaporates and the vapor around it insulates it from the rest of the water (the vapor is a much worse thermal conductor than liquid water). This is why you can stick your hand in liquid nitrogen for a short period of time. However, eventually the nickel ball cools to a point where it’s not hot enough to instantly vaporize the water and it stops.
The Wikipedia article on the Leidenfrost effect has a more detailed description of what’s going on:
The Leidenfrost effect is a phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid’s boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly. This is most commonly seen when cooking; one sprinkles drops of water in a pan to gauge its temperature—if the pan’s temperature is at or above the Leidenfrost point, the water skitters across the metal and takes longer to evaporate than it would in a pan that is above boiling temperature, but below the temperature of the Leidenfrost point. The effect is also responsible for the ability of liquid nitrogen to skitter across floors. It has also been used in some potentially dangerous demonstrations, such as dipping a wet finger in molten lead or blowing out a mouthful of liquid nitrogen, both enacted without injury to the demonstrator. The latter is potentially lethal, particularly should one accidentally swallow the liquid nitrogen.
The article then goes into detailed scientific formulae on calculating the Leidenfrost point, heat transfer correlations and that kind of thing, but for us laymen, the synopsis above is perfectly good enough.
What really interested me afterwards is that, while attempting to hone my rather non-existent cooking skills, I have seen this before; I just didn’t know what it was, or indeed give it any second thought, until now. In fact, this can easily (and relatively safely) be reproduced in your kitchen: heat up a stove plate to full, then put a drop of water on it. (Just don’t try any of the potentially lethal demonstrations with molten lead or liquid nitrogen, unless you’re presenting an episode of Mythbusters.) This free image from the Wikimedia Commons shows what happens when you try this:
You learn something new every day.