I stumbled across a nice little blog post on liquid nitrogen experiments. Most people don’t believe me when I tell them that in first year chemistry during my Bachelor of Science we were able to put our hands in nitrogen if we wanted after a lecture on gases. The lecturer brought out a large thermos full of liquid nitrogen after he’d summed the lecture up and invited people to come down and dip their hand in it briefly and lift it back out. Everyone was relatively hesitant at first from what I can remember, and for good reason considering we’d all done experiments with it in high school where you froze things like bananas, balloons and sausages, and then smashed them with a hammer. But someone finally got the guts to do it and plunged their hand deep down into the thermos and pulled it back out instantly. His hand was fine as the liquid nitrogen had vaporised instantly due to the large temperature difference between it (-198C) and his hand (~25C), of obviously more than 200C. So it would be like pouring cold water onto a read hot iron for a second, it would hardly cool the iron down at all. A layer of gas forms between the object and the liquid due to this temperature differential, and is a phenomenon known as The Leidenfrost Effect. Unfortunately he also managed to fling a lot of droplets of the freezing liquid up in the air and some poor girl had one land in her eye. Although she initially freaked out it evaporated instantly and she was ok, but for the shock.
Anyway, for those of you who are probably still a little skeptical here’s a photo of someone doing it. The same rule also applies for incredibly hot liquids and dipping your hand in them, as Mythbusters showed with molten lead (I’ll post the video below). I should also add that the Leidenfrost Effect works slightly differently with hot liquids, it requires a layer of liquid that can evapourate on the surface of whatever it is your dipping in it.
Below is a really cool video of someone making solid mercury moulds by freezing it with liquid nitrogen.
Mythbusters video of molten lead and The Leidenfrost Effect.