freezing - Dangers of using liquid nitrogen

So, I remember in college physics the prof using liquid nitrogen in a demonstration. When he was done, he threw the container of LN at the front row, and of course it evaporated (or whatever) before it got to the students.

I am watching a cooking show now and they are using LN -- if they touched that, what would happen? Would it be possible for the chefs to pour the LN onto their skin accidentally, or would it evaporate before it reached their skin?

Answer

Liquid nitrogen boils when it comes in contact with skin, so small amounts of spatter are no danger at all-- the droplets just bounce off. I regularly pour a liter or so (a bit at a time) out on a lab table when I do liquid nitrogen demos, with no problems or safety gear.

The biggest risk from the low temperature is getting it into fabric of some sort, which will hold it in closer proximity to skin for longer than the drops by themselves will. I have, on occasion spilled some on my pants, which is annoyingly cold, but not too bad unless you're wearing really tight clothing.

Really, the biggest hazard from any of the nitrogen demos I do is not the temperature but the expansion. When it boils, it expands to something like 700 times the volume of the liquid, so if you put some in a sealed container, it can make a big bang. I know of a case where a grad student at MIT destroyed a bathroom with a 2-liter bottle of liquid nitrogen.

You can use this expansion for a kind of cool demonstration if you take one of those little dropper bottles with the angled spouts that are common in chem labs, and seal a little nitrogen inside. Put it down on the floor, and it spins like a firework. The problem with that is, you almost always end up getting a little water vapor condensing in the spout, which plugs it up, and then the bottle will go bang. I had a student a few years ago who had one go off in his hand, and he said it stung pretty badly.

I have seen videos of people (Jearl Walker in particular) "drinking" liquid nitrogen by taking a small amount into their mouth, and holding it there for a second or so-- the instant boiling will keep it from giving you oral frostbite for a little while, and you can spit the liquid out after breathing out over it, which makes an enormous plume of steam. It's really cool to see, but kind of risky to do. I've never had the guts to try it myself.