Liquid nitrogen is great fun, but how careful do you have to be with it? Here's an email I received that asks a question you may be wondering about, too:
We recently had a teacher from another school visit us and pair his students with our students. At the end he made ice-cream using liquid nitrogen to freeze the milk, sugar, cream mixture.My response:
He had a large container of liquid nitrogen. I was concerned about his treatment of the liquid nitrogen. After making the ice cream he started pouring cups and dishes of it on students hands. Later he threw cups of it up into the air over their heads. He let it splash all over students' bodies. The liquid quickly evaporated and no one got hurt but I thought it showed students very dangerous tactics.
Our students might do that with any chemical. They don't know the difference. And not respecting the danger of liquid nitrogen might lead students to put their fingers or hands into it.
We had about 40 students seeing all this. They were ages 14 to 20.
May I ask if I am being overly concerned?
I think a 14-year-old knows chemicals are not equally safe. I don't believe seeing liquid nitrogen tossed into the air is going to lead to experimentation with airborne sulfuric acid, for example. However, I think you have well-founded concerns about cavalier treatment of liquid nitrogen.
Throwing nitrogen into the air where it could land on students is ill-advised. There is a good chance the nitrogen will vaporize before touching skin, but I don't think the effect is worth risking a student's vision, for example. Plus, I suspect that demonstration would lead to students tossing cupfuls of nitrogen toward each other.
I'm all for people having fun with liquid nitrogen. It's very safe when proper safety precautions are used. Those precautions should include eye protection and skin protection to minimize the risk of injury if a spill occurs. I recommend good air circulation, since adding nitrogen to the air diminishes the partial pressure of oxygen, especially near the ground if the nitrogen is a fog.
Making ice cream with liquid nitrogen is one of my favorite demonstrations, but another excellent demonstration is dipping a rose or hot dog into nitrogen and shattering it on a countertop. That demonstration illustrates how cold the nitrogen is. Ask a student what they think would happen to their finger in the nitrogen.
Make Liquid Nitrogen Ice Cream | Liquid Nitrogen Fog
Photo: Image of solid, liquid, and gaseous nitrogen. (chemdude1, YouTube.com)