Q: Why does liquid nitrogen cause the ping pongs to fly everywhere?

Liquid nitrogen (N2) is very, very cool.  At normal atmospheric pressure it boils at temperatures above -195.8 degrees Celsius, or -320.4 degrees Fahrenheit.  See?  It’s so cool, it’s downright cold.  (Please forgive my bad jokes…)  Like gaseous nitrogen, liquid nitrogen is clear and colorless, resembling water.  Liquid nitrogen is used to preserve biological samples, freeze food, cool supercomputers and other equipment, and remove skin abnormalities, among other things.

And, of course, you can do a lot of really awesome science demonstrations with liquid nitrogen.

I’ve been lucky to run and participate in a lot of science demonstrations over the years, and many of them involve liquid nitrogen, but none of those liquid nitrogen demos have been as explosive as this.

In this demo, a bottle is filled with liquid nitrogen.  The bottle is sealed and placed in a large container holding hundreds or thousands of ping pong balls.  Because liquid nitrogen becomes a gas at room temperature, it boils in the closed bottle.  As the liquid nitrogen turns into gas, it expands- the liquid-to-gas expansion ratio of nitrogen is 1:694, meaning a tiny amount of liquid nitrogen can take up a much larger space as gas.

The bottle is sealed, so the nitrogen is trapped and the pressure builds.  At some point, the bottle can no longer tolerate the pressure, and the bottle bursts open, releasing the nitrogen gas.  This explosion forces all the light ping pong balls to go flying everywhere.

Note that these demonstrations with liquid nitrogen “bombs” take place outdoors or in very large spaces.  If enough liquid nitrogen expands into gas in a smaller room, the composition of the air will change, and a person in the room will end up breathing in more nitrogen and less oxygen.  This could lead to asphyxiation, and the person could pass out or die.  Because of this potential danger, scientists often have oxygen alarms in areas where the store or use liquid nitrogen.  The alarm is triggered when the oxygen level drops too low, alerting researchers to vent the area of excess nitrogen.

If you ever get a chance to see this demo live, do it.  If you’re ever the one actually running this demo- I mean, I’m super jealous, but I do not envy your clean-up.

Keep calm and science on.

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