Cool Stuff: Liquid Nitrogen
In the past I have taught a first-year course (TECH1238) in which we discussed refrigeration and liquid gases. Each term I brought in a dewar of liquid nitrogen (at a cold 77K) and did a few experiments demonstrating cryogenics. The course is now defunct and I've described the demo below. I have also performed this demo for a class of grade 2 children at a local school.

As discussed by James Burke in the Connections series, gases in liquid form are used as a very compact method of storage. By storing a gas in liquid form, vast quantities of the material may be kept in a small space. When gas is required, we simply let some of the liquid boil off.

Here is a view of the large dewar which supplies the liquid nitrogen for our demonstration. The nitrogen is used in both liquid and gas form in our college microelectronics lab where we manufacture thick and thin film integrated circuits. The dewar shown holds enough nitrogen to last the lab about three weeks of operation. If ordinary nirtogen gas were used (in tanks) we'd have a delivery truck arriving virtually each and every day!

The small dewar is used to transport small quantities (4 litres or less) of liquid nitrogen. It is essentially an overgrown thermos flask consisting of an inner and outer shell with vacuum in between. The fog is generated from water vapour in the surrounding air and from large amounts of nitrogen liquid which are boiling away into the environment.

For our demo the liquid nitrogen is poured into a beaker. Looking at the liquid it is evident that it is indeed boiling (it boils at -197 degrees C). Vapour is seen as the cold liquid cools the beaker and surrounding air.

When a balloon containing air is placed into the liquid nitrogen the balloon contracts as air inside is liquified. Nitrogen and oxygen in the air turn to liquid. Carbon dioxide in the air freezes solid into dry ice and we are left with a shrivelled balloon full of dry ice crystals and liquids. When the balloon is allowed to warm to room temperature it expands again to it's original size.