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Note: I was not able to get any more liquid nitrogen to make a video version of this demonstration. The entire change takes only about 30 seconds and the attraction to the magnet is easily observable.
What You Do...
A test tube and balloon full of oxygen gas are suspended from the ceiling. The test tube is carefully lowered into a Dewar flask of liquid nitrogen.
What You See...
The balloon shrinks as the gas cools. The balloon also becomes brittle looking. When you lower the flask away from the test tube, a pale blue liquid is visible in the tube... liquid oxygen. The tube immediately frosts up, so you need to carefully wipe away the frost from the VERY cold test tube.
There is a discussion of why liquid oxygen is blue in Bassam Shakhashiri's demonstration book.This lecture experiment also provides a demonstration of the Gaw Laws (as the temperature lowers, the volume and pressure of the gas decreases) as well as intermolecular forces of attraction (the fact that the nonpolar molecule, O2, is so difficult to liquify signifies a very small attraction between the molecules... London dispersion forces.
Oxygen is an oxidizer. The test tube must be clean so no oxidizable materials (fuels) come in contact with the liquid oxygen. The liquid nitrogen and liquid oxygen are also very cold so they should not be touched. Disposal is easy... I just pour it on the floor.