CO2 and He balloons in liquid nitrogen.
What it shows:
Cooling a gas causes a proportional decrease in volume with the drop in absolute temperature. A gas such as helium, which remains close to ideal at low temperatures, shows a four-fold decrease in volume when taken from room temperature 330K to liquid nitrogen temperature, 77K. Carbon dioxide however, sublimes at 194.5K, so is solid at 77K. Oxygen liquefies at 90K (S.T.P.). A qualitative demonstration of these effects can be shown with gas filled balloons.
How it works:
Helium and CO2: Balloons filled with helium and CO2 are lowered into a flask of liquid nitrogen. Placed just above the surface of the LN2, they are in an atmosphere of boiling nitrogen. The helium balloon will deflate, and the CO2 collapse as the gas solidifies. Both will expand to their former glory when returned to 300K air.
Oxygen: An oxygen filled balloon can be stretched over the mouth of a test tube (which has been purged with O2). When the tube is submerged in LN2, the balloon collapses as the oxygen condenses into a pale blue liquid at the bottom of the test tube. Lifting the tube from the LN2, the liquid boils and reinflates the balloon.
Setting it up:
Use a 4L clear glass dewar 1 about a third full. A pair of insulating gloves should be used by the demonstrator.
For the CO2, a member of the audience could blow up the balloon, there being sufficient carbon dioxide in exhaled breath. There is also water vapor of course.
1 Flask from Kontes & Martin, Evanston, Ill.