LIQUID NITROGEN DEMONSTRATIONS
Liquid nitrogen is a cryogenic liquid (of or related to low temperatures) and is stored at 77K (-199oC). The liquid boils at –196oC. Since liquid nitrogen is so cold it has many uses as a refrigerant (including keeping some electronic equipment cool and preserving simple living organisms for long periods of time) and is also used to form inner atmospheres for the preservation of other materials. Because of its cryogenic properties, the interaction of liquid nitrogen with other materials can lead to reactions which are scientifically interesting, some of which will be described in the following procedures. ( Special Note : Always handle liquid nitrogen and any materials which have been submerged in liquid nitrogen with cryogloves.)
Banana Hammer:
Place a banana in liquid nitrogen. The nitrogen will boil rapidly, and then slow down. At this point the banana is hard enough to hammer a nail into a small piece of soft wood. If removed too early, the banana will be too soft to pound the nail, but if left in the liquid nitrogen too long, it will be too brittle and will break.
Shrinking Balloons:
Have students blow up balloons and place in the shallow pan filled with liquid nitrogen. The balloon flattens as the air condenses and the rubber becomes brittle. You can either shatter the flattened, brittle balloon, or carefully remove it from the liquid nitrogen and allow it to re-inflate as the air inside the balloon warms up to room temperature.
Shattering Materials:
Several materials are good to submerge in liquid nitrogen and then either shatter or crumble. A racquetball bounces, of course, before being placed in liquid nitrogen. After several minutes the ball will be hard enough to shatter when bounced. If the ball is removed from the liquid nitrogen too early, it will bounce like a billiard ball. A small piece of rubber hose will shatter after being in liquid nitrogen for several minutes. Flowers crumble nicely after submerging in liquid nitrogen. Each of these activities takes some practice to get your timing right.
Cloud Nine:
Boil a large beaker full of water, and pour the boiling water into a large bucket. Pour liquid nitrogen into the bucket, and a “cloud” instantly appears.
Superconductivity:
Cut off the bottom 2 cm of a Styrofoam cup. Place a superconducting ceramic disc in the cup and pour liquid nitrogen in the bottom of the cup. Place a small magnet on top of the disc and the magnet will levitate. Since the demonstration materials are rather small, a good way to project this so that your entire class can see it is to use an overhead projector laid on its side. The projection will appear upside down.