Before doing any demonstration with liquid nitrogen see the
Liquid Nitrogen
Safety Page
The Boiling Tea Kettle
Materials: a non-white whistling tea kettle
The boiling tea kettle is a simple but effective demonstration. Simply carefully poor some nitrogen into a whistling tea kettle and put the top back on. The kettle will instantly begin to whistle and "steam" will be visible shooting out of the kettle. At this point the tea kettle looks and acts just like the kind that kids are used to seeing when water has been heated to a boil. The nitrogen is boiling just like water would when heated. What has heated the nitrogen to boiling? The room temperature kettle! If your group is quiet and you remove the lid or the whistle from the kettle you can hear the nitrogen boil.
In a matter of moments after the nitrogen has been added to the kettle frost will begin to accumulate on the side of the kettle. Since the frost gives the kettle a white coating, using a colored kettle works best. After the nitrogen has left the kettle the frost will melt and further water from the air will condense on the kettle and the kettle will be well coated with moisture. So be sure to leave the kettle in a visible place while doing something else to allow time for the frost to melt. Often a student will notice the change.