Before doing any demonstration with liquid nitrogen see the Liquid Nitrogen Safety Page
This is a favorite demo for small groups (20 or so), but I suppose it could be scaled up for large classes.
Make ice cream using liquid nitrogen! It makes good ice cream for two reasons:
1) The mix freezes very quickly, so you get small crystals and a very creamy texture.
2) The evaporating Liquid N2 aerates (nitrogenates?) the mixture, so it doesn't end up as a frozen lump.
This demo ties in nicely with phase changes.
Recipe for ~1/2 gallon of strawberry ice cream (adapted from a Ben & Jerry's recipe):
4 cups heavy cream
1 ½ cups half and half
1 3/4 cups sugar
1 quart frozen strawberries that have been allowed to thaw or fresh strawberries, mashed
½ cup sugar
Use ice cream cones (or bowls with spoons) for serving.
Keep all ingredients cold! Make sure the sugar is dissolved in the cream base. Pour the cream base into a large metal bowl. Add one to two liters of liquid nitrogen and stir vigorously. When the cream has thickened, add the berry mixture and more nitrogen, if necessary. Continue to stir until the nitrogen has evaporated (the fog has disappeared).
This recipe does not keep well, and is best consumed immediately. If it begins to melt before everyone is served, simply add more liquid nitrogen and stir.