Before doing any demonstration with liquid nitrogen see the Liquid Nitrogen Safety Page

The Water Egg

Materials: water balloon, pin

Take a small water balloon and place it into some nitrogen. Use tongs to hold it under.

This take a while. The water on the outside of the balloon will freeze and expand. At some point the expansion, coupled with the fact that the balloon gets brittle when that cold causes the balloon to pop. You want to remove the balloon from the nitrogen when it is very frozen, but before the balloon fails. Practice or additional water balloons may be a good idea.

Use tongs to remove the balloon. If you managed to get the frozen water balloon out poke a hole with a pin in the one place that it probably didn't freeze on the outside - where the air bubble was located. I find it is best to hold the balloon in a gloved hand and use an non-gloved one to do the next step. Poke a hole at the bubble and allow the water to drain out and peel away the balloon. Draining it into the nitrogen produces spectacular cloud action! What you will have behind is a hollow sphere of ice a water egg! This is just plain cool.

It also illustrates a few important things about nature. Many students will have expected the entire water balloon to have frozen. What they may not know is that ice is a good insulator. Point out the examples of the igloo and how ice on the top of a pond insulates the water below and prevents the entire pond from freezing. This protects the life within during winter.