Hands-on Science Carnival 2010 Activity Stations: Material Properties

Liquid Nitrogen

[Shopping List: half & half; sugar; vanilla; popsicle sticks; 3 oz plastic cups; 3 oz Dixie cups; small plastic spoons (Wendy’s ones work best); 2 large pitchers; chocolate/caramel/strawberry syrup; rags; kids gloves; styrofoam bowls in styrofoam insulating bowls; bananas; nails; rough wooden board; flower; penny; rubber band; hammer; balloons. Liquid nitrogen will be provided in a large Dewar.]

Ice Cream

1. Carefully pour liquid nitrogen into small styrofoam bowls for easier dispensing.
2. Mix half & half, sugar, and vanilla in a big pitcher to taste.
3. Place a plastic cup inside a Dixie cup, and fill one third or one half full of the cream mixture.
4. Tell the kids to stir the cream constantly with the popsicle stick, and add one spoonful of nitrogen at a time (add the liquid nitrogen for the kids, don't allow them to get it themselves). Wait to add more nitrogen until the cloud has disappeared to avoid burning the kid with nitrogen splashing. You will need to add nitrogen 5 or 6 times.
5. Add syrup, if desired. You might need to add one more spoonful of nitrogen after adding syrup, as it will warm up the cream.

Flash Freezing

1. Dip a balloon into the nitrogen and watch it shrink
2. Submerge a flower in the nitrogen and then crumble it in your hands. Show the difference between the pliable non-frozen flower and the rigid frozen flower.
3. Drop a penny into a bowl of nitrogen – after it has frozen, retrieve it and smash it with a hammer. It will shatter!
4. Submerge a banana – once it has frozen use it to hammer a nail into the board.
5. Freeze anything else around that you think would be interesting.

Nitrogen and Hot Surfaces

1. Drop some nitrogen from the spoon onto the ground or gently onto the table, and watch the instant cloud and tiny balls of nitrogen that run around on the table.

What's Happening: At atmospheric pressure Nitrogen is a solid below -346°F (its freezing point), a liquid below -321°F (its boiling point), and a gas at any higher temperature. Since the cream is warmer than the liquid nitrogen, it transfers some of its heat to the nitrogen when in close contact, turning the nitrogen into a gas. In this way, the cream solution transfers more and more heat, and is gradually cooled as more nitrogen is added, until the water freezes. This process is much faster than making ice cream in the freezer, which can take 30 minutes or more! Continually stirring the cream will prevent large ice crystals from forming, making the ice cream taste creamier.

Flash freezing something is exposing it to a cryogenic liquid, and letting the water instantly crystallize. Large crystals cannot form in this way, like they would if the object was slowly frozen. Different materials change phase at different temperatures, which is why not everything dipped into the nitrogen thaws out at the same time. The rubber band has very little water, and so can thaw out more quickly than the fruit or flower. The balloon shrinks when exposed to liquid nitrogen because the gas molecules inside transfer heat to the boiling liquid nitrogen, slowing them down and reducing the gas pressure inside the balloon. If the various gas molecules cool enough they may even condense into a liquid (or a solid for carbon dioxide), which you can see by quickly cutting open the balloon.

Any surface around you is so much hotter than the liquid nitrogen that it will instantly boil the nitrogen – make it transition to the gas state. The same thing is happening with your hand – body temperature is hot enough to boil a liquid! The nitrogen won’t “burn” you unless there is prolonged contact (it only takes a couple of seconds though!) Technically it will never burn you like a fire or stove, it will rapidly freeze your skin and cause cryogenic burns. We interpret the sensation as burning because the body’s response to extreme hot and cold temperatures is much the same.