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Liquid Nitrogen Ice Cream!

From:
Beth Stadler, Assistant Professor, Electrical and Computer Engineering, University of Minnesota.

I took a recipe from the Rochester MRS University Chapter's "Science Demonstrations Handbook" initially but have tried many varieties since- most popular is "Make your own flavor." I've found that any tasty mix of cream and/or milk and sugar will work, but of course it is creamier when the ratio is tipped!

(A special thanks to Mr. Yanchunas, chemistry teacher at Chardon High School in Ohio, for passing on his enthusiasm for science! Thanks Doc Yan!)

Jump to: What you need / Directions / Science to discuss and notice


What you need

  • 1 quart (950 ml) heavy cream
  • 1.5 cups (350 ml) sugar
  • 2 cups (475 ml) milk
  • 1 teaspoon (5 ml) vanilla
  • 5 quarts (4.75 L) liquid nitrogen
  • Favorite ice cream additives (Oreo bits, chocolate chips, chocolate syrup, fruit jelly, caramel, mini M&M's, marshmallows, you name it!)
  • Wooden spoon
  • Large bowl
  • Styrofoam cups and plastic spoons
  • Napkins

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Directions

Blend ingredients. Add liquid nitrogen slowly while stirring with spoon until it approaches ice cream consistency.

Scoop about ¼ cup into each student's Styrofoam cup and let them scoop in their favorite additives. You can then add a small amount of liquid nitrogen to each cup to firm up ice cream, or allow them to eat "blizzard" style. The latter is often preferred because the liquid nitrogen can spatter a little (not enough to be harmful, but students don't like it).

I've done this at birthday parties (and many other places) for 5-15 year olds. They all love it. Who wouldn't like ice cream in a demo?

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Science to discuss and notice

1) Temperature effects on the state of different materials:
Pour some liquid nitrogen into the bowl before making ice cream.

Liquid nitrogen is liquid air. Take a deep breath. That's mostly nitrogen at room temperature (68F/20C). Liquid nitrogen is air at 77 Kelvin (about -328F/-200C). See the "steam" coming off? It is evaporating and adding to the room air.

To help think about it, picture water. It's steam (gas) at high temperature (>212F/100C), liquid at room temperature (32-212F/0-100C), solid at low temperature(<32F/0C). When a kettle boils, does the steam go up or down? (up) Is this steam going up or down? (down) That's because it is colder (and therefore more dense) than the rest of the air in the room. If doing this demo in winter (or most of the year in Minnesota), drop some snow into the liquid nitrogen. It will actually cause the liquid nitrogen to boil because the liquid nitrogen thinks the snow is HOT (like 212F/200C hot)!

2) Heat capacity and temperature-effects properties of materials:
Here in Minnesota, just about everyone has gotten their tongue stuck on metal in the winter as a child (ok, that is a slight exaggeration). Anyway, ask students if they know about this phenomenon.

The trouble begins with the fact that putting together two things at different temperatures makes them want to reach the same temperature. So the metal tries to heat up to body temperature to match the tongue, but it takes a lot of energy to heat it up. Also, the metal conducts away each bit of heat as it comes, so the whole metal block is trying to heat up at the expense of the tongue. Only a little bit of surface needs to heat up for ceramic, most plastics and wood, because they are insulators - so the tongue heat doesn't conduct away from the surface.

Back to heat capacity. By definition, that is the amount of energy it takes to heat 1 gram of a material by 1C. Water and metal have a high heat capacity. Foam often has a low heat capacity, as does air.

So... if you drop a strawberry into liquid nitrogen, would you want to pop it right into your mouth? NO!! It will try to suck the energy out of your tongue and cheeks and roof and anywhere else it can to get 200+ degrees out of you!

What about a marshmallow? SURE!! Go ahead and try it. (Drop mini marshmallows into liquid for a minute, then onto the table first so that all liquid nitrogen is off of them.) Your mouth has plenty of energy enough to heat the marshmallow up 200+ degrees! As an aside, did the properties of the marshmallow change when it was 437F/225C cooler than room temperature? (They crunch like the dried marshmallows in cereal, but get soft again as they warm up). This is fun. I usually do a large marshmallow first. I'll pop it into my mouth and crunch it, then exhale so the cold air mists in front of me. I've given preschoolers mini marshmallows, so I'm sure they are safe for older kids too!

It is also fun to lower a blown-up balloon into liquid nitrogen. It shrinks and becomes less flexible. Then, it gets bigger as it warms back up. Also, bounce a racquet ball a bit, then drop it into the liquid nitrogen. When it stops boiling, throw the ball against the wall. It appears to explode as it shatters. Remember the Space Shuttle explosion in 1986? (Probably before middle school kids' time.) It was too cold that day and an o-ring (made of rubber) was below its glass transition temperature so that it wasn't flexible anymore and couldn't perform its job of sealing.



Stirring it up...


Blowing cold steam at each other.
(Don't inhale- it doesn't have the same oxygen
content as air- it's pure nitrogen!)

 

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