Demonstrations 14 – Liquid Nitrogen I – 3
Introduction
Liquid nitrogen is a cryogenic liquid – one that deals with low (and we mean low) temperatures. It is stored at 77 Kelvin (that’s –199 °C, or –326 °F, roughly the same temperature as the surface of Saturn), and boils at –196 °C. Since it is so cold, it has many uses as a refrigerant – it’s used to keep some electronic equipment cool and to preserve simple living organisms for long periods of time. It is also used to form a protective, inert atmosphere for the preservation of other materials, such as paper and film. Because of its cryogenic properties, the interaction of liquid nitrogen with other materials can lead to reactions that are scientifically interesting, like those in the following demonstrations.
Materials and Equipment
Dewar flask of liquid nitrogen “Dewar bucket”
Cryo-Glove(s)® Racquetball
Banana, apple, grapes, or other fruit Tygon® tubing, 3” – 5” length
Tongs Crucible tongs
Safety Considerations
§ As always, you should wear safety goggles/glasses when working in the lab area. If aprons are available, wear one. When working with liquid nitrogen, you should always have a pair of Cryo-Gloves available – and wear them!
§ Liquid nitrogen is some of the neatest stuff you will ever work with, but it’s also one of the most dangerous substances you’ll ever deal with. Treat this material with caution and respect – it can hurt you very badly!
§ Any time you work with liquid nitrogen, you must remember that it is a boiling liquid – despite the fact that its boiling temperature is –196 °C. When you pour it out of the large Dewar flask, you are pouring a boiling liquid. Treat it as if it were boiling water and behave accordingly.
§ As an indication of how potent liquid nitrogen is, stop for a moment and realize that dermatologists use this liquid to “burn off” or “freeze off” moles, warts, skin “tags,” and even skin cancers – a process known as cryosurgery.
§ Don’t get liquid nitrogen on yourself or anyone else! It will cause almost instant frostbite on the skin it comes in contact with – this is quite painful. Add to that the fact that the tissue will die, turn black, and slough off in a matter of days, and you get one really good reason not to play with this stuff – even though working with it seems like playing.
§ Never handle an object that has been immersed in liquid nitrogen barehanded. Use a Cryo-Glove. Think about the kid in the movie The Christmas Story who stuck his tongue to the flagpole. Objects frozen in liquid nitrogen will be just like that flagpole – your skin will stick to them and be damaged.
§ Never reach into the “Dewar bucket” to retrieve objects you’ve frozen. Always use tongs, and don’t leave the tongs in there too long.
Procedure A: Frozen Foods
1. Pour approximately 1½ liters of liquid nitrogen into the “Dewar bucket.” Have your classmates clear their book bags, purses, and other belongings out of the aisles in the room.
2. Using a “sliding” technique, gently place the banana in the liquid nitrogen – don’t just “plunk” it over in there. Let the boiling subside a bit before you try to move the bucket.
3. Gently add the apple and grapes to the bucket.
4. While you’re waiting for the fruit to freeze, carefully carry the bucket around to show your classmates what’s happening. Hold it low enough for everyone to see over into the bucket – it looks like you’re deep-frying a turkey in there! Remember, though, that you’re handling a boiling liquid and should be extremely careful when moving about with this stuff. Return to the front of the room, away from your classmates.
5. After a few minutes – or when the liquid nitrogen has just about boiled out of the bucket – use tongs to re-move the banana from the bucket. Hold it up at the front of the room so that everyone can see the “smoke” (actually water vapor condensing out of the air around the banana) coming off of it. While you’re showing it off, “accidentally” drop the banana onto a hard surface, such as a countertop or desktop (try not to drop it onto the floor).
The banana will sound like a rock hitting the desktop, and will break into several pieces. If any of the pieces fall onto the floor, use the tongs to pick them up immediately, before they turn into a really nasty mess. Throw the “banana shrapnel” into the trash can.
The question always arises: “Can you eat that banana?” Your answer is that (a) you would have to wait
until it thaws so you don’t break your teeth on it, and (b) while you could theoretically eat a banana that had
previously been frozen in liquid nitrogen, you wouldn’t want to, because it’s going to get all slimy and brown
before too long.
6. Repeat the “show and tell” process (Step 5) with the apple and grapes. These will be harder to fish out of the bucket because they’re round, but work with them a bit. The caution box above applies to these objects, too.
Procedure B: The Shattering Ball
1. Bounce a racquetball on the floor several times so that your audience knows it isn’t “rigged.”
2. Pour the Dewar bucket about half full of liquid nitrogen.
3. Immerse the racquetball in the liquid nitrogen for a few minutes. You may have to hold it there with the tongs – tell your audience some of the other properties of liquid nitrogen (see the Introduction) while you’re waiting for the ball to freeze.
4. After a few minutes, remove the ball with tongs. Making sure you’re wearing a Cryo-Glove, drop the ball or gently throw it at the floor. The ball is hard enough to shatter when “bounced” – well away from any innocent bystanders. If you removed the ball too early, it will bounce like a billiard ball (i.e., not very well) – it may not break, but it’ll sound like a rock hitting the floor and just sort of sit there.
A tennis ball will work here, but a racquetball works much better – the tennis ball will bounce like a billiard ball. Ping-pong balls don’t work very well at all. “Bouncy balls” still bounce very well – they just sound like marbles hitting the floor.
Procedure C: No Plastic in Space
1. Take a 3 – 5” length of Tygon clear plastic tubing and show your audience how flexible it is. Bend it, twist it, and explain that this type of tubing has made life much easier and more convenient for chemists.
2. Slip one open end of the tubing over one of the “hooks” of a pair of crucible tongs. Bend and twist the tubing so that the other open end will fit over the other “hook.” You should now have a loop of tubing. Demonstrate its flexibility one more time.
3. Immerse the tongs and tubing in liquid nitrogen for a couple of minutes. While you’re waiting for the fun to start, explain to your audience that Tygon tubing is one byproduct of the space program, along with things like Tang® orange drink, much of our electronics, and much of our language.
4. Remove the tongs and tubing from the liquid nitrogen and hold them up so your audience can see the “smoke” coming off them (actually water vapor from the air condensing around the super-cold parts).
5. Hold the tongs well away from you and your audience. Make sure your safety goggles are in place. Open the jaws of the tongs – the tubing will shatter convincingly, sending bits of plastic shrapnel all over the place. Explain to your audience that because of the extreme temperatures in space, there are some materials that aren’t suitable for use outside a spacecraft.
Procedure D: She Loves Me, She Loves Me Not …
1. Pour the Dewar bucket half full of liquid nitrogen. Be careful to splash as little as possible around the area.
2. Immerse a long-stemmed daisy, chrysanthemum, or other many-petaled flower into the liquid nitrogen and hold it for about 45 seconds (or until the boiling subsides).
3. Hold up the flower so the audience can see it.
4. Using your Cryo-gloved finger, “plink” the petals away a few at a time. For a dramatic finale, slam the flower head down on the desk – petals and frozen bits of flower will fly everywhere.
5. Try to sweep up as many of the petal pieces as you can, before they have a chance to get brown and nasty (and stick to the desktop or floor).
Disposal and Cleanup
§ Make sure you’ve cleaned up any bits of banana, tomato, or whatever that have made it to the floor. They will get really juicy, slimy, and brown rather quickly.
§ Your instructor will tell you what to do with any leftover liquid nitrogen. He or she may have another demonstration in mind. In any case, do not pour it back into the Dewar flask – remember your lab rules about returning chemicals to the stock container!
University of South Alabama Introductory Manual: Demonstrations
Science in Motion 2003 Liquid Nitrogen I: Freezing Things