Sometimes physicists are interested in finding out what happens to things when they get really cold. What’s the coldest thing you can think of? (ice is +32 F, dry ice [CO2] is –109 F) What we have here is liquid nitrogen, its temperature is minus 320 degrees Fahrenheit (pour some liquid nitrogen into a container. Discretely put a banana and racquetball in liquid nitrogen for later use). First off, who knows where you can find nitrogen? (air). The air we breathe is about 80% nitrogen gas. So what we have here is liquid air. It took a really powerful refrigerator to make the air so cold that it turns into a liquid. Most gases turn into a liquid when they get cold enough. What’s all this bubbling, does it remind you of anything? (boiling water). It’s actually a lot like boiling water. When you boil water, you make the pot really hot until it gets to a specific temperature, called the boiling point, then the liquid starts turning into a gas. For water the boiling point is 212 degrees, that’s why we need to heat it on a stove. For nitrogen the boiling point is minus 320 degrees, so the liquid nitrogen is boiling at room temperature to become air. What is this smoke coming off the nitrogen, does that remind you of anything? (fog). It actually is the same as fog. The liquid nitrogen makes the nearby air very cold. The air can’t hold as much water vapor when it’s colder, so some of the water vapor condenses into small water droplets that we see as fog, just like you can see over the ocean in the morning.
Now that we know what liquid nitrogen is, we’re going to use it to make other things cold. First I have a balloon. What might happen when I put the balloon in liquid nitrogen? (get some answers, then dip the balloon into the nitrogen for a few seconds). Making the air in the balloon colder makes the molecules in the air slow down so they put less pressure on the balloon. Since there’s less pressure the balloon shrinks. Remember that making the nitrogen in the air cold enough makes it turn into a liquid, eventually the cold air becomes liquid nitrogen too, and you can see this liquid in the bottom of the balloon (take the balloon out of the nitrogen and show liquid in balloon. this is often hard to see). The balloon is also blowing up again. Now that it’s warming up in the air again, the air inside the balloon is going back to its normal temperature and its normal pressure. Now we also know that the balloon didn’t have a leak in it.
Next I have some flowers. I’m going to need a couple of volunteers for this demonstration. (get two volunteers). What might happen if we dip flowers in liquid nitrogen? (get answers) Well, since we’re scientists, why don’t we do an experiment. You guys can each have a flower. Does it feel like a real flower? Good. Now you can each dip the flowers in the liquid nitrogen. (make sure the volunteers put only the flower part in the nitrogen and don’t put their hands in it. A few seconds is plenty for the flower). Now touch the flowers and see what they feel like. Tell your classmates what they feel like. Try to crush the flowers (You can also give the volunteers extra frozen flowers to take back to their friends in the audience). So that was pretty interesting, the flowers froze. What in the flowers would make them freeze when they get cold? (water). Flowers have a lot of water in them, and we know water freezes into ice when it gets colder than 32 degrees Fahrenheit. Most liquids will turn into solids when they get cold enough.
This next experiment has a story to go along with it. One day I was working in the lab and I wanted to hang up a picture, so I needed to put a nail in the wall. Unfortunately, I didn’t have a hammer, but I did have a banana in my lunch. I thought to myself, maybe I can use this banana to hammer in that nail. Who thinks they can hammer in a nail with a banana? (bring up a volunteer who thinks they can do it. Have them hammer away at the nail, and tell them to hit it harder if they aren’t making enough of a mess). Okay, I guess that didn’t work. Does anyone have a better idea? (use a frozen banana). Why would a banana in liquid nitrogen make a better hammer? (because it’s made of water so it freezes when it’s cold). (have the volunteer use the frozen banana to hammer in the nail. Give them two gloves, so they aren’t tempted to touch the banana with an ungloved hand). So now we all know that if you ever find yourself needing to hammer in a nail and you don’t have a hammer, you just need a banana and some liquid nitrogen.
You’ve probably all seen a racquetball before (bounce a room temperature racquetball). Now I also have a racquetball that’s been sitting in liquid nitrogen for a while. What’s going to be different about this racquetball? Is it going to bounce as well? (get guesses). It couldn’t have frozen like the flowers or the banana, because there’s no water in it, and it’s already a solid. Well, since we’re scientists we’re going to do an experiment to find out. I’m going to drop both racquetballs at the same time, and we’ll see what happens, okay. 1…2…3… (drop). The rubber became very brittle when it got cold so it shattered! (pick up a piece) The pieces are more rigid; they don’t bend like before. The rubber in the racquetball acts very differently when it gets cold. The molecules in the rubber don’t want to move when they’re cold; they want to stay close together, so I can’t bend the rubber as easily. It’s very interesting to do experiments with things at different temperatures because all sorts of properties can change.
Since you’ve been such a great audience, we have a special treat for you. Has anyone tried frozen marshmallows before? We’re going to freeze some marshmallows in liquid nitrogen and everyone will get one to eat.
Safety Note: When using liquid nitrogen on stage always use gloves, even if you don’t think you need to. It is important that we get the message across that this stuff can hurt, so if we do spill some the kids won’t try to touch it.