cryogenics

Here are demonstrations of some of those effects.
I will add more as time and my liquid nitrogen supply permit.

Liquid nitrogen is dangerously cold.
Direct contact with the liquid or gas that is boiling off or with materials/containers cooled by it can cause severe frostbite very quickly.

It expands more than 700 times when it goes from a liquid to a room temperature gas.
If confined it can produce pressures that can burst nearly any container. Pressures of over 40,000 pounds per square inch are possible.
A pop bottle or a thermos with the lid on is an invitation for disaster
Sealed containers are bombs if there is no way for pressure to be released.

Some materials become very brittle and are easily broken when cold.
This includes some steel and many plastics.
Don't use untested materials.

As it boils nitrogen gas is released.
It is not poisonous but it will dilute the oxygen content of air thereby reducing it.
If the oxygen level gets too low you will lose consciousness and at still lower concentrations you will die.
Make sure of adequate ventilation during your experiments.

It is possible to make liquid oxygen by condensation of oxygen from the air. This can result in a fire or explosion hazard if the liquid or the high concentration of oxygen gas that results when it evaporates contacts combustible materials. This includes materials that you wouldn't ordinarily think of as flammable. Eliminate all sources of ignition including flames, sparks, and ground apparatus to avoid static discharges.

The temperature of liquid nitrogen which is boiling at normal atmospheric pressure is:
77 ° Kelvin
-196° Celsius
-320° Fahrenheit
139° Rankine
-157 Reaumur
These are just different ways of representing the same temperature. Just like we can measure the same distance in inches, centimeters, miles or other units.
To give you some sense of how cold it is, it is about as much below room temperature as a pizza oven is above it.

Many things change their physical, chemical and electrical properties when cooled to the temperature of liquid nitrogen. The pictures below show some of these effects.

One of the simplest experiments to try is to just spill a few cc's of liquid nitrogen on a hard smooth floor. You will see that each droplet forms a bead and moves quickly around on the floor as if it were frictionless. Close examination of one of the droplets will show that it isn't touching the floor at all. Instead it is sitting on a thin film of gas that is coming from the bead. This is called the Leidenfrost effect, named for a German experimenter who investigated it using water and a hot metal plate in the mid 1700s. The film of gas that is formed is an effective insulator so the evaporation rate is considerably slower than you might expect. The droplets also have the tendency to pick up dust from the surface and as they evaporate the dust is concentrated often forming a tiny ball when the nitrogen has completely evaporated. These pictures show some liquid nitrogen when it was initially poured on a surface and later when dust and possibly frost has collected on it.

Superconductivity can be demonstrated with a disk of yttrium barium copper oxide (Y Ba2 Cu3 O7) ceramic and a neodymium-iron-boron (or other strong) magnet. What you see in the pictures below are the ceramic disk with a small powerful magnet sitting on it. As the disk is cooled by the liquid nitrogen the Meissner effect forces the magnet's field out of the disk which causes the magnet to rise supported by its magnetic field. The magnet is quite stable (if you poke at it it will bob around and return to its starting position). If you do manage to knock it off it can be picked up and set it back in place by using non magnetic tweezers.

If you use a pin to poke a hole in a Ping-Pong ball tangential to the surface you can use it to make a Hero's engine. Drop it into a container of liquid nitrogen and hold it under the surface for a few seconds. As the air in the ball cools the pressure in the ball drops and atmospheric pressure forces some liquid nitrogen into the ball. Take the ball out and put it on the floor and as the nitrogen boils and is discharged the ball will spin rapidly. You don't need much liquid in the ball for it to spin so don't try to fill it up. There is the possibility that if the ball is too full the pressure could build up to the point that the ball would burst so don't stand too close when you try this. I have done this experiment many times and haven't had one burst yet but I can't be sure that all Ping-Pong balls can take the pressure.

You can demonstrate how the properties of some materials are changed when they are cooled to cryogenic temperatures.
Here an uninflated long skinny balloon like clowns use to make animals and such was dipped into liquid nitrogen and then stretched. The kink will break before it straightens. That is not what some might expect for thin flexible rubber.

When you tap a sheet of lead you probably expect to hear a dull thud. When it is cooled to the temperature of liquid nitrogen the sound is much more bell like.

A spring made from solder doesn't work all that well. When stretched it deforms rather than returning to its original length. The bottom half of this spring was dipped into liquid nitrogen and behaves like you would expect a spring to until it warms up. The top part wasn't cooled and deforms easily and does not come back to it's original shape. This demonstration is more effective if you can find some old lead/tin solder rather than the newer lead free type.

Strips of brass (on left), copper (next to it) and tin (bottom two) can be used to demonstrate the difference in the behavior of metals when subjected to extreme cold. The brass and copper remain flexible but the tin becomes so brittle that it is easily broken when a strip of it is cooled and bent.

Ten cc's or so of liquid nitrogen were put into this bottle then the balloon was put over the top. As the nitrogen boils the gas fills the balloon with the expected result. The difference in volume between the liquid and gas is brought home vividly. It expands about 730 times as it warms to room temperature. The pop can be loud enough to make ears ring if you use a high quality balloon so warn your audience to cover their ears.

Cool temperatures cause chemical reactions to slow down. The chemiluminescent glow of a light stick is quickly extinguished by cooling it. The light returns when the reaction resumes as it warms up.

A simple, dramatic, and messy way to show the change of volume that liquid nitrogen undergoes as it changes to a gas is to dump a small amount of it into a soap and water solution. The nitrogen boils and generates a lot of bubbles rather quickly. It helps if the soap solution is warm and you use only a few cc's of liquid nitrogen. This both makes the change in volume more dramatic and is less likely to result in frozen liquid and bubbles. You can add more later to see just what frozen bubbles are like.

The old frozen flower trick. Take a flower and dip it in the liquid nitrogen. It looks much the same except for the frost and fog. Rapidly crush the bloom and let the fragments fall on a hard surface. Usually someone will say that they sound like broken glass. You can crush the flower with your bare hand if you release it very quickly and use a flower that doesn't have a lot of places that the liquid could be trapped. Chrysanthemums are a particularly bad choice.

A ball of modeling clay will freeze and when dropped or struck will shatter nicely. There are a lot of different kinds of clay available and some probably will be harder to break than others. If you discover one that is particularly durable let me know about it.

Here is a way to produce a few drops of liquid oxygen. Put liquid nitrogen in an aluminum can. The outside of the can will quickly cool to just above the temperature of the nitrogen. Frost will quickly form and then it will appear to be wet. The frost is created from water vapor from the air. The liquid that is wetting it is oxygen that is condensing out of the air. This happens because the temperature at which oxygen changes from a gas to a liquid is about 13° Celsius above that of nitrogen. The oxygen can be seen dripping from the can just as on a humid day you can see drops of water condense and drip from a can you take from the refrigerator. Be sure to observe the special precautions for liquid oxygen in the red text at the top of the page.

The resistance of most conductors goes down as the temperatures falls. Here is a flashlight bulb connected to a battery through a coil of wire. At room temperature the resistance of the coil is high and the bulb barely glows. When the coil is cooled the bulb gets much brighter.

What is the effect of immersing a small flashlight in liquid nitrogen. I usually ask the audience what they think will happen. If I do it right after the demonstration just above or the superconductor experiment, most will predict that it will get brighter. If I do it soon after showing the light stick most will predict that it will get dim or go out. In fact it goes out, but it takes several minutes for the battery to cool down and slow the chemical reactions that make it work. It does come on again when it warms up. It would not surprise me if some batteries or flashlights may be damaged and the light won't come back on but I haven't found it to be a problem with the ones I use. There is also the potential for the flashlight to leak and get some liquid nitrogen inside it which will boil and expand as it warms up causing the flashlight to burst.

An American penny that was minted since 1983 consists of a zinc core clad with copper. When chilled in liquid nitrogen they will become quite brittle and can be broken by striking them with a hammer.

An inflated balloon pushed into a container of liquid nitrogen will slowly collapse as the air inside it contracts and condenses. The part of the balloon that is cooled becomes quite stiff and crinkles like a plastic bag. When it has nearly completely deflated you can take it out and show the liquid air by swirling it in the bottom of the balloon. If you haven't cracked the balloon by bending it when it was frozen and there is a reasonably large part of the balloon that isn't so hard that it won't stretch, it will reinflate to it's original size demonstrating that all of the air is still in there.

Put a marshmallow on a stick and freeze it. Marshmallows are good insulators so they take a couple of minutes to freeze. Take it out and rap it on a hard surface and it will shatter. I recommend that you don't do this where it will be hard to clean up because those shards of marshmallow will quickly warm up and they are incredibly sticky. (The first time I did this it was in my kitchen and my wife hasn't forgotten it yet.)

A light emitting diode (LED) is a semiconductor that converts electrical energy to light. The color of the light depends on the size of the "step" between the valence and conduction bands in the material from which it is made. When an LED is cooled the band gap and therefore the color of light it produces is changed. The change in the band gap also changes the voltage drop across the diode which, depending on the circuit providing the current, may change the current through it. If the current changes, the intensity of the light as well as the color will change. The two pictures below show the same LED, first at room temperature then after it was cooled in liquid nitrogen.

Freeze a banana. It can be used as a hammer to drive a nail. Take care to only hold on to the part that hasn't been immersed in the liquid nitrogen. Bananas that are less than fully ripe are not as likely to break

When one does break you have a new way to examine the structure of a banana. Compare it to a sliced one. Let it warm up and check it again. The results are interesting and you don't need to use liquid nitrogen to check what happens. You can just put a banana in a home freezer and leave it there for at least 24 hours to be sure that it is completely frozen. Take it out and check what happens as it warms up. Be sure to wait long enough (at least 1 day) for it to completely change.

Freeze a rubber ball. The solid high bouncing rubber balls still bounce a little but they sound like marbles or pool balls. Hollow handball type balls are easily broken but if you decide to do that be aware that the fragments can fly quite a distance and could hurt if they hit someone.

Make a candle from vegetable oil or any liquid hydrocarbon. When frozen most will look a lot like paraffin. Alternately dip a wick into the liquid and into the nitrogen. When you get a reasonable size candle you can put it in a holder and light it. Of course as it burns and warms from the air it will entirely liquefy and you will have an oil lamp with a long wick that may fall in any direction. Please be aware of the potential fire hazard and take appropriate precautions. The white candle was made using vegetable oil and the greenish one was made from gasoline (use extreme care).

Wizard ice cream. Use any good homemade ice cream recipe. Put it in a metal bowl and add about an equal volume of liquid nitrogen while stirring vigorously with a wooden spoon. You won't be able to see what is going on in the bowl because of the vapor cloud that is produced. The record time for going from liquid to finished product is 20 seconds for a half gallon of ice cream. Be sure that you don't use too much liquid nitrogen. If you do the ice cream will be rock hard and if you manage to serve some there will be a real chance of frostbite of the tongue.

You can make tiny ice cream balls by dripping the mix directly into a container of liquid nitrogen. It doesn't make much very fast but it is an interesting product similar to Dippin' Dots®.

A water balloon dropped into liquid nitrogen will freeze into a shell of ice with liquid water inside. The balloon may split because it becomes brittle and the water expands as it turns to ice. The picture shows a hole chipped in the ice shell to see if the center was still liquid. It was.

A two liter plastic pop bottle that has been cut off near the top of the wide part makes a good container for liquid nitrogen. It can be seen to be boiling looking through the side of the container. The cloud that forms and cascades from the bottle is like formation of clouds when warm moist air is cooled by rising to higher altitudes where it is cooler. Fog is also formed when the ground is cooled by radiation of heat into space and it cools the air and the moisture condenses out. If you tap on the bottle the frost that has formed on it will fall off and feels just like snow so you have a few elements of a meteorology demonstration.

Styrofoam cups also make good containers if you can be sure that no one will mistake them for a cup of beverage. You shouldn't have cups of beverage in any laboratory setting in any case.