Liquid Nitrogen Classroom Demonstrations

Liquid Nitrogen, being the cheapest, most widely produced, and most common cryogenic liquid, serves as an excellent research tool to demonstrate several thermodynamic and physical processes. Observing how it behaves under certain conditions, and seeing how different solids, liquids, and gases react to being cooled to such low temperatures, gives us incredible insight on the properties of matter. Below are outlined some of the experiments demonstrated on Mr. Carah’s I.B. HL-2 Physics class on 9/11/00, and their significance. The demonstration was divided in 4 parts:
1-Liquid Nitrogen and its Properties
2-Phase Change and thermal expansion
3-Liquid Oxygen
4-Supercooled matter
Acknowledgements

 Liquid Nitrogen is mass-produced in air liquefaction plants. The liquefaction process is very simple: in it, normal, atmospheric air is passed through a dust precipitator and pre-cooled using conventional refrigeration techniques to remove all traces of dirt and water. It is than compressed inside large turbo pumps to about 100 atmospheres. During the compression cycle the air heats up dramatically and has to be cooled constantly, so the compression cycle is actually done in stages, and between each stage there is an intercooler, which cools it down before it is compressed any further. Once the air has reached 100 atmospheres and has been cooled to room temperature, it is allowed to expand rapidly through a nozzle, into an insulated chamber. Just as air heats up during the compression cycle, it cools down during decompression, since the energy for the rapid escape of gas has to come from the molecules themselves. By running several cycles, the temperature of the chamber reaches low enough temperatures that the air entering it starts to liquefy. Liquid Nitrogen is removed from the chamber by fractional distillation and is stored inside well-insulated Dewar Flasks until it needs to be used. A dewar flask consists in two walled bottle. Between the two walls there is a high vacuum which prevents any heat to be transmitted by convection currents. Both walls are silver coated so as to prevent heat from being transmitted by radiation.

 The following table illustrates the most important properties of nitrogen gas, and Liquid Nitrogen (LN2):
(Note: data compiled using data from CRC Handbook of Chemistry and Physics 80th edition¹, and The Merck Index 12th edition²).

Molecular formula: N2, Atomic weight = 4,0067 Molar weight = 28.02 ².
Nitrogen constitutes 75.5% by weight (78.06% by volume) of the atmosphere².
Boiling point:  -195.79C (77,36K), N2 freezes into a snow-white mass at -210,01C (63.14K)¹.
Critical temperature: -147,1C, Critical pressure: 33.5atm; Critical density: 0,311g/cm^3¹.*
Density of liquid at -195.8C = 0.807g/cc, gas = 1.2506g/l at stp (4.622g/l at boiling point)¹.
Specific heat capacity = 1.341j/g/K (gas), 2.042j/g/K (liquid), enthalpy of vaporization: 198.8j/g¹

* Critical values are those values below with a gas will not form into a liquid. Notice how, no matter how pressurized, Nitrogen will not become a liquid at non cryogenic temperatures. This makes it very useful as a means of producing high pressure, which we will come back to later.

 Looking at the 3 pictures above two properties of Liquid Nitrogen can be observed: One is that it is continuously “smoking” in air… The causes of this smoke have been discussed in the previous page, and are known to be due to water condensing out of the air due to super cooling. The last picture demonstrates Leidenfrost effect: A droplet of liquid nitrogen can circle a round glass flask for a very long time at very high speeds, since the layer of vapor it forms between itself and the walls keeps it isolated from the glass, and hence it can move with very little friction.

As liquid nitrogen warms up from its liquefaction temperature of -196C to room temperature (25C), it undergoes first a phase change (from liquid to gas), which results in an expansion of (0.807g/cc / 0.004622g/cc) 174.6 times the original volume of liquid. The resulting nitrogen gas is than warmed by 221degrees Celsius, expanding an extra (0.00462g/cc / 0.00125g/cc) 3.7times. The net expansion for liquid nitrogen is hence (174.6 x 3.7) 645.3 times the original volume when heated to room temperature. This means that 1 liter of liquid nitrogen will occupy 645.3 liters as a gas once it has all vaporized. Due to this dramatic expansion, when liquid nitrogen is placed in a closed container and allowed to vaporize, the pressure in the container will rise very quickly, and if it is not allowed to escape, the container is very likely to burst (Due to the non ideal gas behavior of N2 at such extreme pressures, the theoretical pressure of the container would be over 30 000PSI!).
This phenomena is used to pump crude oil from deep oil pits.

Liquid Nitrogen in a sealed container:
In this classic demonstration, liquid nitrogen is put in a bottle containing a small amount of water (to speed up the vaporization) and a cork is used to seal the bottle. As the liquid vaporizes and the cold gas is warmed to room temperature, the volume expansion results in a great increase in pressure inside the bottle, which quickly builds up to a point where the cork is shot out with a loud “pop”, indicating the release of gas.
Unfortunately in this particular demo the cork was pushed too far into the bottle, which, being made of thin gas, could not withstand the pressure and burst. Hover the mouse pointer over the pictures for a description.

Balloons in Liquid Nitrogen:

 Here the opposite of what happens when liquid nitrogen vaporizes is demonstrated: We have seen on the previous demo that LN2 expands greatly when vaporized and heated to room temperature. But what happens when ordinary air is cooled to Liquid Nitrogen temperatures?
Since the pressure of a gas is directly proportional to its temperature, we can assume that all the Nitrogen inside the balloon will contract 235times, leaving the balloon 1/235th of its original volume. But since air is made up (by weight) of 75.5% Nitrogen, 23% oxygen, 1% argon and the remainder is noble gases and carbon dioxide, and since oxygen liquefies at -186C, what actually happens is once the nitrogen has cooled and contracted, all the Oxygen inside the balloon is liquefied, together with the carbon dioxide and any water vapor (both of which freeze), and the actual contraction is in the order of 900 times the original volume of gas, making the balloon shrink rapidly into a solid, shriveled up piece of hard rubber. The picture sequence below demonstrates this (hover your mouse button over the picture for a description):

A balloon filled with Helium was also dipped in the liquid nitrogen. Being much closer to an ideal gas than Air, Helium does not liquefy at atmospheric pressure, even under very low temperatures, hence, the balloon didn’t shrink nearly as much as the air filled balloons.

 Liquid oxygen (or LOX) is a pale blue liquid used as an oxidizer for rocket fuel formulations (such as those on the space shuttle’s main engine). Being 600 times as dense as the oxygen in air, it allows the violent combustion of large amounts of fuel that is needed to power a rocket. When air is cooled down to liquid nitrogen temperatures, at -186C all the oxygen condenses out. For this demonstration liquid oxygen was produced inside a test tube that was immersed in liquid nitrogen. The LOX was than poured onto a magnet, where it sticks, demonstrating paramagnetism:

As a general rule, any solid that is cooled to liquid nitrogen temperatures will become hard and brittle. Under very low temperatures the molecules in the solid vibrate so slowly that they are not able to move when the solid is subjected to a great force. Hence, it breaks. Some solids actually become stronger when cooled. For this demonstration, 3 solids were dipped into liquid nitrogen, held there for about a minute, and than removed and subjected to impact. The first object was a flower. After 1 minute of cooling, it became so hard that when dropped onto the floor it shattered into thousands of pieces, as though it was made from glass. The second solid was a rubber tube, which, once cooled, became inflexible and so brittle that when hit against a table, it broke into several fragments. Finally, a banana was dipped into liquid nitrogen. After a minute of cooling, it became hard enough to hammer a nail onto a piece of wood. The reason why the banana becomes harder is because although it is made up mostly of water, it contains fibers, and these fibers hold the ice crystals together. While ice itself may be very hard and brittle, when arranged with fibers it looses a lot of its brittleness. A similar phenomena occurs with glass fibre, which is made up of brittle glass, and a somewhat flexible polymer, making it extremely tough.
Because it had a soft core (the ice formed is an insulator and hence slows down any further freezing of the banana), after a few good hits the banana broke.

 Acknowledgements:

The presentation was planned and carried out by myself, Sam Barros. I would like to thank:
Mr. Carah for giving me time during his class to present it,
Michell Zappa for taking the pictures presented on this page.
Miss. Brehms and the Graded Chemistry Department, for supplying the flasks (including the one that was broken), the liquid nitrogen container, and the banana used for this presentation.