Science Express

Liquid Nitrogen Cryogenic Demonstration

“The Land of the Super Cold”

This demonstration is appropriate for K through 12. The older students would receive more scientific explanation for the various phenomena shown.

There are safety issues for LN2. It can cause near instantaneous frost bite. Correct gloves during handling should be worn during decanting from the large Dewar flask. While transporting liquid N2 the windows of the vehicle should be open to prevent asphyxiation.

Liquid nitrogen’s boiling point is -320F, -196C and 77K

One liter of LN2 will expand to 694 liters of gaseous N2

List of supplies

  1. Large 10L metal Dewar flask for bulk supply of N2
  2. Large 4L glass table top Dewar for the demonstrations
  3. Small .5 to 1.0 L Dewar for demonstrations
  4. Tongs for immersion of objects in the LN2
  5. 250ml beaker for dipping out small amounts of LN2
  6. Red rubber to cut out triangular “nails”
  7. Wood block to hammer rubber nails into
  8. Hammer for nails
  9. Balloons for deflating
  10. Sweet Valley High book for dipping
  11. Racquet balls
  12. Half inch surgical tubing
  13. Quarter inch surgical tubing, long
  14. Rubber band
  15. Lead spring
  16. Iron pipe cannon with matching rubber stopper
  17. Steam engine with syringe funnel
  18. Cotton balls
  19. Saltine crackers
  20. Ping pong balls with tangential holes with 500ml beaker
  21. Go purchase fresh flowers with largish petals.

Demonstrations

1. Simply filling the table top Dewar’s from the 10 L Dewar is an interesting event to watch. Pour somewhat slowly to prevent over boiling and wasting N2. You can fill to the brimby pouring very slowly when nearly full. Simply filling the Dewar’swill cause massive boiling due to the fact that they are initially “hot” relative to the LN2’s temperature.

2. Explain the characteristics of LN2. Boiling pt. % of atmosphere etc.

3. Take an inflated balloon and immerse it in the larger glass Dewar. It will deflate down to near zero volume! Take the balloon out and it will re-inflate to original volume. Because the air we breathe is almost 80% N2the balloon respondsaccordingly. Make sure the balloon is inflated to a point were it will still fit in the Dewar.

4. The iron “cannon” should be filled with a few ml of N2 from the 250 ml dipping beaker. Rapidly place the rubber stopper and then tighten it by hitting it with your fist on the iron pipe. It will blow the stopper off the pipe and fly about 10 to 20 feet. Do not aim it at your bad kid! Obviously due to the enormous expansion of the LN2 (1 to 694)the pressure will blow the cork off.

5. While on this subject of expansion we can run a steam engine. The room temp is so high that the nitrogen will boil in the boiler thus expanding to run the engine. Unscrew the pressure relief valve off the boiler so two holes are exposed on the boiler. Use the syringe as a funnel to pour LN2 into the boiler using the 250 ml beaker, and then plug both holes with fingers and the engine will run. If it does not you might be at a dead center so spin the fly wheel and it will take off spinning.

6. Ping pong ball Hero’s engine showing gas expansion will demonstrateNewton’s third law of motion. The ping pong balls have had a tangential hole poked into them. Take a ball and fully immerse into the large glass Dewar until boiling stops. The gas in the ball contracts thus drawing LN2 into the ball. Take the ball out and set on the table. As the LN2 inside the ball expands the jet action will cause the ball to spin. Repeat placing the cooled ball into the 500ml beaker filled with hot water. The LN2 will expand at a much higher rate so the ball will spin faster and possibly get airborne!

7. Cotton balls can be dipped into LN2 to make little poof balls when thrown down on a table or floor. The LN2 in the balls will instantly be turned into a gas causing a small report when striking a surface. A ball can be thrown down many times poofing until no more LN2 is left in the ball. Let the kids do this.

8. While the students are up poofing cotton balls, it is time to feed them saltines dipped into LN2. Dip a saltine into the LN2 and fling off any excess. Tell the students to eat the cracker as fast as possible. Then sent the students to the office because smoking is not allowed in school!

9. Time for the fire hose. Take the 18” half inch dia. surgical tubing and dip one end down to the bottom of the Dewar. The opposite end will spray out a mixture of LN2 and N2 up to about 5 feet. You can spray the kids in the front row. When it settles down take the hose out and lightly tap it on the table. The tube will be rock solid. Explain to the students how the properties of substances change at cryogenic temps. As temperature drops, brittleness increasesas well as hardness.

10. Hammering rubber nails can be done by cutting red rubber pieces at acute angles and then freezing them by dipping them into the LN2 with the tongs. Take the hammer and you can beat the nails into the wood block!

11. Now it is time to read a really hot steamy romance novel to your audience! Immerse the Sweet Valley High book into the large glass Dewar. Pull it out and fan through the pages. A prodigious amount of vapor will be produced. You can really ham this one up. What is going on is LN2 will make things wet including the low paper quality of a cheap book. The rapid evaporation of the LN2 and the interaction of the moisture in the air will produce the fog.

12. Freezing racquet balls is rather dramatic. Immerse a blue racquet ball deep into the LN2 to the point where the boiling quits. Not only is the ball rock hard but also has a vacuum inside due to the gas inside liquefying. Now throw the ball (wearing gloves) hard against the side wall. Not only will the ball shatter into many fragments, there will be a loud report due to the vacuum in the ball (like breaking a light bulb).

13. Some rather bizarre thawing events can be observed with elastic materials. Take the three foot one quarter inch dia. surgical tubing and wrap it pulling hard to stretch it around a cylinder and then immerse into LN2 until boiling stops. Take it out and slide the tubing off the cylinder and place the tubing on the table. The thawing will not be even so the tubing will go through many interesting gyrations. This can also be done with a rubber band. Stretch the rubber band with the tongs and immerse. Remove and watch it thaw!

14. Once again the change in materials properties can be observed. Take the lead “spring” which could not support even 100g and immerse it into LN2 until boiling ends. Now hang from the spring a 1000g weight. For a short time the spring will support the weight and be springy. Very soon the spring will warm and become soft so it will extend and the weight will fall from it.

15. Now it is time to get your heart broken. Take a fresh flower and immerse into the LN2. Pull the flower out after boiling and shatter it on the table. Very dramatic!

Advanced Demonstrations

1. Cryogenic temperatures can decrease resistance in a conductor. Take a 1” 1foot long Dow rod and wrap 200 turns of AWG 23 copper wire and place in series with a 60 watt bulb. When plugged in the bulb will be dim. Immerse the wire coil into the LN2 and the bulb will brighten considerably due to the lowered resistance of the wire.

2. Light bulbs exist to isolate the hot filament from the oxidizing effects of air. LN2 can isolate the filament too. Take a new 60 watt bulb and carefully break it in a towel. Do not damage the delicate filament structure. Screw the bulb into a socket. Place the filament structure into a half filled Dewar in very calm conditions. Do not immerse the filament structure; we want it in an all N2 environment. Plug the bulb in and the filament will light up! Next slowly lift the bulb out of the N2 environment. When air (02) gets to the filament it will burn out quickly.

3. Lenz’s law can be more dramatically demonstrated when the aluminum ring has been cooled in LN2. Due to the greater conductivity at cryogenic temperatures the ring will go considerably higher from the coil.