In Demo Veritas

Liquid nitrogen diamagnetic, liquid oxygen paramagnetic.

Demonstration of the inadequacies of Lewis dot structures and valence bond theory.  Both oxygen and nitrogen look to have their unbonded electrons as pairs.  :N:::N: and ::O::O::.  Pairs of electrons have magnetic spins which effectively cancel, making it a reasonable prediction that nitrogen and oxygen are both diamagnetic.

The experiment uses liquid nitrogen and liquid oxygen, because the effect is hard to see with gases, and easy with liquids.

A big magnet with a small gap is handy, so I use an old  synchrotron magnet, with steel washers to narrow the gap even further.  Takes a bit of doing to get those washers in place.  To take these pictures, we used a lightbox to illuminate.  For the demonstration in class, the magnet is placed on an overhead projector, with a plastic sheet protecting the glass.  The gap is shadow projected on a screen.

Liquid nitrogen goes in both Dewars, pre-cooling the small one so we don’t waste oxygen.  An oxygen cylinder is connected to a copper tube, which enters the big Dewar, coils six or seven times, and then exits up and over and down into the small Dewar.   With the oxygen flowing, the coil is immersed in the colorless LN2, and begins condensing liquid oxygen.   Pale blue LO2 squirts into the small Dewar.

With about 100 ml. of LO2, I remove the coil from the LN2  and close off the oxygen tank.  Time to pour and compare. LN2 first, because we have lots of it and the pole pieces need cooling, and (spoiler alert) it is diamagnetic and won’t stick around.

Then I pour LO2 into the gap.

The LO2 sticks in the gap.  Attracted to the strong magnetic field is paramagnetic, and paramagnetic means unpaired electrons.

So two bonding schemes give way to molecular orbital theory, and this simple experiment is conclusive proof that the oxygen molecule has unpaired electrons.

Last picture is a flash shot close-up.  The LO2 is boiling away, spitting out blobs of liquid, and cold vapor that hits the humid air and forms a fog.  The washers are white from frost.  As the liquid boils off, the oxygen gas still prefers the high field, and forms a torus of cold gas, visible by condensed water vapor at its periphery (not visible in this picture, the effect shows up clearly with the shadow projection.)

Making liquid oxygen gives the professor the chance to do another demonstration.  On a fire-proof surface is an 800 ml pyrex beaker.  I char a cardboard tube from a roll of paper towels with a torch, blow out the flame, and put it smoldering end down in the beaker.

A quick pour of LO2 into the beaker turns the tube into a flaming dart.  The effect is like an upside down rocket engine, with the beaker full of orange fire, and a tongue of fire shooting from the top of the tube.  The tube is entirely consumed, leaving only a bit of ash and a cracked beaker.

Ansari X Prize winner, Scaled Composites SpaceShipOne, uses a rocket engine with liquid oxidizer and solid fuel, like the demo, but way more sophisticated.