Paramagnetism of Liquid Oxygen

As a result of the parallel spin of unpaired electrons, molecular oxygen is paramagnetic and is attracted by a magnetic field. This is demonstrated by bringing a large test tube with a small amount of liquid oxygen close to a powerful magnet. The test tube is visibly attracted to the magnet.

Curriculum Notes 

This demo could be used when Hund's Rule and it's relation to paramagnetism is being discussed. But it is probably best used as an illustration of the predictive power of Molecular Orbital theory. It could also be used much earlier when the concept of physical properties is being introduced. Allow about 5 minutes for this demo.

Lead Time 
One day of lead time is required for this project.
Discussion 

The two highest energy electrons in the oxygen molecule could either reside with one in each of two degenerate pi orbitals with parallel spins or with both electrons in one of the pi orbitals with anti-parallel spins. Since molecules, as well as atoms, obey Hund's rule, the electrons reside in separate orbitals. As a result of the parallel spin of the unpaired electrons, molecular oxygen is paramagnetic and is attracted by a magnetic field. If desired, this can be contrasted with the behavior of liquid nitrogen, which exhibits no such attraction. This is a good demonstration for linking the new concept of molecular orbitals with the established concept of atomic orbitals.

Materials 
  • large test tube with a string tied around it containing 1/4 to 1/3 full of liquid oxygen immersed in a small Dewar flask of liquid nitrogen to keep it cold
  • a very powerful magnet
Procedure 

Withdraw the test tube containing liquid oxygen from the Dewar flask using the attached string. Still holding the tube by the string, bring the test tube close to the magnet. The liquid oxygen is visibly attracted to the magnetic field.

Safety Precautions 
  • Liquid oxygen and the liquid nitrogen in which it is immersed in the Dewar flask are extremely cold. There is a risk of frostbite if they come in contact with your skin. Handle the test tube only by its attached string.
  • Liquid oxygen is a very powerful oxidizer. If flammable materials come in contact with it they are capable of quite vigorous combustion. The vapor that results from the vaporization of liquid oxygen is almost pure oxygen gas, which is also a very powerful oxidizer. Make sure that this demonstration is performed away from sources of combustion.
  • The powerful magnet used in this demo can attract iron and steel objects with enough force to pinch and smash fingers if they are in the way. It will also do a number on credit cards and electronic equipment if you get them too close.
Prep. Notes 
  • Liquid oxygen for this demonstration is condensed from gaseous oxygen from the tank in the demo prep room. Fill the small wide-mouth Dewar flask with liquid nitrogen from Science Stores in the basement of Onyx Building. Place the test tube in the Dewar flask and run a rubber hose from the oxygen tank to the bottom of the test tube. Adjust the regulator so that a very slow flow of oxygen runs into the test tube. Liquid oxygen will begin to condense in the test tube. Turn off the oxygen flow when the test tube is 1/4 to 1/3 full.
  • Liquid oxygen should be condensed not more than 15 minutes or so before the demonstration and test tube should be loosely capped with foil to prevent nitrogen from the air from condensing in the tube and contaminating the oxygen.
  • Be careful with the "mondo" magnet. Besides being really strong (see above) it's also heavy !

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group