Discussion

The attraction of liquid oxygen by a magnet indicates that oxygen is paramagnetic and suggests that the oxygen molecule contains unpaired electrons. The absence of such an interaction between liquid and nitrogen and the magnet indicates that nitrogen is diamagnetic and that all of the electrons in nitrogen molecules are paired.

Valence bond theory predicts that both nitrogen and oxygen molecules are diamagnetic. However, as the demonstration shows, oxygen is paramagnetic. Whereas a fairly simple valence-bond description of dinitrogen adequately accounts for the behaviour of liquid nitrogen observed in this demonstration, it is insufficient to explain the behaviour of liquid oxygen.

To account for the magnetic properties of oxygen, a molecular orbital description of the bonding of O can be used. On drawing a simple molecular orbital diagram for O, it is possible to see that unpaired electrons produce a triplet ground state.

This demonstration of the paramagnetism and colour of liquid oxygen can be used along with a demonstration of the red chemiluminescence of the singlet state of O. The red chemiluminescence is due to a process which is the reverse of the absorption responsible for the blue colour of liquid oxygen. Both the colour and the paramagnetism of oxygen are intrinsic properties of its molecules and not bulk properties of the liquid phase.

As liquid oxygen evaporates, it produces a region of high oxygen gas concentration in its container. The effect of this high concentration on the rate of combustion is demonstrated by tossing a smouldering combustible into a beaker containing a small amount of liquid oxygen. The gas in the beaker is nearly pure oxygen, and its high concentration causes the combustible to be consumed almost instantaneously.

The blue colour of the liquid oxygen is the result of its absorption of light in the red portion of the spectrum.