Liquid O2 is Blue

Liquid O2 is Blue

I didn’t know that liquid oxygen is blue until last Friday, when my physics professor brought it to class and let us play with the three-hundred-Celsius-below-zero substance.

A bar magnet (ferromagnetic), liquid oxygen (paramagnetic), andliquid nitrogen (diamagnetic)

I did have a vague recollection of being told before that liquid oxygen is magnetic, but in class, we briefly learned why. Although liquid oxygen has an even number of electrons, molecular orbital theory decrees that two of these electrons abstain from pairing up and canceling out each other’s spins. In other words, liquid oxygen is paramagnetic. Each “spinning” charge looks like a loop of electrical current and produces a magnetic field. In the presence of another magnet, the electrons turn so that their magnetic fields align with the external field.

In a magnetic field, such as that from a bar magnet, the tiny magnets of liquid oxygen’s unpaired electrons align (left). Normally, they point in random directions (right). (Image fromWikimedia)

Now, under the influence of an external magnet, the paramagnetic material has itself a net magnetic field. Further, its North and South poles are aligned with those of the external magnet. Voila, we have magnetic attraction.

The liquid oxygen in the beaker behaves like another bar magnet pointing in the same direction as the real one, and is thus attracted to it.

One thing I really like about my physics professor is that every time he performs demos, he invites everyone to come play with the equipment after class. When we do, it’s practically unsupervised—except for maybe an offhand warning to wear gloves when handling the freezing substances. It makes me feel like an adult and exemplifies either his personal trust for students or trust deriving from Caltech’s Honor Code—or maybe both.