Paramagnetism of Oxygen

What It Shows

A large magnet with a small cylindrical gap allows a stream of liquid nitrogen to pass over and through. Poured liquid oxygen hangs between the poles in the strong field until it boils away.

Watch the Paramagnetism of Oxygen demo on YouTube.

How It Works

Pour the liquid nitrogen (LN2) first, slowly over the pole pieces. The result is nothing but vapor condensation and cooling of the magnet; the liquid does not interact with the large magnetic field.

Next pour the liquid oxygen slowly over the pole pieces, and it collects in the gap between the poles. It might take two or three short pours to get the liquid oxygen to stick around, as it has to cool the poles first. When the liquid oxygen boils off the first time, pour liquid nitrogen again to show no interaction. The next slosh of oxygen will last for 30 or 40 seconds, suspended in the strong magnetic field. Nitrogen, nothing; oxygen, stickiness.

Setting It Up

The magnetron magnet is big and heavy, with conical pole pieces. The poles are extended with washers to leave about a one centimeter gap.

The magnet is placed on the lecture bench or a metal cart with lighting to illuminate the poles.

Place a non-ferrous object between the poles and set up a camera so that the object and the poles of the magnet are magnified and in focus.

Before class starts, get 4 L of liquid nitrogen. The liquid oxygen [Lox] is condensed in a copper coil, which is immersed in a large, glass Dewar flask of liquid nitrogen [LN2]. It is caught in the tall, thin glass Dewar, which should be pre-cooled with liquid nitrogen immediately before. When ready, pour the LN2 in the small Dewar into the large Dewar, then place the outlet tube of the condenser within the thin Dewar at the same moment the coil part of the condenser is being lowered into the larger one. Top off the large LN2 Dewar from the 4 liter flask as necessary.

A good flow of oxygen will fill the Lox Dewar to three-quarters full in about five minutes, but we usually only make about 2 fingers above the metal part of the thin dewar. With the oxygen still flowing, slowly lift the entire condenser vertically, to catch the remaining Lox in the coils in the Lox Dewar. Shut off the gas flow from the O2 tank, and remove it and the condenser from the room.

Pour the remaining LN2 in the large dewar back into the 4 liter (non-magnetic aluminum) flask, to be used later to pour onto the magnet.

When demonstrating, first show that a metal object containing iron, such as a key, is attracted to the poles of the magnet. Then generously pour liquid nitrogen over the poles of the magnet, both to cool down the poles and to show that nitrogen is not attracted to the magnet. Next, show the dewar of liquid oxygen, pointing out its pale blue color. Pour the liquid oxygen on the poles of the magnet showing that it sticks and stays attracted as it boils away.

If the demonstrator still has liquid oxygen remaining, and wishes to demonstrate the acceleration of combustion, do the following. On a fireproof surface, place a 1 liter beaker. Carefully light the bottom edge of a cardboard cylinder from a paper towel roll (industrial-grade, thick cardboard cylinder from janitorial maintenance is what we typically use). The object is to get it smoldering evenly all around the bottom edge. Place the tube smoldering end down in the beaker, and carefully and quickly pour a good slosh, say 50 ml or so, into the beaker. Quickly stand back.

Comments

An additional way that we make liquid oxygen is by filling a metal tea kettle with liquid nitrogen. Suspend the kettle at an angle from a ring stand, and place a dry sponge underneath the kettle on the base of the ring stand. Add a small amount of liquid nitrogen. Liquid oxygen will start dripping from the lower edge of the kettle onto the sponge, creating scorch marks. The sponge may even catch on fire, so have a bowl of water nearby. To stop making liquid oxygen, pour the liquid nitrogen out of the kettle.