A few weeks ago, we looked at a classic experiment of putting a lit candle under a glass. We saw that water was drawn into the glass, not by the oxygen being burned up, but by the cooling of the air in the glass after the candle went out. This experiment will allow us to do what many people thought the first experiment was doing, measure the amount of oxygen in the air.

You will need:

- a pie pan
- water
- a clear glass or jar
- steel wool
- vinegar
- tape

Carefully fit a wad of steel wool into the bottom of the glass. If it does not fit tightly, then use some tape to hold it in place. Pour a little vinegar into the glass, to wet the steel wool. Pour off any excess vinegar. Pour about two inches of water into the pan. Place the glass upside down in the pan of water. The jar will be full of air, with the steel wool at the top.

Check the water level and the steel wool after about ten minutes. You should notice rust on the steel wool, and should also see that the water level has risen. Let it sit overnight, checking periodically, to see how high the water has risen.

What happened? You should find that the water has risen into the glass. You should also find that the steel wool has rusted. The two things are connected. When iron or steel rust, they combine with oxygen to form iron oxide (rust). As the steel wool rusted, it took the oxygen out of the air. This lowered the air pressure in the glass, allowing the outside air pressure to push water into the glass to balance the pressure. When the rusting steel wool has removed all of the oxygen from the air in the glass, the water should have risen 1/5 of the way up the glass, as oxygen makes up about 1/5 of the air around us. Almost all of the rest of the air is made up of a gas called nitrogen.

When iron rusts, it gets heavier, as the iron is bonding with the oxygen in the air. When iron filings are burned, they also get heavier, for the same reason. This weight gain from burning iron was one of the first proofs of the existence of the gas oxygen.