Which part of the air?
Next, find which component of the air is needed for rusting.
Activity 12.3.2
You will need: two boiling tubes, steel wool, a basin full of water
- Fix a ball of steel wool at the closed end of one boiling tube, as in the diagram. Stand it upside down in the basin of water.
- Stand the other, empty tube upside down in the basin in the same way.
- Observe after a few days.
Figure to be added
The observation
In the tube with the steel wool the water level has risen by about 1/5 of the original volume of gas. So part of the air has been used up in rusting. By the composition of air, the part that makes up about 1/5 is oxygen. So it can be concluded that what rusting needs from the air is oxygen.
Why the water climbs
As the steel wool rusts, it takes oxygen out of the air trapped in the tube and locks it into solid rust. With less gas inside, the pressure of the outside air pushes water up the tube to fill the space. The water stops rising when all the oxygen is used, which is why the rise is about one fifth: the other four fifths, mostly nitrogen, do not take part. The empty tube is the control — its water level does not change, so the rise cannot be caused by temperature or anything else about the set-up. Steel wool is used instead of a nail because its fine threads have a very large surface, so the rusting finishes in days rather than weeks.
Rusting uses the oxygen of the air
Rusting steel wool uses up about 1/5 of the air in a tube — the oxygen.