A student pushed some damp iron wool into the closed end of a test tube. The tube was turned upside down and stood in a beaker of water, trapping a column o...

This tests using rusting to measure the fraction of oxygen in air. As the iron rusts it removes oxygen from the trapped air, so the gas volume falls and water is pushed up to fill the space.

(a) Why the water level rises [2]. The oxygen in the trapped air is used up as the iron rusts [1]; this reduces the volume (and pressure) of the trapped gas, so water rises up the tube to take the place of the used-up oxygen [1].

(b) What the one-fifth rise tells you [1]. Since the oxygen has been removed and the water rose by about one fifth, about one fifth (20%) of the air is oxygen [1].

(c) Why damp iron wool, not a dry nail [1]. Iron wool has a much larger surface area and is damp, so it rusts faster and more completely, using up the oxygen in a reasonable time [1]. A single dry nail would rust far too slowly and would not remove all the oxygen.

(d) Comparing rusting in salt solution and water [4].

1. set up two identical tubes, one with iron wool dampened with water and one dampened with salt solution [1];
2. keep the amount of iron wool, the tube size and the temperature the same [1];
3. measure how far the water rises up each tube over the same time [1];
4. the tube with the greater / faster rise shows the faster rusting [1].

Examination tip: the fair-test comparison changes only the one variable under test (water vs salt solution) and keeps surface area, temperature and time constant - salt solution is expected to give the faster, larger rise.