Damp iron wool is placed inside an inverted test-tube standing in water. After several days the water level has risen to fill about one fifth of the tube, a...
Question 1 Report
Damp iron wool is placed inside an inverted test-tube standing in water. After several days the water level has risen to fill about one fifth of the tube, as shown. Which gas in the trapped air has been used up?
Answer Details
Rusting needs both oxygen and water. The damp iron wool supplies the water and the trapped air supplies the oxygen, so as the iron rusts the oxygen is removed from the air and locked into solid hydrated iron(III) oxide. Since gas disappears, water rises up the tube to take its place.
The water rose by about one fifth of the tube, and oxygen is about 21% of air, so the two figures match. Hydrogen is not present in air in any measurable amount. Carbon dioxide is only about 0.04%, so losing it could never move the level by a fifth. Nitrogen at 78% is far too unreactive to rust iron. A one fifth rise is the fingerprint of oxygen.
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