An inverted gas jar of air stands over water with a piece of moist iron inside, as shown. After several days, which change would you expect in the volume of...
Question 1 Report
An inverted gas jar of air stands over water with a piece of moist iron inside, as shown. After several days, which change would you expect in the volume of gas in the jar?
Answer Details
Rusting needs both oxygen and water, and the moist iron supplies the water while the trapped air supplies the oxygen. As the iron rusts, oxygen is taken out of the air and locked into hydrated iron(III) oxide, a solid. The volume of gas in the jar therefore falls, and water rises to fill the space left behind.
Oxygen is about 21% of air, close to one fifth, so that is roughly how far the level moves. No gas is produced by rusting, so the volume cannot rise, and it cannot stay the same because a reaction is clearly happening. Nitrogen is far too unreactive to be used up, so a fall of four fifths is impossible. This is the standard way of measuring the oxygen content of air.
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