CBSE Class 10 Science 2020 · Paper 31/1/2 · Question 15

Rusting is the slow corrosion of iron in moist air: iron reacts with oxygen and water and gets coated with a brown, flaky substance, rust, which is hydrated iron(III) oxide, ${\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$. Activity (see the labelled diagram): place clean iron nails in three corked test tubes: A with ordinary water (air and water); B with boiled distilled water under a layer of oil (water, no air); C with anhydrous calcium chloride (dry air, no water). After a few days only the nail in A rusts, so iron rusts only when both air and water are present.