Video Transcript
Which gas, present in air, is necessary for iron to rust?
To answer this question, we must understand what rust is and under what conditions it
is formed. Chemically, rust is hydrated iron(III) oxide, with the chemical formula
Fe2O3·𝑛H2O. The 𝑛 indicates that the number of water molecules in each chemical unit of hydrated
iron(III) oxide may vary. The chemical process of converting iron into hydrated iron(III) oxide is known as
rusting. Rusting involves several chemical reactions.
Rusting begins with the dissolution of iron metal in water, which produces iron two
plus ions and two electrons. As iron is losing electrons to form the two plus ion, this is oxidation. Whenever one species is oxidized, another must be reduced. The species that undergoes reduction during rusting is oxygen in the air. The oxygen gas combines with hydrogen ions dissolved in the water as well as the
electrons from the oxidation of iron to produce water. As well as reacting to form water, the hydrogen ions and the oxygen further oxidize
iron two plus ions into iron three plus ions. The iron three plus ions can then react with water to form iron(III) hydroxide. This species then dehydrates to form hydrated iron(III) oxide, what we can call
rust.
This diagram can help us to see the processes involved during rusting, but it does
not show complete balanced reactions. The overall process of rusting can be described by the equation four Fe plus three O2
plus 2𝑛H2O react to produce two Fe2O3·2𝑛H2O. We can see that the formation of rust requires three things: iron, water, and
oxygen. So the gas present in air that is necessary for iron to rust is oxygen.
