Source: Extra Practice

When a piece of iron is left exposed to moist air for a few days, it develops a reddish-brown flaky substance on its surface. What is this process called, and what is the chemical nature of the reddish-brown substance?

Options

Option A

Burning, Iron oxide

Option B is correct

Rusting, Hydrated iron(III) oxide

Option C

Evaporation, Iron hydroxide

Option D

Sublimation, Iron carbonate

Explanation

The process described is called rusting. Rusting occurs when iron reacts with oxygen and moisture (water vapor) in the air. The reddish-brown flaky substance formed is hydrated iron(III) oxide (Fe2O3.xH2OFe_2O_3.xH_2O), commonly known as rust. This is a slow oxidation process. Burning involves rapid combustion and significant heat, which is not what happens here. Evaporation and sublimation are physical changes, not chemical reactions that form new substances like rust. Iron hydroxide and carbonate are not the primary products of typical iron rusting.