International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What is the chemical reaction that leads to the formation of rust?

1. Iron + Hydrogen -> Water

2. Iron + Oxygen + Water -> Hydrated Iron(III) Oxide

3. Iron + Sulfur -> Iron Sulfide

4. Iron + Carbon Dioxide -> Iron Carbonate

The correct answer is: Iron + Oxygen + Water -> Hydrated Iron(III) Oxide

The formation of rust is specifically characterized by the reaction of iron with oxygen and water. Rust is primarily composed of hydrated iron(III) oxide, which can be represented as Fe2O3 · nH2O. In this reaction, oxygen from the air reacts with iron in the presence of water (either from humidity or liquid water), leading to the oxidation of iron and the formation of the rust compound. The presence of water is crucial as it facilitates the electrochemical reactions required for rusting, allowing iron to corrode and combine with oxygen from the environment. This process is an example of oxidation, where iron loses electrons and forms iron ions in the presence of moisture and oxygen. When looking at the other possible reactions, they do not involve the correct combination of reactants needed to produce rust. The other options either involve different elements or compounds that do not contribute to the rusting process, or they result in different products unrelated to the formation of hydrated iron(III) oxide. Thus, the reaction of iron with oxygen and water resulting in hydrated iron(III) oxide accurately represents the chemical process leading to rust.