Rust, commonly seen on iron objects, is formed when iron reacts mainly with which component of moist air?

Introduction / Context:
This everyday chemistry question asks about the formation of rust on iron. Rusting is a classic example of corrosion and oxidation that students encounter frequently, from rusty gates to old tools. The process occurs when iron reacts with oxygen in the presence of water or moisture. Knowing that oxygen is the main reacting element helps learners connect real world observations to basic redox chemistry.

Given Data / Assumptions:
- The metal involved is iron, which forms rust over time when exposed to air and moisture.
- Options list carbon, oxygen, hydrogen, and all of the above as possible reactants.
- We assume typical atmospheric conditions where air contains oxygen and water vapour.

Concept / Approach:
Rust is primarily a mixture of hydrated iron oxides, often written in simplified form as Fe2O3·xH2O. The key chemical change is oxidation of iron metal to iron(III) ions by oxygen. Water acts as a medium and participates in hydration but is not the main element combining with the iron in the new compound. Carbon is not involved in rust formation under normal conditions, and hydrogen from water does not directly form a stable iron hydride in this context. Thus, the element that reacts with iron to form rust is oxygen.

Step-by-Step Solution:
Step 1: Recognise that rust is a type of iron oxide, which means iron has combined with oxygen.
Step 2: In moist air, oxygen dissolves in the thin layer of water covering the metal surface and reacts with iron atoms.
Step 3: The iron is oxidised, losing electrons and forming iron(II) and then iron(III) ions, while oxygen is reduced.
Step 4: These iron ions combine with oxygen and water molecules to form hydrated iron oxides, the flaky reddish brown material we call rust.
Step 5: Carbon is present in some steels and in atmospheric carbon dioxide, but it does not directly form rust with iron.
Step 6: Hydrogen is part of water but does not form a major solid compound with iron in the normal rusting process.

Verification / Alternative check:
Chemistry textbooks typically write simplified equations for rusting such as 4Fe + 3O2 + xH2O gives 2Fe2O3·xH2O. These expressions emphasise that oxygen from air is essential, and rust does not form in the complete absence of oxygen. Experiments show that iron immersed in oil or kept in an oxygen free environment does not rust, even if water is present. This evidence confirms that oxygen is the central element reacting with iron to form rust.

Why Other Options Are Wrong:
- Carbon: Although carbon can alloy with iron to form steel, it is not the main element combining with iron during the rusting process in air.

Why Other Options Are Wrong (continued):
- Hydrogen: Hydrogen is part of water but does not form the main solid product of corrosion on iron under normal conditions. The visible rust is an iron oxide, not an iron hydride.
- All of the above: Since neither carbon nor hydrogen play the primary role in rust formation, this combined option is incorrect.

Common Pitfalls:
Some learners incorrectly think that water alone causes rust, forgetting the crucial role of oxygen. Others may confuse carbon in steel with atmospheric reactions and assume carbon must be part of rust. To keep the concept clear, remember that rust is a type of iron oxide, so it must involve iron and oxygen, with water acting as a necessary medium and hydrator but not as the elemental reactant that defines the compound.

Final Answer:
Oxygen is the element in moist air that reacts with iron to form rust.