In corrosion chemistry, rust forms when iron reacts with which gas in the presence of moisture in air?

Introduction / Context:
Rusting is a common example of corrosion and is often seen on iron objects exposed to air and moisture. It is a chemical process in which iron is oxidised to form hydrated iron oxides. Understanding which components of air are actually responsible for rusting helps clarify basic concepts in environmental chemistry and everyday science. This question focuses on identifying the main gas that reacts with iron during rust formation.

Given Data / Assumptions:
- Rust is commonly observed on iron and steel when they are exposed to damp air. - Air is mainly a mixture of nitrogen, oxygen, and small amounts of other gases including carbon dioxide and water vapour. - Rust is chemically a form of hydrated iron oxide. - We consider normal atmospheric conditions, not special industrial environments.

Concept / Approach:
Rusting is an oxidation reaction in which iron reacts with oxygen in the presence of water to form iron oxides, typically Fe2O3·xH2O. Oxygen is the key oxidising agent that combines with iron. Water acts as a medium that facilitates the flow of electrons and ions in electrochemical corrosion. Nitrogen is largely inert under these conditions, and carbon dioxide plays only a secondary role in some corrosion processes. Therefore, oxygen gas from the air is the main gas responsible for rust formation on iron.

Step-by-Step Solution:
Step 1: Recall the basic rusting reaction, often simplified as iron plus oxygen plus water gives hydrated iron oxide. Step 2: Recognise that the chemical change involves oxidation of iron atoms to iron ions, which requires oxygen as the electron acceptor. Step 3: Note that water is necessary for rusting but is not the primary gas that reacts; it provides the medium in which ions can move. Step 4: Identify oxygen gas as the component in air that reacts chemically with iron to form iron oxides. Step 5: Compare with nitrogen, which does not usually react with iron at ordinary temperatures and pressures, and carbon dioxide, which plays a minor role only in specific conditions. Step 6: Conclude that oxygen gas from the air is the correct answer.

Verification / Alternative check:
The chemical equations representing rust formation show iron reacting with oxygen. For example, 4Fe + 3O2 + xH2O gives 2Fe2O3·xH2O. Industrial corrosion studies and school experiments that remove oxygen from water demonstrate that rusting is greatly slowed or prevented when oxygen is absent, even if water is present. On the other hand, removing nitrogen or small amounts of carbon dioxide does not stop rusting. These observations confirm that oxygen is the critical gas in rust formation.

Why Other Options Are Wrong:
Option A (Water vapour only) is incomplete because water alone without oxygen does not produce rust; oxygen is needed for oxidation. Option C (Nitrogen gas) is incorrect because nitrogen is relatively inert at room temperature and does not significantly react with iron to cause rust. Option D (Carbon dioxide gas) may contribute to some forms of corrosion in the presence of water, but the classic rusting of iron is primarily due to oxygen, not carbon dioxide.

Common Pitfalls:
Students sometimes overemphasise the role of water and think that moisture alone causes rust. While water is necessary, it is only part of the process; oxygen from the air is equally essential. Another common misconception is to blame carbon dioxide or other minor air components without recognising that rust is an oxidation process requiring oxygen. To correctly identify the cause of rusting, always remember the involvement of iron, oxygen, and water together.

Final Answer:
The correct answer is: Oxygen gas from the air.