Case Study:

In many parts of India, large iron bridges are built over rivers and highways to support heavy traffic. Engineers regularly inspect these bridges because iron structures slowly undergo corrosion, commonly known as rusting.

Consider an iron bridge built over a river in a humid coastal region. During the monsoon season, the bridge is frequently exposed to moisture, oxygen from air, and dissolved salts from water droplets carried by wind. Over several years, reddish-brown patches begin appearing on the surface of the bridge girders.

This reddish-brown layer is rust, which forms when iron reacts with oxygen in the presence of water. The reaction produces hydrated iron(III) oxide (Fe₂O₃·xH₂O). The overall chemical reaction can be represented as:

4Fe + 3O₂ + xH₂O --> 2Fe₂O₃·xH₂O (Rust)

Rust is porous and flaky. When it forms, it does not protect the metal beneath. Instead, the rust layer falls off easily and exposes fresh iron to air and moisture, allowing corrosion to continue. Over time, this weakens the metal structure and may compromise the safety of the bridge. 

To prevent such damage, engineers apply protective coatings like paint, galvanization (zinc coating), or anti-corrosion oils on iron surfaces. Regular maintenance is essential, especially in areas with high humidity and pollution.

This case highlights how chemical reactions directly affect real-life structures and why understanding corrosion is important for both chemistry and engineering.

Questions

Section A - MCQs

1. Why does corrosion occur faster on the iron bridge during the monsoon season?

A. Iron reacts faster at low temperature
B. Presence of moisture and oxygen increases rusting
C. Iron dissolves in rainwater
D. Sunlight directly forms rust

2. Which substance is mainly responsible for the reddish-brown layer on the bridge?

A. Iron sulphide
B. Hydrated iron(III) oxide
C. Iron carbonate
D. Iron chloride

3. Why does rusting continue even after rust is formed?

A. Rust acts as a catalyst
B. Rust layer is porous and exposes fresh iron
C. Rust absorbs oxygen from air
D. Rust reacts with sunlight

4. Which method is commonly used to protect iron bridges from corrosion?

A. Heating the metal
B. Painting or galvanizing the metal
C. Washing with water
D. Cooling the metal regularly

Section B - Short Answer Questions

1. Explain why both oxygen and water are necessary for rusting of iron.

2. Why is rust considered harmful for large iron structures like bridges?

3. Suggest two methods used by engineers to prevent corrosion in iron bridges.

Section C - Long Answer Question

1.An old iron bridge shows severe rusting because its protective paint layer has worn out.

a) Explain the chemical process responsible for rust formation.
b) Describe two preventive techniques that could increase the life of the bridge.
c) What might happen if corrosion is not controlled for many years?

Answer Key

MCQ Answers

1. B - Moisture and oxygen accelerate rusting.
2. B - Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O).
3. B - Rust is porous and flakes off, exposing fresh iron.
4. B - Protective coatings like paint or galvanization prevent rust.

Short Answer Solutions

1.Rusting requires both oxygen and water. Oxygen reacts with iron while water acts as a medium that allows electron transfer and formation of hydrated iron oxide.

2.Rust weakens iron because the flaky rust layer does not protect the metal. It continuously exposes fresh iron to air and moisture, causing gradual structural damage.

3.Two preventive methods:

• Painting the iron surface
• Galvanization (coating with zinc)

Long Answer Solution

a) Chemical process

Rusting is an oxidation reaction in which iron reacts with oxygen and water to form hydrated iron(III) oxide.

Reaction:

4Fe + 3O₂ + xH₂O --> 2Fe₂O₃·xH₂O

b) Preventive techniques

1. Painting - Creates a barrier preventing air and moisture from contacting iron.
2. Galvanization - Zinc coating protects iron and prevents oxidation.

c) Consequences

If corrosion continues:

• Metal strength decreases
• Structural cracks may form
• Bridge safety may be compromised.