Question
What is rusting of iron? Write the chemical equation for the rusting process. List the methods used to prevent rusting.
(NCERT Class 10, Chapter 3 — Metals and Non-metals)
Solution — Step by Step
When iron reacts with oxygen and moisture (water) from the air over time, it forms a reddish-brown, flaky substance called rust. The chemical name of rust is hydrated iron(III) oxide.
Both oxygen and water are necessary — iron doesn’t rust in dry air or in oxygen-free water.
Rusting needs three things simultaneously:
- Iron (the metal)
- Oxygen (from air)
- Moisture/Water
The process is accelerated by: salt water (coastal areas), acid rain, and higher temperatures. That’s why ships and bridges near the sea rust much faster.
| Method | How it works | Example |
|---|---|---|
| Painting | Creates a barrier between iron and air/moisture | Bridges, gates, railings |
| Oiling/Greasing | Coating of oil prevents contact with moisture | Machine parts, bicycle chains |
| Galvanisation | Coating iron with a layer of zinc | Iron buckets, pipes, roofing sheets |
| Electroplating | Coating with chromium, nickel, or tin using electricity | Car bumpers, bathroom fittings |
| Alloying | Mixing iron with other metals to make it rust-resistant | Stainless steel (iron + chromium + nickel) |
Galvanisation (zinc coating) works even if the coating is scratched. Why? Zinc is more reactive than iron — so even if water reaches the iron through a scratch, zinc corrodes first (sacrificial protection). The iron stays protected as long as any zinc remains.
This is different from paint — if paint scratches, the exposed iron starts rusting immediately.
Why This Works
Rusting is an oxidation reaction — iron loses electrons to oxygen. It’s a slow process (unlike burning), happening over days or weeks. The rust that forms is porous and flaky, so it doesn’t protect the iron underneath — instead, more and more iron keeps getting exposed and rusting. That’s why rusting can eventually destroy an entire iron structure if not prevented.
All prevention methods work on one principle: block the contact between iron and oxygen/water. Whether it’s paint, oil, zinc, or chrome — the goal is always to create a protective barrier.
Alternative Method — The experiment to prove both air and water are needed
Classic CBSE experiment: take three test tubes with iron nails. Tube 1: nail in plain water (air + water). Tube 2: nail in boiled water topped with oil (water but no air). Tube 3: nail with calcium chloride/silica gel (air but no water). After a week, only the nail in Tube 1 rusts — proving both oxygen and water are needed.
Common Mistake
Students write the formula of rust as just (iron oxide). The correct formula is (hydrated iron oxide) — the "" part indicates that water molecules are chemically bound into the structure. This distinction between rusting (which needs water) and simple oxidation matters in board exams.