Question
Iron is left out in the open for a few days. A reddish-brown coating forms on its surface. What is this process called? Is it a physical change or a chemical change? Justify your answer.
Solution — Step by Step
Iron reacts with oxygen from the air and moisture (water vapour) to form iron oxide — commonly called rust. The chemical formula of rust is — hydrated iron(III) oxide.
Yes. Iron () is a shiny grey metal. Rust () is a reddish-brown, brittle, flaky solid. These are completely different substances with different properties — new substance formed means chemical change.
You cannot “un-rust” iron by simple physical means — no amount of heating, cooling, or cutting will give you back the original iron. Irreversibility is a strong indicator of a chemical change.
Iron, oxygen, and water combine to produce rust. Bonds break and new bonds form — the definition of a chemical reaction.
Rusting is a chemical change.
Why This Works
A physical change only alters the form or appearance of a substance — the substance itself remains the same. Cutting iron, melting iron, bending iron: all physical changes because we still have iron at the end.
Rusting is different because the iron is consumed in the reaction. The product (rust) has different properties: it’s brittle, non-magnetic, doesn’t conduct electricity, and has a completely different colour. When properties change this fundamentally, a new substance has formed.
The key test we always use in Class 7: Can you get the original substance back easily? For physical changes, yes. For chemical changes, no — and rusting is permanent without industrial chemical treatment.
Alternative Method
Use the property checklist approach — no equation needed.
| Property | Iron | Rust |
|---|---|---|
| Colour | Shiny grey | Reddish-brown |
| Texture | Smooth, strong | Flaky, brittle |
| Magnetic? | Yes | No (weakly) |
| Conducts electricity? | Yes | No |
Every single property changed. When a substance’s fundamental properties change, it’s a chemical change. This method works brilliantly for MCQs where you need to decide quickly.
Common Mistake
Many students write: “Rusting is a physical change because only the surface of iron changes — the inside is still iron.”
This is wrong on two counts. First, given enough time, rust penetrates deeper. Second, even surface rusting produces a new substance (iron oxide). The amount of iron converted doesn’t determine whether it’s physical or chemical — the nature of the change does.
Three conditions are needed for rusting: iron + oxygen + water. Remove any one of them and rusting stops. This is exactly why we paint iron railings (blocks oxygen/water), oil iron tools (blocks water), or use galvanisation with zinc (blocks both). This “three conditions” fact is a guaranteed 1-mark question in CBSE Class 7 and Class 10 board exams.