Why Iron Rusts
Iron needs BOTH air and water to rust.
What this lesson covers
Why it matters
Leave a shiny iron nail out in the damp for a few days and a flaky orange-brown coat creeps over it. That coat is rust — and iron only grows it when air and water team up together.
Predict first
Three iron nails are kept for a week: nail 1 in dry air (with a drying agent), nail 2 in boiled water sealed off from air, nail 3 in ordinary water open to the air. Which nail rusts?
Rusting needs BOTH oxygen (from air) AND water at the same time. The nail with air and water together rusts; the others stay shiny because each is missing one thing.
- Only nail 1, because dry air is full of oxygen
- Only nail 2, because water alone causes rust
- Only nail 3, which has both air and water — correct
- All three rust equally
What you do
Run the reaction on the bench: expose iron to both oxygen and water. Watch the grey metal slowly turn into a flaky orange-brown layer of rust. Notice that both the air (oxygen) and the water are needed for the change to happen.
Check yourself
What two things must both be present for iron to rust?
Rusting needs air and water together. Remove either one and iron stays shiny.
Which of these helps STOP iron from rusting?
Painting, greasing, or galvanising (coating with zinc) keeps air and water away from the iron, so it cannot rust.
How does rust compare with the iron it forms from?
Rust is hydrated iron oxide — a soft, crumbly orange-brown solid. It is a brand-new substance, quite different from the hard, grey iron.
- Oxygen (from air) and water — correct
- Only water
- Only oxygen
- Sunlight and heat
- Painting or greasing the iron — correct
- Sprinkling water on it every day
- Leaving it outside in the rain
- Scratching off its coating
- Rust is a soft, flaky orange-brown powder, unlike the strong grey iron — correct
- Rust is even stronger and shinier than iron
- Rust and iron are exactly the same substance
- Rust is a clear liquid