Chemistry · 9.5 Corrosion of metals · Paper 5/6 practical
Rusting of Iron. Prove the cause.
Four test tubes test what iron needs to rust. Remove air, remove water, or add salt, then run the clock over a week. Only the tube with both air and water rusts — salt water rusts fastest.
0620 Topic 9.5 — Corrosion of metals
iron needs O₂ + H₂O
Paper 5/6 — Practical
Day 0
Tube B's boiled water has no dissolved air; the oil layer keeps air out. Tube C's drying agent removes water vapour.
The four tubes
Conclusion
Run the clock to compare the tubes.
📋 Method & ideas
- Tube A (control): iron nail in tap water, open to air → rusts.
- Tube B: nail in boiled (de-aerated) water with a layer of oil on top to keep air out → no rust (no oxygen).
- Tube C: nail in a tube with anhydrous calcium chloride (drying agent) and a stopper → no rust (no water).
- Tube D: nail in salt water, open to air → rusts fastest (salt speeds up rusting).
- Comparing A, B and C shows iron needs both oxygen and water to rust. Rust is hydrated iron(III) oxide.
⚠ Fair test & precautions
- Use identical iron nails (same size, cleaned) so the only differences are the conditions.
- Leave for the same time in the same place.
- Boil the water to expel dissolved oxygen, then cover with oil before it re-absorbs air.
🎯 Syllabus reference (0620)
- 9.5 Corrosion of metals — state that iron rusts in the presence of oxygen and water; describe barrier and sacrificial methods of rust prevention.