The Rusty Nail Investigation
A group places four identical iron nails in separate test tubes. Tube 1: nail in dry air, sealed. Tube 2: nail half in water, open to air. Tube 3: nail fully under boiled water with a layer of oil on top, sealed. Tube 4: nail in salty water, open to air. After one week, the nails in tubes 2 and 4 are rusty, tube 4 more so. Tubes 1 and 3 show no rust. A pupil says 'water causes rust'.
1. Use the results to explain what conditions are needed for iron to rust.
Read the worked answer
Rust needs both water and oxygen. Tube 1 has dry air but no water, so no rust. Tube 3 has water but the boiled water and oil layer keep out dissolved oxygen, so no rust. Tube 2 has both water and air, so rust forms. This shows that neither water alone nor air alone is enough.
The pupil's claim is incomplete. Water is necessary but not sufficient; oxygen must also be present. The comparison of tubes 1, 2, and 3 is a fair test because the nail and tube are the same, and only the water and air conditions differ.
2. Explain why tube 4 rusted more than tube 2, and suggest one way to slow rusting on a garden gate.
Read the worked answer
Salt in the water speeds up the rusting reaction. The salty water in tube 4 allows rust to form faster than the plain water in tube 2, so more rust appears in the same week. Salt does not replace the need for oxygen; it just makes the reaction quicker.
To slow rusting on a garden gate, coat the iron with paint or oil so water and oxygen cannot reach the metal surface. Keeping the gate dry and free of salt spray would also help. Any coating must be kept in good repair, because a scratch lets water and air reach the iron again.