The Rusting Gate
A school gate is made of iron and rusts badly near the hinge and along the bottom edge. The caretaker paints the gate, but within a year rust appears at a scratch and at the hinge, which is not painted because paint would stop it moving. A student sets up four iron nails: Nail 1 in dry air, Nail 2 in boiled water covered with oil, Nail 3 in tap water exposed to air, and Nail 4 in salt water exposed to air. After a week, Nail 3 shows rust, Nail 4 shows more rust, and Nails 1 and 2 show none. The student concludes that salt causes rusting and that water is not needed.
1. Evaluate the student's two conclusions using the results and the chemistry of rusting.
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The first conclusion is partly right but overstated. Nail 4 rusted more than Nail 3, so salt speeds up rusting, but salt is not the cause: Nail 3 rusted in tap water without added salt. The second conclusion is wrong. Nail 2 had water but no air (boiled water, oil layer) and did not rust; Nail 1 had air but no water and did not rust. Rusting needs both water and oxygen. Nail 3 had both and rusted. So the correct statement is that water and oxygen are both required, and salt accelerates the process. The student looked at which nail rusted most rather than which conditions were necessary, which is a common error in judging cause.
2. Explain why the hinge and the scratch rust first, and judge whether painting alone is an adequate protection plan.
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The hinge is unpainted, so bare iron contacts air and water. It also moves, so any protective layer is worn away. The scratch exposes bare iron while the surrounding paint film stays intact; water can collect at the edge of the paint and rust spreads under it. Painting alone is not adequate. Paint is a barrier coating, and any break in it lets rust start. Better protection combines methods: use a rust-resistant or galvanised hinge, apply a rust converter and primer before painting, and use a grease or oil film on moving parts. Regular inspection and touching up scratches also help. The judgement is that barriers work only while intact, so a plan must include maintenance and suitable materials for parts that cannot be painted.