Rust product

Rust is hydrated iron(III) oxide, often represented generally rather than by one simple fixed formula. Rusting is an oxidation process involving iron, oxygen and water.

Required conditions

Both oxygen and water are necessary. Dry air without water gives no rust, while boiled water protected from oxygen by an oil layer also gives no rust.

Salt and rate

Dissolved ions, especially in seawater, increase conductivity and can accelerate electrochemical corrosion. Salt is not required for rusting but can make it faster.

Experimental proof

Use identical clean nails, equal times and sealed tubes. A control containing both oxygen and water confirms that the nails and conditions can produce rust.

High-value recall and connections

Essential recall: What is rust? Hydrated iron(III) oxide. What two conditions are essential? Oxygen and water. Why add oil above boiled water? To prevent oxygen from re-entering the water. Practical connection: Plan the three-tube rust investigation. Explain how anhydrous calcium chloride removes water and why boiled water is covered with oil. Examination connection: State both oxygen and water. Writing only “air” may be insufficient.

Review questions and suggested answers