Both air and water are necessary for iron to rust.

The idea

Rusting is the corrosion of iron, forming hydrated iron(III) oxide. It requires both oxygen, supplied by air, and water. Remove either and rusting stops. Classic experiments compare four tubes: nail in water with air; nail in boiled water under oil, so no air; nail in dry air with a desiccant, so no water; nail in salt water, which rusts fastest. Only the first and fourth rust, showing both air and water are needed. Salt water speeds rusting because ions help electrons move, but salt is not itself required. The overall idea is that iron is oxidised, losing electrons to oxygen in the presence of water.

What earns the marks

• Rusting needs both oxygen from the air and water together; removing either prevents it.
• Rust is hydrated iron(III) oxide, formed when iron is oxidised.
• A control tube with both air and water rusts; tubes missing air or water do not.
• Salt water accelerates rusting but is not a requirement.
• Boiling removes dissolved air, and an oil layer stops air re-entering.
• A desiccant such as calcium chloride keeps air dry so no water is present.

Common mistakes and how to put them right

• Saying only water or only air causes rust.

  Put it right: Both are needed together.

• Believing salt causes rust.

  Put it right: Salt only speeds up rusting, which still needs oxygen and water.

• Thinking rust is iron oxide only.

  Put it right: It is hydrated iron(III) oxide.

Exam tips

• Describe the control and each changed variable when explaining the experiment.
• Use the phrase 'both air and water' explicitly in your answer.
• Link salt water to faster rusting, not to a different product.