Describe how the conditions needed for rusting can be demonstrated experimentally, and state what the results would show.

Cambridge International IGCSE Chemistry (0620) — 9.5 Corrosion of metals · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Rusting is a chemical process that requires specific conditions to occur. A student sets up three test tubes, each containing an iron nail, to investigate which conditions cause rusting.

Model answer (4 marks)

1. Test tube 1: iron nail in tap water, tube open to air – the nail rusts.
2. Test tube 2: iron nail in boiled/distilled water, tube sealed with a layer of oil – the nail does not rust.
3. Test tube 3: iron nail in a dry tube with a drying agent (e.g. CaCl₂ or silica gel) – the nail does not rust.
4. Conclusion: rusting requires both water (moisture) and oxygen from the air; without either, no rust forms.

Examiner tips

Use the word ‘rusts’ or ‘does not rust’ to show the result. Mention the sealing oil in test tube 2 to emphasise the absence of air. State the conclusion clearly: both water and oxygen are necessary.
Include the type of drying agent in test tube 3 to demonstrate lack of moisture.

Common mistakes

Failing to mention the oil seal in test tube 2. Using ‘dry’ instead of ‘no water present’ for test tube 3. Omitting the conclusion that both water and oxygen are required.

Mark scheme (4 marks)

One test tube contains a nail in water (with air/oxygen present) — this nail rusts
One test tube contains a nail in boiled/distilled water sealed with oil (no air/oxygen) — this nail does not rust
One test tube contains a nail in a dry tube with a drying agent (e.g. calcium chloride/silica gel) — no water present — this nail does not rust
Conclusion: both water (moisture) and air (oxygen) are required for rusting to occur

Key terms in this question

rusting