Corrosion and rusting, their prevention, and alloys sheet5-mark answer frameWhat is rusting? Describe an experiment, with a labelled diagram, to show the conditions needed for it, and give three ways to prevent it.
How to lay out the answer. Follow the frame, then write it in your own words.
- 1 · Define. Rusting is the corrosion of iron by oxygen and moisture, forming rust, Fe₂O₃·xH₂O.
- 2 · Set up three tubes.
- A: nail half in water, corked (air and water)
- B: nail fully under boiled water with oil on top, corked (water only)
- C: nail with anhydrous calcium chloride, corked (dry air only)
- 3 · Draw. Draw the three labelled tubes, and shade the nail in A reddish-brown.
- 4 · Result. After a few days only nail A rusts. So rusting needs both air and water.
- 5 · Prevention.
- Painting, oiling or greasing
- Galvanisation: a coat of zinc
- Alloying, as in stainless steel
- 6 · Real life. Gates and grills are painted before the monsoon, when moist air rusts iron fastest.
- 7 · Close in one line. So keeping air or water away from iron stops rust.
Scoring tip: In the diagram, label what is in each tube, not just A, B and C. The examiner checks the conditions.
