Science 7–10 · Year 8
What makes iron rust: nails in air, water, both and neither
Change
The idea
Rusting is a slow chemical change that needs both oxygen and water, and a controlled comparison shows which conditions matter.
What you need
- five clean iron nails, five test tubes in a rack, cotton wool
- anhydrous calcium chloride (drying agent), 2 spatulas; boiled and cooled water; cooking oil; salt
- test-tube bungs, labels
How to do it
- Tube 1: nail with air only, plus calcium chloride under a cotton-wool plug and a bung (dry air).
- Tube 2: nail fully covered with water that has been boiled and cooled, with a 1 cm layer of oil on top and a bung (water, no air).
- Tube 3: nail half-covered in tap water, open to the air (water and air).
- Tube 4: nail half-covered in salt water, open to the air. Tube 5: nail in air only, open (room air, usually humid).
- Leave for at least 3 days (a week is better); record the rust seen on each nail on a scale of 0 (none) to 3 (heavy) each school day.
What you should see
Tube 3 (water and air) rusts clearly; tube 4 (salt water) rusts most; tubes 1 and 2 show no rust; tube 5 shows little or none unless the room is humid. The learner knows it worked when the two single-condition tubes stay bright while the water-plus-air tubes turn orange-brown, so the conclusion "both are needed" follows from the data rather than from the textbook.
What changes
- What you change
- conditions around the nail (dry air, water without air, water and air, salt water and air)
- What you measure
- amount of rust after a fixed time
- What you keep the same
- identical nails
- same test-tube size
- same temperature and time
- nails cleaned the same way
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Rust is dirt that settles on the metal.
- Water alone causes rust.
- Salt causes rust by itself.
Safety card
Hazards
- anhydrous calcium chloride is an irritant
- sharp nails
Controls
- eye protection when handling calcium chloride
- oil and salt water to the bin, not the sink
Note
NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-CHG-01SC4-WS-03
- Australian Curriculum v9AC9S8U07AC9S8I02
Sources
The pages the author read to write this activity.