Science 7–10 · Year 8
How much of the air is used up when iron wool rusts
Change
The idea
Rusting uses only the oxygen in trapped air, so the water rises to replace about one fifth of the air column and no more.
What you need
- test tube, 100 mL beaker two-thirds full of water, ruler, clamp stand
- iron wool (steel wool without soap), a wad the size of a walnut, wetted with water and pushed to the base of the tube
How to do it
- Push the wet iron wool to the bottom of the test tube and invert the tube into the beaker of water; clamp it so the mouth stays under water.
- Measure the length of the air column (water level to the closed end) and record it as the start value.
- Leave for a week. Without lifting the tube out of the water, measure the air column again each day if possible.
- Calculate the fraction of the air column lost: (start length minus final length) divided by start length.
What you should see
Water rises steadily for several days and then stops. The final air column is about one fifth shorter than at the start: dry air is 20.95 percent oxygen by volume, and because the liquid water keeps the water vapour pressure in the tube steady (2.34 kPa at 20 °C) while the oxygen is used, the column shrinks by that same 20.95 percent, so a 15 cm column loses about 3.1 cm. The iron wool turns orange-brown. The learner knows it worked when the rise stops well short of filling the tube, which shows one component of the air, not all of it, was consumed.
What changes
- What you change
- time
- What you measure
- length of the trapped air column
- What you keep the same
- tube left in the water
- room temperature
- same wad of iron wool
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The water rises because the tube sucks it up.
- All of the air is used when something rusts or burns.
- Rust weighs less than the iron because iron has been lost.
Safety card
Hazards
- iron wool fibres can prick fingers and irritate eyes
Controls
- eye protection
- wash hands after handling the wool
Note
Iron wool only; no hazardous reagents. Basic risk assessment per CSIS Section 1.7.
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-06
- Australian Curriculum v9AC9S8U07AC9S8I05
Sources
The pages the author read to write this activity.