What is air made of? Measuring the oxygen
Trap air in a tube with damp iron wool over water, watch the water rise as the iron rusts, and compare fresh air with breathed-out air.
Core concept
Air is a mixture: about 78% nitrogen, 21% oxygen, 1% argon and a little carbon dioxide
Topic: Air and gases
Air is a mixture: Two or more substances mixed together, which can be separated again. All key words of gases, not a single substance. About 78% is nitrogen and about 21% is oxygen: A gas in the air that living things need to breathe and that fire needs to burn. All key words. The remaining 1% is mostly argon, with about 0.04% carbon dioxide: A gas we breathe out. Plants take it in to make their food. All key words and varying amounts of water vapour. Iron rusts by combining with oxygen, so damp iron wool trapped in a tube of air slowly removes all the oxygen. Water rises up the tube to take the place of the oxygen used up, and the rise shows what fraction of the air was oxygen. Breathed-out air has less oxygen and more carbon dioxide, because our bodies use oxygen in respiration.
What you will learn
- State the approximate composition of air.
- Measure the percentage of oxygen in air using rusting iron.
- Compare fresh air and breathed-out air.
Before you start
- Know that rusting needs oxygen and water.
Materials
- Damp iron wool (washed in vinegar to remove grease)
- Water
Equipment
- Graduated tube, trough or beaker, clamp stand
Procedure
- Trap fresh air in the tube with damp iron wool. Record the water level after 1, 2, 4 and 7 days.
- Repeat with breathed-out air and record after 7 days.
- Compare the final rise with the pie chart of the gases.
How this simulation is simplified
- The iron removes 55% of the remaining oxygen each day; the temperature and pressure stay the same.
- Breathed-out air is taken as 16% oxygen and 4% carbon dioxide (dry, ignoring water vapour).