Project: Rust and the Oxygen Content of Air


In this experiment you have an opportunity to learn about several topics in Chemistry. First, you will learn about a type of chemical reaction called an oxidation-reduction reaction. Second, you will learn about the air you breathe and find out that it is actually a mixture of different gases. Finally, you will be able to make a rough measurment of what fraction of the air is oxygen.


Iron is a very important element. It is most often used to make steel, an alloy of iron, manganese, chromium, and carbon. The steel is used to make everything from cars, to knives and forks, to bridges and large buildings. When iron rusts it changes color and begins to flake apart. Iron objects that are allowed to rust no longer function the way they were intended. Bridges may crumble if rust damage is not taken care of in time. Or maybe your bicycle chain will get so rusty you can’t pedal anymore. In 2002 a federal study backed by the U.S. Federal Highway Administration and carried out by NACE, the Corrosion Society, found that annual costs due to corrosion were $276 billion. This is more than a quarter of a trillion dollars. (source: accessed Mar 6, 2013). In other words, rust is a big deal with huge real-world costs.

Rusting is a complex chemical reaction involving iron, oxygen, and water. In order for the reaction to work, water must be present: iron that never gets wet rusts extremely slowly or not at all. Iron that is wet and is exposed to air will quickly obtain a surface layer of rust. If the water is salty, the reaction may be even faster because the salt serves as an electrolyte which makes it easier for the iron atoms to give electrons to the oxygen atoms. Chemical educators classify this reaction as an oxidation-reduction reaction. Some may add that it is an example of a synthesis reaction. Here is one way to write the reaction symbolically:

4Fe(s) + 3O2(g) —> 2Fe2O3(s)

In this reaction the iron gives electrons to the oxygen changing from the pure metal (Fe) to the ion iron(III) (Fe3+). The oxygen gains electrons and changes from pure oxygen (O2) to the oxide ion (O2–). The oxygen in this reaction comes from the air. Air is actually a mixture of many small components but only three major components. The Earth’s atmosphere is 78% nitrogen gas (N2), 21% oxygen gas (O2) and 1% argon gas (Ar). In this activity you will be able to measure the oxygen content of the air by causing iron to rust in a container that is closed off from the rest of the atmosphere by a water barrier. As the oxygen is used up water will rise in the container to take its place. The fraction of the container that is filled with water when the iron has reacted with all of the oxygen trapped in the container is the same as the fraction of the air that is oxygen.


  1. 2 identical tall jars (olive jars work well)
  2. fine steel wool with no soap
  3. 2 small food storage containers, at least 1.5 inches deep
  1. 2 rubber bands
  2. metric ruler
  3. water
  4. salt


Read all instructions completely before beginning your experiment. By knowing why you are doing each step you will be able to do it better. There are few hazards in this activity but as always, be careful when carrying out chemistry experiments. Keep the materials away from mouth, nose and eyes and do not eat or drink while working on the experiment. The rust may be handled but wash hands thoroughly after handling the rusty steel wool. When you are finished, throw the rusty steel wool away in the trash. Place the other items in your recycle bin.

  1. Prepare a salt-water solution by dissolving about 2 tablespoons of table salt in a cup of water. Stir until completely dissolved then pour into one of the small food-storage containers. Fill the other container with an equal amount of tap water. Mark each container so you know which one contains the salt water. You could place them on a piece of paper with a label or write on a piece of clear tape that you can stick to the container.
  2. Tear off two pieces of steel wool that are large enough to be wedged in the bottom of the jar without falling out when you tip the jar upside-down. Dip one in the tap water and the other in the salt water until they are both wet through. Then wedge each one into the bottom of a jar, letting the excess water drain out.
  3. Tip each jar upside-down into one of the food-storage containers. Make sure the steel wool dipped in salt water is in the salt water container.
  4. Tip each jar up a bit to let out a small bubble of air so the jar won’t tip over. Mark the inner water level by putting a rubber band around the glass at the current level.
  5. The rusting process will take several days to be complete. Here are some observations you should make (and write down). Check the jars every day.
    • Has the water level inside the jar changed? Measure the height of the water inside the jar that is above the rubber band. Measure the entire height of the jar and calculate the percentage of the original air space that is now filled with water.
    • Observe the steel wool: how rusty is it? How will you know when it is as rusty as it can get?
    • Which jar fills with water faster, the one with the tap water or the one with the salt water? Why do you think there is a difference?
  6. In your report be sure to give a definitive answer about the percentage of the air in the jar that was oxygen.
  7. Also, do not fail to mention which jar filled with water sooner and to explain why.
  8. Finally, in your report answer this question: how did you know that rusting was complete and that no more rust would form?

The Report

Write the usual Real Life Chemistry report for this experiment.

Here are a few web links to useful sources of further information about rust:
See also the link in the Background section about the corrosion cost study
Here are a few web links to useful sources of further information about the atmosphere:
This project was inspired by an activity published by the American Chemical Society on their Science for Kids site. Their version is on this page.
Last updated: Mar 08, 2013       Home