A rusting experiment – an easy practical that students can try at home

This experiment, which uses the rusting of iron to measure the percentage of oxygen in the air, can be done at home using easily available items.

Each student will need:

• A small transparent plastic water bottle
• Iron wool
• Ruler
• A jug or tall plastic container

Student instructions

Iron reacts with oxygen and water in the air to form hydrated iron oxide, otherwise known as rust. This is an example of an oxidation reaction.

You can use the reaction to find out the amount of air that is oxygen.

1. Cut the top of the bottle off. You might need to ask an adult to help you with this.
2. Create a ball of iron wool slightly larger in diameter than the bottle and wet it. Place it inside the bottle so it sits at the bottom. Turn the bottle upside-down and check the wool does not fall out. If it does, you will need to place more in so it fits snugly.
3. Place about 300 ml of water into the jug and place the bottle upside down into it.
4. Use a ruler to measure the length of the column of air inside the bottle.
5. Leave this in a place where it will not get disturbed. Every day, observe any changes in the appearance of the iron wool and the length of the column of air inside the bottle. Be careful not to take the bottle out of the water.
6. Stop when the length of air column no longer changes. This will be about a week.

Now try to answer these:

What happened to the iron wool?

Students should see that parts of the iron wool went rusty. This is because the iron was reacting with the oxygen in the air and water to make hydrated iron oxide which is rust. Not all of the iron will rust though because the iron is in excess.

What happened to the length of the column of air inside the bottle?

The length would have decreased as the water rose up the bottle. The iron wool was reacting with the oxygen inside the bottle. This removal of the oxygen meant that the volume of air would decrease and the water would rise up to fill the space left.

Calculate the decrease in the length of the column of air as a percentage. This will tell you how much of the air is oxygen.

If the column of air went from 15 cm to 5 cm then the percentage decrease would be calculated by: (5/15) x 100 = 33%.

The percentage of air that is oxygen is around 21%. How accurate are your results?

Students can decide this by looking at how close their result is to 21%. Inaccurate results may be due to not leaving the experiment until the oxygen was all used up and the water level had stopped rising or errors in measurement.

Related content

Your students can test their understanding of chemical reactions and oxides by completing Structured questions: Properties of oxides.

They can see how rusting led to an environmental disaster by reading Oops! When science goes wrong: Fuel tanker disaster.