Does rusting need air?
First, test whether rusting needs air.
Activity 12.3.1
You will need: two boiling tubes, ordinary cold water, coconut oil, two iron nails, a burner, dilute hydrochloric acid
- Nails from the market have a zinc coating. To remove it, dip the two nails in dilute hydrochloric acid for about 10 minutes and wash them with water.
- Fill the two boiling tubes about half full of cold water.
- Boil the water in one tube for about five minutes. Drop one cleaned nail into each tube. To stop air dissolving again in the boiled water, pour a little coconut oil onto it.
- Leave both tubes for about a day, observe and record.
Figure to be added
In both tubes the nail is in contact with water. But the water in tube B has been heated, so the air that was dissolved in it has been driven out; and the oil layer keeps the water from touching the air. So the nail in tube B gets no air, while the nail in tube A gets air (dissolved in the water). All other factors are the same in both tubes.
The observation
The nail in tube A has rusted and the nail in tube B has not. This confirms that rusting needs air.
Why it is a fair test
The two tubes differ in only one way: whether the nail can get air. The same kind of nail, cleaned the same way, sits in the same amount of water for the same time. So any difference in rusting must come from the air. Removing the zinc first matters too: zinc protects iron, as you will see at the end of the chapter, and a zinc-coated nail might not rust in either tube.
Rusting needs air
A nail in boiled water under oil does not rust; a nail in ordinary water does.