To explain corrosion from evidence, compare tubes that differ in one condition at a time. Iron rusts only when both oxygen and water are present, and the tubes that lack one of them show it.
This lesson follows relating extraction to reactivity and uses the same idea: a reactive metal changes back towards its compounds.
What is rusting, and what does it need?
Rusting is the corrosion of iron and steel. Iron reacts with oxygen and water to form hydrated iron(III) oxide, which is rust. Word form: iron + oxygen + water → hydrated iron(III) oxide.
An investigation is a set of tubes where one condition changes and the rest are kept the same. The tube with both oxygen and water is the test. The others are controls that each remove one condition.
The investigation below is shown as data for reasoning. A real classroom version is done only under a teacher’s supervision with the school’s safety rules.
How to read the evidence, step by step
- List what is present in each tube: oxygen (air), water, any extra such as salt.
- Record the result in each tube: no rust, some rust, a lot.
- Find the tube with everything present and check that it rusted.
- Compare each control with it. Remove one condition and see whether rusting stops.
- State the conclusion that links the condition to the result, naming the tubes.
Worked example
These results are invented for practice. An iron nail was placed in each tube for the same time.
| Tube | Conditions | Result |
|---|---|---|
| A | tap water, air above the water | rust |
| B | boiled water (air removed), oil layer on top | no rust |
| C | dry air, drying agent in the tube | no rust |
| D | salt solution, air above the water | more rust than A |
Step 1, tube A: water and oxygen are both present, and the nail rusted.
Step 2, tube B: water is present but dissolved oxygen was removed and the oil stops fresh air entering. No rust, so oxygen is needed.
Step 3, tube C: oxygen is present but water is absent. No rust, so water is needed.
Step 4, tube D: the same conditions as A plus dissolved salt gave more rust, so salt increases the rate.
Answer: rusting needs both oxygen and water, and dissolved salt speeds it up.
The mistake to watch for
A common slip is to draw a conclusion from one tube only.
Mistaken conclusion: “Nail A rusted in water, so water causes rust.”
The student ignored that air was present too. Tube A cannot show which condition mattered.
The correction is to use the controls. B keeps the water and removes the oxygen, and C keeps the oxygen and removes the water. Only the comparison between tubes can show that both conditions are needed.
How is rusting prevented?
Barriers such as paint, oil or plastic keep oxygen and water away from the iron. If the barrier is scratched, rusting can start at that point.
Sacrificial protection uses a more reactive metal. Zinc on iron reacts in place of the iron, so the iron stays protected even where the zinc is scratched. A coating of a less reactive metal, such as tin, protects only while the layer is unbroken.
Check yourself
1. In the table above, which tubes together show that oxygen is needed?
Show answer
Tubes A and B. Both have water, and only A has oxygen. A rusted and B did not, so oxygen is needed.
2. A steel bridge is painted and then scratched down to the metal. Explain why rust can begin at the scratch.
Show answer
The paint was a barrier that kept oxygen and water from the steel. The scratch exposes the iron to both, so rusting can begin there.
3. A galvanised (zinc-coated) iron sheet is scratched. Why does the iron still not rust quickly?
Show answer
Zinc is more reactive than iron, so it corrodes in place of the iron. This is sacrificial protection, and it works while zinc remains in contact with the iron.
Where this leads next
The next lesson moves from protecting metals to changing them: comparing an alloy with a pure metal. Water, air and environmental chemistry continues the air and water ideas, and the metals and reactivity practice set tests them.
Some students write the right facts but not the comparison between tubes. Online one-to-one Chemistry tuition can help, because a teacher can read the conclusion sentence and show where the evidence should appear.