Rusting and preventing it
Key idea: Iron needs both oxygen and water to rust. Paint, grease and plastic coatings protect it by keeping these substances away from its surface.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Compare A with B for oxygen and A with C for water.
- Choose the temperature pair and count the B/C differences.
- Explain which comparisons change only one condition.
- Choose a barrier and consider scratches and trapped moisture.
- Explain how paint protects iron and how a scratch changes this.
- Check how the dry wire is protected and whether coating reverses rusting.
- Explain rust beneath an intact coating.
- Compare paint and grease for a moving chain.
- Predict rust when negligible oxygen reaches the nail.
- Explain how boiling, oil and the stopper keep oxygen away.
- Describe the wet control needed to test whether water matters.
- Explain the boat’s rust pattern and how to restore the barrier.
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ContinueIron and steel are strong and cheap, but left outdoors they slowly turn into flaky brown rust. The iron is gradually converted into compounds, weakening the metal. Corrosion is damage caused by a material reacting with its surroundings; rusting is the corrosion of iron that forms rust. In this lesson you will use a controlled experiment to find out what iron needs in order to rust, and use that to explain how paint, grease and plastic coatings protect it.
Rust is hydrated iron(III) oxide. It forms when iron reacts with substances from its surroundings:
The word equation names oxygen and water. The experiment below tests whether each is needed.
The experiment: one change at a time
Three identical iron nails are rubbed clean with sandpaper and set up as shown. Each tube differs from tube A in one condition only.
Swipe or scroll sideways to inspect the complete overview.
- Tube A has both air and water.
- Tube B has water, with oxygen excluded from the nail. Boiling removes most dissolved air, and the oil layer restricts air dissolving back in. There is still air above the oil; the aim is to keep it from reaching the submerged nail.
- Tube C has air but no water. The stopper keeps out moist air, and anhydrous calcium chloride absorbs any water vapour inside.
The tubes are left side by side at room temperature for a week. Assume B is prepared carefully enough that negligible oxygen reaches its nail, and C stays dry.
| Tube | Oxygen available at the nail? | Water present? | Nail after one week |
|---|---|---|---|
| A | yes | yes | covered in brown rust |
| B | no | yes | no rust |
| C | yes | no | no rust |
Worked example 1
What do the three tubes show?
Problem
Use the results table to state the conditions iron needs in order to rust. Say which tubes you compared for each condition.
Show full solution
Compare A with B
Method
Compare whether oxygen can reach the nail.Reason
The setups are designed to keep water present while excluding oxygen from B’s nail. The result supports oxygen being needed for rusting.
Working
Answer
A rusts, B does not → oxygen at the nail is needed.
Compare A with C
Method
Only water differs: A has it, C does not.Reason
Again, one condition changed and everything else stayed the same.
Working
Answer
A rusts, C does not → water is needed.Combine the comparisons
Working
Answer
Iron rusts only when both oxygen and water are present. Removing either one stops it.
Quick check
Tube A is the control. Without it, tubes B and C would only show that nothing happened; comparison with A shows what each missing condition does.
Guided practice 2
Pick the fair comparison
Problem
A student adds tube D: it has the same clean nail, oxygen and water as A, but is kept at a higher temperature for the same time. Which pair tests the effect of temperature on rusting? First select the pair and identify what changes between B and C. Give your complete comparison in the next activity.
Change one condition only
Hints
Hint 1
Fair test
A fair comparison changes one condition and keeps the rest the same.
Hint 2
Read the table
Keep oxygen and water the same while changing temperature.
Show solution step by step
Find the temperature-only difference
Method
Choose A and D.Reason
Both have oxygen and water, with equivalent nails and the same duration. Temperature is the only intended difference.
Working
Answer
Compare the extent of rusting in A and D after the same time. The setup tests temperature; results are needed to judge its effect.
Reject B with C
Working
Answer
B and C differ in both air and water, and neither rusts, so the pair cannot show which condition matters.
Check your understanding 1 Explain the controlled comparison
A and D have equivalent nails, oxygen, water and duration, but D is warmer. Explain why A and D test temperature and why B and C cannot isolate either oxygen or water. Can you judge the temperature effect before reading the results?
Show answer
A and D differ only in temperature, so their rusting after the same duration can test its effect. B excludes oxygen but has water; C has oxygen but is dry. Both oxygen and water availability change between B and C, and neither rusts. Compare A with B for oxygen, and A with C for water. Results for D are needed before judging the effect of temperature.
When a result looks wrong
Suppose the nail in tube B shows a few spots of rust. Investigate before drawing a conclusion. Was the nail already rusty? Was the water boiled thoroughly, and was air kept from dissolving back in as it cooled? Repeat with clean nails and carefully prepared controls. If the unexpected result persists, investigate it further rather than ignoring it or forcing it to fit the prediction.
Stopping rust with a barrier
If rusting needs oxygen and water together, keeping either of them away from the iron surface stops it. The simplest way is a barrier: a layer around the metal that oxygen and water cannot pass through.
| Barrier | Typical uses | Limitation |
|---|---|---|
| paint | bridges, railings, car bodies | chips and scratches expose the iron; needs repainting |
| grease or oil | moving parts such as bicycle chains and hinges | rubs or washes off; must be reapplied |
| plastic coating | wire fences, dish racks, garden wire | a cut or tear exposes the iron |
The choice depends on the object. Paint suits a still surface that is seen and should last for years. Grease suits parts that rub against each other, where paint would crack off. A flexible plastic coating can suit wire that bends.
An intact barrier protects; a broken one does not. A barrier works only where it covers the iron. At a scratch, the iron is exposed to oxygen and water again, so rust starts there. It can then spread under the paint around the scratch. A barrier also cannot turn rust back into iron. Prepare a clean, dry surface before coating: trapped moisture and existing loose rust can undermine the coating. The barrier reduces further contact with oxygen and water; it does not reverse the reaction already completed.
Exam-style question 1
Explain why paint prevents rusting
Examination question
State the conditions needed for iron to rust, and explain why painting an iron railing prevents rusting until the paint is scratched. [4 marks] Work on paper or in the optional notes, then compare all four criteria.
Write conditions, barrier action and limitation
Show solution step by step
State both conditions
2 marksMethod
Name oxygen and water.Reason
Iron needs both for rusting.Working
Answer
oxygen and waterExplain the barrier
1 markMethod
Paint stops oxygen and water reaching the iron surface.
Reason
Without contact with both, the iron cannot rust.Working
Answer
paint → oxygen and water kept away from the iron
Explain the scratch
1 markMethod
A scratch exposes the iron to air and water.Reason
At that point the barrier no longer separates the metal from both conditions.
Working
Answer
scratch → exposed iron → rusting can start
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Award oxygen and water as separate conditions, then the barrier and scratch explanations.
Try it yourself 3
Rust beneath an intact coating
New situation
Two clean steel wires receive intact plastic coatings. Wire P was dried first; wire Q had droplets of water and some air trapped beneath its coating. Small rust spots later appear on Q but not P. First choose how the dry wire is protected and whether painting reverses rusting. In the next activity, explain why rust beneath Q’s coating does not mean that the steel became more reactive.
Compare conditions beneath the coatings
Hints
Hint 1
Conditions
Rusting needs oxygen and water at the iron surface.
Hint 2
Where is the water?
A barrier prevents entry; it does not remove what was already trapped beneath it.
Show solution step by step
Explain the dry wire
Method
The intact plastic is a barrier.Reason
On properly dried P, the coating prevents the conditions for rusting from being brought together at the steel surface.
Working
Answer
Dry surface + intact barrier → no rust expected.
Explain the trapped conditions
Method
Q already has water and oxygen beneath the coating.
Reason
The coating cannot remove these trapped substances, so some rust can form without a break in the coating or a change in the steel’s reactivity.
Working
Answer
Trapped water + oxygen from trapped air → rust can form.
Prevention, not reversal
Working
Answer
An intact coat on properly cleaned, dried iron can protect it. Coating cannot turn rust that has already formed back into iron.
Quick check
The plastic does not change the iron itself. Its protection lasts only as long as it covers the metal.
Check your understanding 2 Explain rust beneath an intact coating
Wire P was dry before an intact plastic coating was applied. Wire Q had water droplets and air trapped beneath its intact coating, and later developed rust spots. Explain why the plastic did not make Q more reactive, why P is protected, and why painting cannot turn rust back into iron. Work on paper or in the optional notes.
Show answer
The intact coating keeps oxygen and water from reaching properly dried P. Q already has oxygen from trapped air and water at the steel surface, so rust can form without a break in the coating or a change in reactivity. A barrier prevents contact; it neither removes trapped substances nor reverses rust already formed. Clean and dry the iron before coating to protect it from further rusting.
Common mistakes
| Mistake | Why it fails | Correct reasoning |
|---|---|---|
| “Iron rusts in water.” | Tube B had water but no oxygen, and did not rust. | Rusting needs oxygen and water. |
| “Iron rusts in air.” | Tube C had air but no water, and did not rust. | Both conditions are needed together. |
| “Paint makes iron unreactive.” | Iron under paint is unchanged; it rusts once exposed. | Paint keeps oxygen and water away. |
| “A scratched coating still protects the scratch.” | The scratch has no barrier over it. | Exposed iron can rust. |
| “All metals rust.” | Rust is hydrated iron(III) oxide. | Only iron and steel rust; other metals corrode in other ways. |
Try these
Check your understanding 3 Choose and maintain a chain barrier
Compare paint and grease for protecting an iron bicycle chain from rusting. Which better suits the chain, and what maintenance does it need?
Work on paper or in the optional notes. Compare suitability for movement, barrier action and maintenance.
Show answer
Grease better suits the moving chain. Both act as barriers to oxygen and water, but movement can crack or wear paint off. Grease coats moving parts and needs reapplying when it rubs or washes off.
Check your understanding 4 Predict before explaining
A clean nail is submerged in boiled water beneath oil in a stoppered tube. Assume negligible oxygen reaches the nail. Predict whether visible rust is expected.
Show answer
No visible rust is expected under the stated oxygen-exclusion assumption. Explain how the setup maintains that condition in the next activity.
Check your understanding 5 Explain the oxygen-exclusion setup
A clean iron nail is submerged in boiled water under oil in a stoppered tube. Assume negligible oxygen reaches the nail. Predict whether visible rust forms and explain.
Work on paper or in the optional notes. Compare oxygen exclusion and the need for both oxygen and water at the nail.
Show answer
No visible rust is expected. Boiling removes dissolved air; the oil and stopper restrict oxygen returning. Rusting requires both water and oxygen at the nail, so water alone is insufficient.
Check your understanding 6 Design the missing control
An iron nail stays rust-free in dry air. A learner uses this tube alone to conclude that water is needed for rusting. What comparison is missing, and why does it matter?
Work on paper or in the optional notes. Compare the control, the one condition changed and what the paired results tell you.
Show answer
Compare it with an otherwise equivalent nail exposed to both air and water, which rusts. The dry tube alone shows only no rust under its conditions. The paired result isolates the effect of water while oxygen remains available.
Check your understanding 7 Explain local damage and a suitable barrier
A painted steel boat rusts only along a strip where its side rubs against a wooden jetty. Explain the pattern, and explain why coating the whole hull with grease instead of paint would be a poor choice.
Work on paper or in the optional notes. Compare local exposure, suitability of the coating and maintenance.
Show answer
Rubbing scrapes the paint off along the strip, exposing the steel to oxygen and water, so it rusts there; elsewhere the intact paint still keeps them out. Grease suits moving parts, but on a large fixed surface in water it would wash or rub off and need constant renewal. Clean and dry the strip, then repaint it.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
Last reviewed: