Why is "Giving only one condition for rusting, or writing 'air' instead of 'oxygen'" a common mistake in Iron rusting and preventions?

Why it happens: Students remember rusting needs 'air and water' and assume one word is enough, or use 'air' loosely. How to avoid it: Always write **both water AND oxygen**. Use the word 'oxygen' (not 'air'), because it is the oxygen in air that is needed. Source: Edexcel Chemistry examiner reports — recurring error

Why is "Naming rust as iron oxide or iron(II) oxide" a common mistake in Iron rusting and preventions?

Why it happens: Students forget it is the (III) oxide and that it is hydrated. How to avoid it: Rust is **hydrated iron(III) oxide** (Fe₂O₃·xH₂O). Include 'hydrated' and '(III)' where the question asks for the name. Source: Edexcel Chemistry examiner reports

Why is "Explaining a barrier method as 'it protects the metal' without saying what is kept out" a common mistake in Iron rusting and preventions?

Why it happens: Students know the idea but do not state the mechanism. How to avoid it: Say the layer **stops water and oxygen reaching the iron**. Naming both substances is what earns the explanation mark. Source: Edexcel Chemistry examiner reports

Why is "Saying sacrificial protection works without mentioning that the protecting metal is MORE reactive" a common mistake in Iron rusting and preventions?

Why it happens: Students learn 'zinc protects iron' as a fact but miss the reason. How to avoid it: State that the metal (Zn/Mg) is **more reactive than iron**, so it **corrodes / is oxidised in preference to** the iron. Source: Edexcel Chemistry examiner reports

Why is "Thinking tin-plating protects iron the same way as galvanising" a common mistake in Iron rusting and preventions?

Why it happens: Both are metal coatings, so students assume they behave identically when scratched. How to avoid it: Zinc is **more** reactive than iron (sacrificial — protects when scratched); tin is **less** reactive (barrier only — iron rusts, even faster, at a scratch). Source: Edexcel Chemistry examiner reports

Why is "Not explaining HOW oxygen or water is removed in the three-tube experiment" a common mistake in Iron rusting and preventions?

Why it happens: Students describe the tubes but skip why B has no oxygen and C has no water. How to avoid it: State that **boiling removes dissolved oxygen and oil seals out air** (tube B) and that the **drying agent removes water** (tube C). Source: Edexcel Chemistry examiner reports

Reactivity seriesEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Iron rusting and preventions

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Iron Rusting and Prevention — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

Why iron and steel rust, and how to stop it. Rust is hydrated iron(III) oxide, and it only forms when BOTH water AND oxygen are present — proved by the classic three test-tube experiment. Prevention works in two ways: keeping water and oxygen out (barrier methods), or using a more reactive metal that corrodes instead (sacrificial protection, galvanising). Covers Double Award Chemistry spec 2.20–2.21, assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

Mapped to the Edexcel IGCSE 4WSD0-1C syllabus (2026 onwards).

2.20 — Describe the conditions needed for iron to rust (water and oxygen).
2.21 — Describe and explain methods of rust prevention: barrier methods, galvanising and sacrificial protection.

What rust is and why it matters (spec 2.20)

Rust is hydrated iron(III) oxide. It is flaky, so it falls off and lets the iron underneath keep rusting.

What is rust? Rust is hydrated iron(III) oxide — formula Fe₂O₃·xH₂O. It is the flaky orange-brown solid you see on old gates, nails, cars and ships. Only iron and steel rust (steel is mostly iron). The word "rusting" is reserved for iron — other metals "corrode" but do not "rust".

The overall reaction can be written as:

$4\text{Fe} + 3\text{O}_2 + 2x\text{H}_2\text{O} \rightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$

Why rusting is such a problem. Rust is flaky and does not stick firmly to the iron beneath it. So as it forms it falls off, exposing fresh iron to the air and water, which then rusts too. Over time the iron is eaten away completely — rusted bridges, car bodies and pipes can weaken and fail. This is different from aluminium, whose oxide layer is tough and stays put, protecting the metal underneath.

Rusting is oxidation. The iron gains oxygen, so it is oxidised. (In electron terms, iron atoms lose electrons to form iron ions — you do not need the full mechanism for Double Award, but knowing rusting = oxidation helps you explain prevention.)

Rust = hydrated iron(III) oxide, Fe₂O₃·xH₂O.
Only iron and steel rust; the orange-brown solid is flaky.
Rust falls off and exposes fresh iron, so rusting keeps going.
Rusting is an oxidation reaction (iron gains oxygen).

See the full worked example for iron rusting and preventions →

The conditions needed for rusting — water AND oxygen (spec 2.20)

Both water and oxygen must be present. The three test-tube experiment proves each one is essential.

The key fact: iron rusts only when BOTH water AND oxygen are present. Take away either one and rusting stops. This is the most-tested idea in the whole subtopic — never write just "air" or just "water".

The classic way to prove it is the three test-tube experiment. An identical iron nail is placed in each tube under different conditions and left for several days.

Tube	Contents	Result	Why
A	iron nail + tap water + air	RUSTS	both water and oxygen present
B	iron nail + boiled water + oil layer on top	no rust	water present, but no oxygen (boiling drives out dissolved air; oil seals the surface)
C	iron nail + dry air + anhydrous calcium chloride (drying agent)	no rust	oxygen present, but no water (calcium chloride absorbs it)

Only tube A — where both water and oxygen are present — shows rust. This is the experimental proof that both are needed.

The conclusion you must write: because the nail rusts only in tube A (where both are present) and not in B or C (where one is missing each time), both water and oxygen are needed for rusting.

Salt water makes it worse. Dissolved salt (ions) speeds rusting up — which is why cars rust faster near the coast or where roads are gritted in winter, and why ships need extra protection.

Iron rusts ONLY when water AND oxygen are both present.
Tube A (water + air) rusts; tube B (boiled water + oil, no O₂) does not; tube C (drying agent, no water) does not.
Boiling removes dissolved oxygen; oil seals out air; calcium chloride removes water.
Salt water (dissolved ions) speeds rusting up.

See the full worked example for iron rusting and preventions →

Prevention 1 — barrier methods (spec 2.21)

Barrier methods put a physical layer over the iron to keep water and oxygen out.

The idea. If rusting needs water and oxygen to reach the iron, the simplest prevention is to put a barrier in the way so neither can get to the surface.

Common barrier methods:

Painting — cars, gates, railings, bridges. Cheap and easy to apply to large areas.
Oiling or greasing — tools, bike chains, moving machine parts (the barrier also lets parts move).
Coating with plastic — garden furniture, dish racks, the plastic-coated handles of tools.
Plating with another metal — e.g. tin-plating on the inside of food cans, or chrome-plating on taps for a shiny, protective finish.

How they work (the marking point): the layer stops water and oxygen reaching the iron, so the iron cannot rust.

The big limitation. Barrier methods only work while the layer is complete and unbroken. If the coating is scratched or chipped, water and oxygen reach the exposed iron and it rusts at that spot. This is the weakness that sacrificial methods and galvanising are designed to fix.

Barrier methods = a physical layer that keeps water and oxygen out.
Examples: paint, oil/grease, plastic coating, plating (tin, chrome).
Marking point: the layer stops water and oxygen reaching the iron.
Limitation: if scratched, the exposed iron rusts.

See the full worked example for iron rusting and preventions →

Prevention 2 — sacrificial protection and galvanising (spec 2.21)

A MORE reactive metal corrodes instead of the iron. Galvanising adds a zinc coat that is both barrier and sacrificial.

These methods use the reactivity series: a metal that is more reactive than iron loses its electrons (corrodes) more readily than iron does. So if you connect a more reactive metal to the iron, that metal is "sacrificed" — it corrodes first and protects the iron.

Sacrificial protection.

What you do: attach a block of a more reactive metal — usually zinc or magnesium — to the iron object (in contact with it).
Why it works: the more reactive metal is oxidised in preference to the iron — it loses electrons more easily, so it corrodes away instead of the iron. The iron is protected even if its surface is exposed to water and oxygen.
Examples: zinc blocks bolted to ship hulls; magnesium blocks attached to underground steel pipelines and inside hot-water tanks.
Limitation: the sacrificial metal is used up and must be replaced from time to time.

Galvanising.

What you do: coat the iron or steel in a layer of zinc (often by dipping it in molten zinc).
Why it is the best general method — it works in TWO ways at once:
1. Barrier: the zinc layer keeps water and oxygen away from the iron.
2. Sacrificial: zinc is more reactive than iron, so even if the coat is scratched, the surrounding zinc corrodes instead of the exposed iron — the iron is still protected.
Examples: galvanised nails, corrugated roofing sheets, chain-link fences, dustbins, crash barriers.

Zinc is more reactive than iron, so it corrodes in preference to the iron — protecting it both as a sacrificial block and as a galvanised coating.

The crucial comparison. A scratch in paint lets the iron rust. A scratch in galvanising does NOT — the zinc keeps protecting the iron because zinc is more reactive than iron. This is why galvanising is so widely used.

Sacrificial protection: a MORE reactive metal (Zn/Mg) corrodes instead of the iron.
It works because the more reactive metal loses electrons / is oxidised in preference to iron.
Sacrificial blocks must be replaced when used up.
Galvanising = zinc coat that is BOTH barrier AND sacrificial.
A scratch in galvanising is still protected; a scratch in paint is not.

See the full worked example for iron rusting and preventions →

How it’s examined

Double Award Chemistry is assessed on one untiered paper — Chemistry Paper 1 (4WSD0/1C), 2 hours, 110 marks, 33.3% of the Double Award (calculator allowed). Rusting questions reliably appear as: state the two conditions needed for rusting (2 marks — must say water AND oxygen); explain the three test-tube experiment and draw a conclusion (3–4 marks); describe and explain a prevention method (2–4 marks); and the top discriminator — explain why sacrificial protection or galvanising works in terms of a more reactive metal being oxidised / corroding in preference to the iron (3–4 marks). The single most common lost mark is writing just 'air' or just 'water' instead of both.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4 (valid for 2026 onwards); Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4 (shared Chemistry Paper 1 content); Pearson Edexcel 4SD0 / 4CH1 examiner reports 2022–2024. Last reviewed 2026-06-07.

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Step-by-step worked examples — Iron rusting and preventions

Step-by-step solutions to past-paper-style questions on iron rusting and preventions, written exactly the way a tutor would explain them at the board.

1Name rust and state the two conditions (Getting started)
Getting started• 4WSD0/1C style — short answer• rusting, spec-2.20
Question
(a) Name the chemical compound formed when iron rusts. (b) State the two substances that must both be present for iron to rust. (2 marks)
Step-by-step solution
1. Step 1
  (a) Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O).
2. Step 2
  (b) Both water and oxygen must be present.
3. Step 3
  Watch the wording. Writing only 'oxygen' or only 'water', or writing 'air' instead of 'oxygen', loses the mark — you must name BOTH.
Answer
(a) Hydrated iron(III) oxide. (b) Water and oxygen — both must be present.
Examiner tip
The single most common lost mark in this topic is naming only one condition. Always write water AND oxygen. 'Air' on its own is not accepted because air also contains other gases.
2Explain how painting prevents rust (Getting started)
Getting started• 4WSD0/1C style — short answer• prevention, barrier, spec-2.21
Question
A steel gate is painted. Explain how painting stops the gate from rusting. (2 marks)
Step-by-step solution
1. Step 1
  Identify the method (1). Paint acts as a barrier (a physical layer) on the surface of the iron.
2. Step 2
  Explain the effect (1). The barrier stops water and oxygen reaching the iron, so the iron cannot rust.
Answer
Paint forms a barrier on the surface that stops water and oxygen reaching the iron, so it cannot rust.
Examiner tip
For full marks you must say WHAT the barrier keeps out — water and oxygen. 'It covers the metal' alone is too vague.
3Conclusion from the three test-tube experiment (Building confidence)
Building confidence• 4WSD0/1C style — structured• rusting, required-practical, spec-2.20
Question
Three identical iron nails are set up: Tube A (tap water + air), Tube B (boiled water with a layer of oil on top), Tube C (dry air with a drying agent). After one week only the nail in Tube A has rusted. Explain why the nails in B and C did not rust, and state the conclusion. (4 marks)
Step-by-step solution
1. Step 1
  Tube B (1). Boiling removes the dissolved oxygen and the oil seals the surface, so there is water but no oxygen — no rust.
2. Step 2
  Tube C (1). The drying agent (anhydrous calcium chloride) absorbs the water, so there is oxygen but no water — no rust.
3. Step 3
  Tube A (1). Tube A has both water and oxygen, so the nail rusts.
4. Step 4
  Conclusion (1). Because rust forms only when both are present, both water and oxygen are needed for rusting.
Answer
Tube B has water but no oxygen (boiled out, sealed by oil), so no rust. Tube C has oxygen but no water (removed by the drying agent), so no rust. Tube A has both, so it rusts. Conclusion: both water and oxygen are needed for iron to rust.
Examiner tip
Each tube is a separate mark, plus a mark for the conclusion. State clearly what is MISSING from B and C — that is the link to 'both are needed'.
4Why sacrificial protection works (Building confidence)
Building confidence• 4WSD0/1C style — structured• prevention, sacrificial, spec-2.21
Question
Blocks of zinc are bolted to the steel hull of a ship to stop it rusting. Explain how the zinc protects the steel. (3 marks)
Step-by-step solution
1. Step 1
  Reactivity (1). Zinc is more reactive than iron (it is higher in the reactivity series).
2. Step 2
  What happens (1). The zinc is therefore oxidised / corrodes in preference to the iron — it loses electrons more readily than the iron does.
3. Step 3
  Protection (1). So the zinc corrodes instead of the steel, protecting the iron even where it is exposed to water and oxygen. This is called sacrificial protection.
Answer
Zinc is more reactive than iron, so the zinc loses electrons / corrodes in preference to the iron. The zinc is 'sacrificed' and corrodes instead of the steel, protecting it.
Examiner tip
The mark-winning idea is 'more reactive metal corrodes/is oxidised in preference to the iron'. Just saying 'the zinc protects it' without the reactivity reason scores only part marks.
5Why galvanising beats paint when scratched (Stretch)
Stretch• 4WSD0/1C style — extended• prevention, galvanising, spec-2.21
Question
Galvanised steel (steel coated with zinc) and painted steel are both used outdoors. Explain why galvanised steel still resists rusting after the coating is scratched, but painted steel rusts at a scratch. (4 marks)
Step-by-step solution
1. Step 1
  Paint when scratched (1). Paint is only a barrier. A scratch lets water and oxygen reach the iron, so the iron rusts at that spot. (1 for the consequence)
2. Step 2
  Galvanising is dual (1). Galvanising works in TWO ways: a barrier (the zinc layer keeps water and oxygen out)...
3. Step 3
  ...and sacrificial (1). ...and sacrificial protection, because zinc is more reactive than iron.
4. Step 4
  Scratched galvanising (1). So even when scratched, the surrounding zinc corrodes in preference to the exposed iron — the iron is still protected.
Answer
Paint is only a barrier, so a scratch exposes the iron to water and oxygen and it rusts. Galvanising is both a barrier AND sacrificial: zinc is more reactive than iron, so where the coat is scratched the surrounding zinc corrodes in preference to the iron, keeping it protected.
Examiner tip
Top answers explicitly contrast 'barrier only' (paint) with 'barrier + sacrificial' (galvanising) and use 'more reactive' to justify why a scratch is still protected.
6Choose and justify a prevention method (Stretch)
Stretch• 4WSD0/1C style — extended• prevention, evaluation, spec-2.21
Question
An underground steel pipeline cannot easily be repainted once buried. Suggest a suitable rust-prevention method and explain why it is suitable. Give one limitation of your chosen method. (4 marks)
Step-by-step solution
1. Step 1
  Method (1). Use sacrificial protection — attach blocks of a more reactive metal such as magnesium to the pipe.
2. Step 2
  Why suitable (1). Magnesium is more reactive than iron, so it corrodes / is oxidised in preference to the iron...
3. Step 3
  Why suitable (1). ...and it keeps protecting the buried pipe without needing the surface to be re-coated, which would be impractical underground.
4. Step 4
  Limitation (1). The sacrificial metal is used up over time and must be replaced periodically.
Answer
Use sacrificial protection: attach blocks of magnesium (a more reactive metal) to the pipeline. Magnesium corrodes in preference to the iron, protecting the buried pipe without needing a surface coating that could not be reapplied underground. Limitation: the magnesium is consumed and must be replaced.
Examiner tip
'Suggest' questions need a method that FITS the context (you cannot easily re-paint buried pipes) plus a reactivity-based justification and a genuine limitation. Galvanising or sacrificial blocks both earn credit if justified.

Model Answers — Iron rusting and preventions

High-scoring sample answers for iron rusting and preventions on the Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1C style2 marks
State the two conditions needed for iron to rust. (2 marks)
Model answer
Water (1)

Oxygen (1)

Both must be present together for rusting to occur.
Why this scores
1 mark each. Naming only one, or writing 'air' instead of 'oxygen', loses a mark. This exact 2-mark question appears repeatedly.
Question 2
4WSD0/1C style3 marks
(a) Name the substance produced when iron rusts. (b) Name one barrier method used to prevent rusting. (c) State what a barrier method does. (3 marks)
Model answer
(a) Hydrated iron(III) oxide. (1)

(b) Any one of: painting / oiling (greasing) / coating with plastic / plating. (1)

(c) It forms a layer that stops water and oxygen reaching the iron. (1)
Why this scores
Part (c) must say what is kept out — water and oxygen. 'It protects the metal' is too vague for the mark.
Question 3
4WSD0/1C style4 marks
Describe an experiment using three test tubes to show that both water and oxygen are needed for iron to rust. Give the contents of each tube and the expected result. (4 marks)
Model answer
Set up three identical iron nails:

Tube A: iron nail in tap water with air present → the nail rusts (both water and oxygen present). (1)

Tube B: iron nail in boiled water (oxygen removed) with a layer of oil on top to keep air out → no rust (water but no oxygen). (1)

Tube C: iron nail in dry air with a drying agent (anhydrous calcium chloride) to remove water → no rust (oxygen but no water). (1)

Conclusion: rust forms only in Tube A, so both water and oxygen are needed for rusting. (1)
Why this scores
Marks for the correct conditions and result in each control tube, plus the conclusion. Make clear what is REMOVED in B and C and how (boiling/oil for oxygen, drying agent for water).
Question 4
4WSD0/1C style4 marks
Magnesium blocks are attached to a steel ship's hull to prevent rusting. Explain how this protects the steel. (4 marks)
Model answer
Magnesium is more reactive than iron (higher in the reactivity series). (1)

So magnesium loses electrons / is oxidised more readily than the iron. (1)

The magnesium corrodes (is sacrificed) instead of the iron, even where the steel is exposed to water and oxygen. (1)

The magnesium blocks are gradually used up and must be replaced periodically. (1)
Why this scores
The key marks are 'more reactive' and 'corrodes/oxidised in preference to the iron'. The replacement point shows full understanding of the limitation.
Question 5
4WSD0/1C style — extended6 marks
Compare how galvanising and painting protect iron from rusting. In your answer explain what happens to each coating when it is scratched. (6 marks)
Model answer
Painting.

Paint works as a barrier only — it forms a layer that stops water and oxygen reaching the iron. (1)

If the paint is scratched, water and oxygen reach the exposed iron and it rusts at the scratch. (1)

The paint provides no protection once the layer is broken. (1)

Galvanising (coating with zinc).

Galvanising works in two ways at once: as a barrier (the zinc layer keeps water and oxygen out)... (1)

...and by sacrificial protection, because zinc is more reactive than iron, so the zinc corrodes in preference to the iron. (1)

If the zinc is scratched, the surrounding zinc still corrodes instead of the exposed iron, so the iron is still protected — unlike paint. (1)
Why this scores
A 6-mark compare question rewards a clear barrier-vs-(barrier+sacrificial) contrast and a correct account of the scratch outcome for both. The discriminator is explaining the scratch case for galvanising using 'more reactive zinc corrodes first'.
Question 6
4WSD0/1C style — extended6 marks
Steel food cans are coated on the inside with tin, while steel buckets are coated with zinc (galvanised). Explain why a scratch in the tin coating is more damaging than a scratch in the zinc coating. (6 marks)
Model answer
Both coatings are barriers.

While unbroken, both tin and zinc stop water and oxygen reaching the iron, so the steel does not rust. (1)

Zinc (galvanised bucket) when scratched.

Zinc is more reactive than iron. (1)

So at a scratch the zinc corrodes in preference to the iron — the iron is still protected (sacrificial protection). (1)

Tin (food can) when scratched.

Tin is less reactive than iron. (1)

So tin offers no sacrificial protection — it is only a barrier. (1)

At a scratch the iron is exposed and rusts, and rusting can even be faster because the exposed iron corrodes in preference to the less-reactive tin. (1)
Why this scores
This is a high-discrimination question. The whole answer turns on the reactivity comparison: zinc (more reactive than iron) protects when scratched; tin (less reactive) does not. Tie every consequence back to which metal corrodes first.

Key Definitions and Keywords — Iron rusting and preventions

Definitions to memorise and the exact keywords mark schemes credit for iron rusting and preventions answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) sitting.

Rusting
Examiner keyword
The corrosion of iron (and steel) to form hydrated iron(III) oxide, which happens only when both water and oxygen are present.
Rust
Examiner keyword
Hydrated iron(III) oxide, Fe₂O₃·xH₂O — the flaky orange-brown solid formed when iron rusts.
Corrosion
The gradual destruction of a metal by reaction with substances in its surroundings (e.g. oxygen and water). Rusting is the corrosion of iron specifically.
Oxidation (in rusting)
Examiner keyword
Rusting is an oxidation reaction: the iron gains oxygen (and loses electrons) to form iron(III) oxide.
Barrier method
Examiner keyword
A method of rust prevention that places a physical layer (paint, oil, plastic or another metal) over the iron to stop water and oxygen reaching it.
Sacrificial protection
Examiner keyword
A method of rust prevention in which a more reactive metal (e.g. zinc or magnesium) is attached to the iron and corrodes in preference to it, protecting the iron.
Galvanising
Examiner keyword
Coating iron or steel with a layer of zinc. It protects in two ways: as a barrier and, because zinc is more reactive than iron, by sacrificial protection if the coat is scratched.
Drying agent (desiccant)
A substance such as anhydrous calcium chloride that absorbs water; used in the rusting experiment to keep the air in a tube dry.
Control (in an experiment)
A test set up to remove one variable (e.g. oxygen, or water) so its effect can be tested — as in tubes B and C of the rusting experiment.
Reactivity series
Examiner keyword
A list of metals in order of how readily they react / lose electrons. It explains why a more reactive metal (zinc, magnesium) corrodes in preference to iron in sacrificial protection.

Common Mistakes and Misconceptions — Iron rusting and preventions

The traps other students keep falling into on iron rusting and preventions questions — taken from recent Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) examiner reports and mark schemes — and how to avoid them.

✕Giving only one condition for rusting, or writing 'air' instead of 'oxygen'
Edexcel Chemistry examiner reports — recurring error
Why it happens
Students remember rusting needs 'air and water' and assume one word is enough, or use 'air' loosely.
How to avoid it
Always write both water AND oxygen. Use the word 'oxygen' (not 'air'), because it is the oxygen in air that is needed.
✕Naming rust as iron oxide or iron(II) oxide
Edexcel Chemistry examiner reports
Why it happens
Students forget it is the (III) oxide and that it is hydrated.
How to avoid it
Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O). Include 'hydrated' and '(III)' where the question asks for the name.
✕Explaining a barrier method as 'it protects the metal' without saying what is kept out
Edexcel Chemistry examiner reports
Why it happens
Students know the idea but do not state the mechanism.
How to avoid it
Say the layer stops water and oxygen reaching the iron. Naming both substances is what earns the explanation mark.
✕Saying sacrificial protection works without mentioning that the protecting metal is MORE reactive
Edexcel Chemistry examiner reports
Why it happens
Students learn 'zinc protects iron' as a fact but miss the reason.
How to avoid it
State that the metal (Zn/Mg) is more reactive than iron, so it corrodes / is oxidised in preference to the iron.
✕Thinking tin-plating protects iron the same way as galvanising
Edexcel Chemistry examiner reports
Why it happens
Both are metal coatings, so students assume they behave identically when scratched.
How to avoid it
Zinc is more reactive than iron (sacrificial — protects when scratched); tin is less reactive (barrier only — iron rusts, even faster, at a scratch).
✕Not explaining HOW oxygen or water is removed in the three-tube experiment
Edexcel Chemistry examiner reports
Why it happens
Students describe the tubes but skip why B has no oxygen and C has no water.
How to avoid it
State that boiling removes dissolved oxygen and oil seals out air (tube B) and that the drying agent removes water (tube C).

Past paper style quiz

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Iron Rusting and Prevention — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

Tube

Contents

Result

Why

iron nail + tap water + air

RUSTS

both water and oxygen present

iron nail + boiled water + oil layer on top

no rust

water present, but no oxygen (boiling drives out dissolved air; oil seals the surface)

iron nail + dry air + anhydrous calcium chloride (drying agent)

no rust

oxygen present, but no water (calcium chloride absorbs it)

Sacrificial protection.

What you do: attach a block of a more reactive metal — usually zinc or magnesium — to the iron object (in contact with it).
Why it works: the more reactive metal is oxidised in preference to the iron — it loses electrons more easily, so it corrodes away instead of the iron. The iron is protected even if its surface is exposed to water and oxygen.
Examples: zinc blocks bolted to ship hulls; magnesium blocks attached to underground steel pipelines and inside hot-water tanks.
Limitation: the sacrificial metal is used up and must be replaced from time to time.

Galvanising.

What you do: coat the iron or steel in a layer of zinc (often by dipping it in molten zinc).
Why it is the best general method — it works in TWO ways at once:
1. Barrier: the zinc layer keeps water and oxygen away from the iron.
2. Sacrificial: zinc is more reactive than iron, so even if the coat is scratched, the surrounding zinc corrodes instead of the exposed iron — the iron is still protected.
Examples: galvanised nails, corrugated roofing sheets, chain-link fences, dustbins, crash barriers.

Zinc is more reactive than iron, so it corrodes in preference to the iron — protecting it both as a sacrificial block and as a galvanised coating.

Iron rusting and preventions

Detailed Notes

What you’ll learn

How it’s examined

1Name rust and state the two conditions (Getting started)

2Explain how painting prevents rust (Getting started)

3Conclusion from the three test-tube experiment (Building confidence)

4Why sacrificial protection works (Building confidence)

5Why galvanising beats paint when scratched (Stretch)

6Choose and justify a prevention method (Stretch)

Rusting

Rust

Corrosion

Oxidation (in rusting)

Barrier method

Sacrificial protection

Galvanising

Drying agent (desiccant)

Control (in an experiment)

Reactivity series

✕Giving only one condition for rusting, or writing 'air' instead of 'oxygen'

✕Naming rust as iron oxide or iron(II) oxide

✕Explaining a barrier method as 'it protects the metal' without saying what is kept out

✕Saying sacrificial protection works without mentioning that the protecting metal is MORE reactive

✕Thinking tin-plating protects iron the same way as galvanising

✕Not explaining HOW oxygen or water is removed in the three-tube experiment

Past paper style quiz

Iron rusting and preventions

Detailed Notes

What you’ll learn

How it’s examined

1Name rust and state the two conditions (Getting started)

2Explain how painting prevents rust (Getting started)

3Conclusion from the three test-tube experiment (Building confidence)

4Why sacrificial protection works (Building confidence)

5Why galvanising beats paint when scratched (Stretch)

6Choose and justify a prevention method (Stretch)

Rusting

Rust

Corrosion

Oxidation (in rusting)

Barrier method

Sacrificial protection

Galvanising

Drying agent (desiccant)

Control (in an experiment)

Reactivity series

✕Giving only one condition for rusting, or writing 'air' instead of 'oxygen'

✕Naming rust as iron oxide or iron(II) oxide

✕Explaining a barrier method as 'it protects the metal' without saying what is kept out

✕Saying sacrificial protection works without mentioning that the protecting metal is MORE reactive

✕Thinking tin-plating protects iron the same way as galvanising

✕Not explaining HOW oxygen or water is removed in the three-tube experiment

Past paper style quiz