AQA GCSE Chemistry Foundation (8462), Foundation tier · Using resources › Using materials
Practise Corrosion and its prevention. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What corrosion is, the experiment that shows iron needs both air and water to rust, and how barrier methods and sacrificial protection prevent corrosion. This is in GCSE Chemistry only, not in Combined Science. Expect practical-based questions and 'explain' questions using the reactivity series on Paper 2.
Key facts
Corrosion = destruction of materials by chemical reactions with substances in the environment
Rusting needs air (oxygen) AND water
iron + oxygen + water → hydrated iron(III) oxide (rust)
Boiling removes dissolved air; an oil layer stops air re-dissolving; anhydrous calcium chloride dries the air
Aluminium is protected by a layer of aluminium oxide
Galvanising = coating iron with zinc (sacrificial protection)
Notes
Corrosion and rusting
Corrosion is the destruction of materials by chemical reactions with substances in the environment.
Rusting is the corrosion of iron and steel. Iron needs both air (oxygen) and water to rust.
iron + oxygen + water → hydrated iron(III) oxide (rust)
Experiment: what does iron need to rust?
diagram
Tube 1: iron nail in tap water, open to the air. Result: rusts.
Tube 2: nail in water that has been boiled (to remove dissolved air), with a layer of oil on top (to stop air dissolving back in). Result: no rust.
Tube 3: nail in air dried by anhydrous calcium chloride (a drying agent that absorbs water), sealed with a bung. Result: no rust.
Iron rusts only when both air (oxygen) and water are present.
Conclusion: iron rusts only when both air and water are present.
Preventing corrosion
Barrier methods stop air and water reaching the metal: greasing or oiling, painting, coating with plastic, and electroplating (using electrolysis to coat the object with a thin layer of another metal).
Aluminium is more reactive than iron but does not corrode away: a layer of aluminium oxide forms on its surface and protects the metal from further corrosion.
Sacrificial protection: iron is coated with, or connected to, a more reactive metal. Galvanising coats iron with zinc; blocks of zinc or magnesium are attached to ships' hulls and underground pipes.
How to answer each type of question
Interpret the rusting experiment
3 to 4 marksGrade 5
Say which tube rusted and what each of the other tubes lacked.
Give the purpose of each part: boiling removes air, oil stops air dissolving, calcium chloride removes water vapour.
Conclude that both air and water are needed.
Example. A student set up three test tubes, each containing an iron nail. Tube A: nail in tap water, no bung. Tube B: nail in boiled water with a layer of oil on top. Tube C: nail with anhydrous calcium chloride, sealed with a bung. After one week only the nail in tube A had rusted. (a) Why was the water in tube B boiled? (1 mark) (b) What is the purpose of the anhydrous calcium chloride in tube C? (1 mark) (c) What do the results show? (2 marks)
Show the model answerHide the model answer
(a) To remove dissolved air (oxygen) from the water (1) (b) To absorb water / to dry the air in the tube (1) (c) Iron needs air (oxygen) to rust, shown by tube B, and water to rust, shown by tube C (1); both must be present for rusting (1).
Don’t lose marks
Writing that iron rusts with air or water. It needs both.
Saying the water is boiled to kill bacteria. It is boiled to remove dissolved air.
Saying zinc protects iron because it is 'stronger' or 'less reactive'. The reason is that zinc is MORE reactive.
Using 'rust' for other metals. Only iron and steel rust; other metals corrode.
Saying aluminium does not corrode because it is unreactive. It is fairly reactive, but its oxide layer protects it.
More tips
Memory tricks
Rust recipe: iron + air + water. Take away either air or water and there is no rust.
Sacrificial = the more reactive metal 'sacrifices' itself to save the iron.
Zinc and magnesium are above iron in the reactivity series (sacrificial); tin is below it (barrier only).
The oil layer in tube 2 works like a lid that keeps air out of the water.
Exam technique
In sacrificial protection answers, use the words 'more reactive' and 'loses electrons' or 'is oxidised instead of iron'.
For the three-tube experiment, give the purpose of each condition, not just the result.
For 'suggest a method' questions, match the method to how the object is used, e.g. moving parts, underground, needs to look shiny.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define corrosion and give an exampleCorrosion is the destruction of materials by chemical reactions with substances in the environment; rusting is an example.
Grade 4
State the conditions needed for rustingIron needs both air (oxygen) and water to rust.
Grade 4
Name barrier methods of preventing corrosionGreasing, painting and electroplating keep air and water away from the metal.
Grade 5
Describe the rusting experiment with three tubesNails in water open to air, in boiled water under oil, and in dry air; only the first rusts.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the brown / orange compound formed when iron rusts.
hydrated iron(III) oxide
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is corrosion?[1]
(b) Name the two substances that are needed for iron to rust.[2]
(c) Which method of preventing rusting provides sacrificial protection? Tick (✓) one box.[1]
Greasing
Painting
Galvanising with zinc
Plating with tin
Show the answer and mark scheme
(a)
the destruction of materials by chemical reactions with substances in the environment
(b)
air / oxygen
water
(c)Answer: Galvanising with zinc
Question 2Medium5 marks
A student set up three test tubes, each containing an iron nail.
Tube A: the nail in tap water, open to the air.
Tube B: the nail in water that had been boiled and cooled, with a layer of oil on top.
Tube C: the nail in air with anhydrous calcium chloride, with a stopper in the tube.
After one week, only the nail in tube A had rusted.
(a) Why was the water in tube B boiled?[1]
(b) Why was a layer of oil put on top of the water in tube B?[1]
(c) What was the purpose of the anhydrous calcium chloride in tube C?[1]
(d) What do the results show about rusting? Explain your answer.[2]
Show the answer and mark scheme
(a)
to remove dissolved air / oxygen
(b)
to stop air / oxygen dissolving in the water again
(c)
to absorb water (vapour) / to keep the air dry
(d)
both air / oxygen and water are needed for rusting
the nail with both air and water (A) rusted, but the nail without air (B) and the nail without water (C) did not