4 high-yield, syllabus-aligned questions on Metals & corrosion, covering 12 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define3 marks
Define corrosion and state the two conditions necessary for the rusting of iron.
TopMarks model answer
Corrosion is the gradual destruction or eating away of a metal by chemical or electrochemical reaction with substances in its environment. The two conditions necessary for the rusting of iron are the presence of oxygen, usually from the air, and the presence of water or water vapour. Both must be present; iron does not rust in dry air, nor in water that has been boiled and sealed off from the air.
Suggested marking guide
1Definition of corrosion
2Oxygen or air required
3Water or moisture required
Why it scores
The word BOTH is the point of the question. Either condition alone leaves the iron untouched, which is why the classic experiment uses three test tubes — dry air, boiled water, and both — to prove it. Mention that both are needed together and the examiner can see you understand the experiment behind the fact.
A common incomplete answer
“Rusting needs air and water.”
What it costs: One mark of three. Both conditions are credited, but corrosion itself is never defined, and the question asked for a definition first.
Key ideas to include: corrosion, oxygen, moisture, electrochemical, rusting.
Explain4 marks
State four methods of preventing the rusting of iron and explain how each works.
TopMarks model answer
Painting or greasing the surface excludes air and moisture from the metal. Galvanising, that is coating the iron with zinc, excludes air and also gives sacrificial protection, since zinc is more electropositive and corrodes in place of the iron. Attaching blocks of a more reactive metal such as magnesium to the iron gives cathodic or sacrificial protection. Alloying the iron with chromium and nickel to make stainless steel produces a coherent oxide film that resists further attack.
Suggested marking guide
1Painting or greasing to exclude air and moisture
2Galvanising with zinc
3Sacrificial or cathodic protection
4Alloying to form stainless steel
Why it scores
Two of these methods work by a BARRIER and two by ELECTROCHEMISTRY, and the command word “explain” means you must say which. Galvanising is the interesting case because it does both — the zinc coating excludes air, and if the coating is scratched the zinc still corrodes preferentially.
A common incomplete answer
“By painting the iron and by galvanising it.”
What it costs: Three marks of four. Galvanising is credited, but painting is given without saying that it excludes air and moisture, and two further methods are missing.
Explain why aluminium, although high in the electrochemical series, does not corrode readily.
TopMarks model answer
Aluminium reacts rapidly with the oxygen of the air to form a thin, coherent and impervious layer of aluminium oxide over its surface. This oxide layer adheres firmly to the metal and seals it off from further contact with air and moisture, so corrosion stops.
Suggested marking guide
1An oxide layer forms on the surface
2The layer is coherent and protects the metal beneath
Why it scores
The second mark is the whole answer: iron also forms an oxide, but rust is flaky and porous, so air reaches the metal underneath and corrosion continues. Aluminium oxide is COHERENT and IMPERVIOUS. The contrast with rust is what the examiner is testing.
A common incomplete answer
“Because a layer of aluminium oxide forms on its surface.”
What it costs: One mark of two. The oxide layer is credited, but nothing says why it protects — iron forms an oxide too and corrodes away entirely.
Key ideas to include: oxide layer, coherent, impervious, adherent, passivation.
Explain3 marks
State three general physical properties of metals and explain each in terms of metallic bonding.
TopMarks model answer
Metals conduct heat and electricity because they contain delocalised electrons that are free to move through the lattice and carry charge and energy. Metals are malleable and ductile because the layers of positive ions can slide over one another without breaking the metallic bond, which acts equally in all directions. Metals have high melting points because the electrostatic attraction between the lattice of positive ions and the sea of delocalised electrons is strong and requires much energy to overcome.
Suggested marking guide
1Conductivity explained by delocalised electrons
2Malleability explained by layers sliding
3High melting point explained by strong attraction
Why it scores
Metallic bonding is non-directional — the electrons belong to the whole lattice rather than to any pair of atoms — and that single fact explains all three properties at once. Say it, and every mark follows. Listing the properties without the bonding earns nothing under an “explain” command.
A common incomplete answer
“Metals conduct electricity because they have free electrons.”
What it costs: Two marks of three. This one is correctly explained, but malleability and melting point are not attempted.
Key ideas to include: delocalised electrons, lattice of positive ions, non-directional bond, electrostatic attraction.
Practise Metals & corrosion on real questions
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