Metal extraction is one of those Combined Chemistry topics where the marks are very predictable. Examiners ask about the blast furnace almost every year in some form, and the mark schemes reward students who use precise words like "reduced", "reducing agent" and "impurities" rather than vague descriptions. If you can recite five equations and explain three roles, you can pick up almost every mark in this section.
Here is how our tutors, who sat these exact papers recently, break it down.
Start with the reactivity series, not the furnace
Before anything else, be clear on why iron can be extracted with carbon at all. The method of extraction depends on how reactive the metal is:
- Very reactive metals (potassium, sodium, calcium, magnesium, aluminium) are extracted by electrolysis of their molten compounds, because carbon is not reactive enough to displace them from their oxides.
- Metals of medium reactivity (zinc, iron, lead) are extracted by reduction with carbon or carbon monoxide.
- Unreactive metals (silver, gold) occur native and need little or no chemical treatment.
A very common question is "Explain why aluminium is not extracted using carbon." The mark-scheme answer is short: aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide. Do not write about cost or temperature as your main reason.
The four raw materials, and what each one does
Write these out until you can do it from memory:
- Haematite (iron(III) oxide, Fe₂O₃), the source of iron.
- Coke (carbon), which burns to produce heat and produces carbon monoxide, the reducing agent.
- Limestone (calcium carbonate, CaCO₃), which removes acidic impurities, mainly silicon dioxide.
- Hot air, which supplies oxygen for the combustion of coke.
Notice that coke has two jobs. Questions worth 2 marks about coke almost always want both: it burns exothermically to provide the high temperature needed, and it is the source of carbon monoxide which reduces the iron oxide.
The five equations you must be able to write
Balance them, and include state symbols if the question asks.
- Combustion of coke (exothermic, provides heat): C(s) + O₂(g) → CO₂(g)
- Formation of the reducing agent: CO₂(g) + C(s) → 2CO(g)
- Reduction of iron(III) oxide (the key step): Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)
- Thermal decomposition of limestone: CaCO₃(s) → CaO(s) + CO₂(g)
- Slag formation: CaO(s) + SiO₂(s) → CaSiO₃(l)
Equation 3 is the one students lose marks on. Remember the iron is produced as a liquid, because the furnace temperature is above the melting point of iron. Molten iron runs to the bottom and is tapped off, and molten slag floats on top of it because it is less dense.
Phrasing that earns the mark
Combined Chemistry mark schemes are strict about redox language. Use these sentence patterns:
- "Carbon monoxide is the reducing agent. It removes oxygen from iron(III) oxide, so the iron oxide is reduced and the carbon monoxide is oxidised to carbon dioxide."
- "Limestone decomposes to form calcium oxide, which is basic. This reacts with silicon dioxide, an acidic impurity, to form calcium silicate (slag)."
- "The slag is less dense than molten iron, so it floats on top and can be removed separately."
If a question asks you to identify what is oxidised and reduced, always name the substance and say what happened to the oxygen. "Fe₂O₃ is reduced because it loses oxygen" is a full answer. "There is a redox reaction" is not.
Common traps in past papers
- Writing carbon instead of carbon monoxide as the main reducing agent. Carbon does reduce some iron oxide directly at high temperature, but the expected answer for the main reduction is CO.
- Saying limestone "neutralises impurities" without naming silicon dioxide or calcium silicate. Name the substances.
- Forgetting the environmental angle. Be ready to state that carbon dioxide contributes to the greenhouse effect and global warming, and that sulfur dioxide from impurities causes acid rain.
- Ignoring recycling questions. Standard answers: recycling saves finite ore reserves, saves energy, reduces cost and reduces land used for mining and landfill.
- Mixing up steel questions. Cast iron from the furnace is brittle because of its high carbon content. Oxygen is blown through molten iron to remove excess carbon, and other metals are added to make alloys such as stainless steel.
A 10-minute revision drill
Cover your notes. On a blank page, sketch the furnace, label the four inputs and the two outputs (molten iron, slag, waste gases), then write the five equations underneath. Finally write one sentence each on the role of coke, limestone and hot air. If you can do all of that in ten minutes, you are exam ready on this topic.
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