The blast furnace: explain each material's role
Connect coke, carbon monoxide and limestone with heat, iron reduction and slag formation using words.
Core and Extended · Chemistry 0620, 2026–2028. Detailed blast-furnace symbol equations and aluminium electrode reactions are taught in Extended lessons.
Before you begin
- Explain thermal decomposition and reduction as oxygen loss.
- Identify iron oxide, carbon dioxide and a basic oxide by name.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does limestone's thermal decomposition produce?
What you will learn
- Describe iron extraction from hematite through the five required stages.
- Distinguish the reducing role of carbon monoxide from limestone's impurity-removal role.
Make heat and the reducing gas
The blast furnace extracts iron from hematite, an iron(III) oxide ore, using coke and limestone with hot air. Coke burns in oxygen from the air to provide heat and form carbon dioxide. Carbon dioxide then reacts with further hot carbon to form carbon monoxide. Keep coke's heating role and the formation of the reducing gas as related but separate stages.
Carbon monoxide reduces iron(III) oxide to iron while becoming carbon dioxide. In the Core oxygen-transfer model, the iron oxide loses oxygen and carbon monoxide gains it. The iron is produced from the ore; it is not an ingredient supplied by limestone. Symbol equations for these steps are required in the Extended lesson, while the Core objective can be explained correctly through substance names and word reactions.
Limestone leads to a removable impurity product
Limestone is calcium carbonate. Heating decomposes it to calcium oxide and carbon dioxide. Calcium oxide reacts with acidic silicon dioxide impurity to form calcium silicate slag. This impurity-removal role is different from carbon monoxide reducing iron oxide: the calcium compound does not itself turn into iron or become the main reducing gas.
Molten slag and molten iron are separated in the furnace context, with slag lying above the denser iron in the stated school description. Keep the five roles in order: coke combustion, carbon-monoxide formation, iron-oxide reduction, limestone decomposition and slag formation. A furnace diagram can help organise inputs and outputs, but the explanation needs each chemical purpose rather than a list of three raw materials only.
Pause and explain
What is calcium oxide's stated role in the furnace?
Worked example
Explain why both coke and limestone are used, identifying which substance reduces hematite.
Show the worked solution
- Coke combustion provides heat and its subsequent chemistry produces carbon monoxide.
- Carbon monoxide removes oxygen from iron(III) oxide and is the stated reducing substance.
- Limestone produces calcium oxide, which reacts with silicon dioxide impurity to form slag for removal.
Answer Coke supplies heat and CO; CO reduces hematite; limestone leads to impurity-removing slag.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the three linked carbon stages from coke to iron reduction using words.
Give me a hint
Separate heat formation, CO formation and the ore reaction.
Compare my reasoning
- Carbon burns in oxygen, producing heat and carbon dioxide.
- Hot carbon reacts with carbon dioxide to form carbon monoxide.
- Carbon monoxide reduces iron(III) oxide to iron and becomes carbon dioxide.
Coke combustion → carbon monoxide formation → carbon monoxide reduction of hematite.
Look for these in your work
- All three stages are present.
- The reducing substance is identified.
Explain the limestone-to-slag route and distinguish it from iron reduction.
Give me a hint
Start with calcium carbonate and track its calcium-containing product.
Compare my reasoning
- Calcium carbonate decomposes to calcium oxide and carbon dioxide.
- Calcium oxide reacts with silicon dioxide impurity to form calcium silicate slag.
- Slag removes impurity; carbon monoxide performs the separate iron-oxide reduction.
Limestone gives CaO, leading to slag from silica; CO separately reduces hematite.
Look for these in your work
- The two limestone stages are linked.
- Impurity removal and reduction are distinguished.
A report says replacing all coke with limestone should keep the furnace chemistry unchanged because both are solids. Assess the roles without requiring symbol equations.
Give me a hint
Physical state does not make their chemical roles identical.
Compare my reasoning
- Coke is a carbon source for combustion heat and carbon-monoxide formation.
- Limestone decomposes to the oxide used in slag formation.
- Replacing one with the other removes the stated heating/reducing-gas role; shared solid state cannot justify identical chemistry.
The replacement does not preserve the stated route: coke and limestone have different chemical roles.
Look for these in your work
- Both roles are compared.
- Physical state is not used as a chemical equivalence rule.
Common mistakes
- Calling limestone the substance that reduces iron oxide to iron.
- Skipping the carbon-dioxide-to-carbon-monoxide stage.
- Saying calcium carbonate directly becomes iron.
- Listing inputs without explaining their roles.
What can you explain now?
Which substance reduces hematite in the required blast-furnace description?
Compare with the explanation
Carbon monoxide.
It removes oxygen from iron(III) oxide while becoming carbon dioxide.
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Make it stick
Explain all five furnace stages with substance names. Label heat, reducing gas and slag, and state which feed supports each purpose.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.