ChemistryICSE Class 10 · Metallurgy

Roasting, Calcination and Reduction of Metal Oxides (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Roasting heats a sulphide ore strongly in excess air to form the oxide and sulphur dioxide; calcination heats a carbonate or hydrated ore in the absence or limited supply of air to form the oxide. The oxide is then reduced by carbon, carbon monoxide or hydrogen, or, for very reactive metals, by electrolysis.

Both roasting (excess air) and calcination (limited air) give the oxide; then reduce it with C, CO or H₂, or use electrolysis for reactive metals.

Illustration drawn for QuestionBuddy.

Understand Roasting, Calcination and Reduction

After an ore has been crushed and concentrated, it is usually changed into the oxide, because oxides are easier to reduce to the metal than sulphides or carbonates.

Roasting is heating the concentrated ore strongly, below its melting point, in excess of air. It is used for sulphide ores. Sulphur is removed as sulphur dioxide and the metal is left as its oxide; volatile impurities such as arsenic oxides are driven off too.

$2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$ (zinc blende roasted)

Calcination is heating the ore strongly in the absence of air or in a limited supply of air. It is used for carbonate and hydrated ores: carbon dioxide or water of crystallisation is driven off and the ore becomes porous.

$Z n C O_{3} Δ Z n O + C O_{2}$ (calamine calcined)

$A l_{2} O_{3} \cdot 2 H_{2} O Δ A l_{2} O_{3} + 2 H_{2} O$ (bauxite calcined)

Reduction of the oxide depends on how strongly the metal holds oxygen, which follows the activity series.

Metals in the middle and lower part of the series (zinc, iron, lead, copper). Their oxides are reduced on heating with carbon (coke), carbon monoxide or hydrogen:
$Z n O + C Δ Z n + C O$
$F e_{2} O_{3} + 3 C O Δ 2 F e + 3 C O_{2}$
$P b O + C O Δ P b + C O_{2}$
$C u O + H_{2} Δ C u + H_{2} O$

In each, the reducing agent takes oxygen and is itself oxidised.

Highly reactive metals (potassium, sodium, calcium, magnesium, aluminium). They have a very strong affinity for oxygen, so their oxides, such as Al₂O₃ and MgO, are not reduced by carbon, carbon monoxide or hydrogen. These metals are extracted by electrolysis of their fused (molten) compounds: sodium, potassium and calcium from their fused chlorides, aluminium from alumina dissolved in molten cryolite. The cathode supplies electrons, the strongest possible reducing agent. For example, $C a C l_{2} \to C a + C l_{2}$ on electrolysis of the fused chloride.

Aqueous solutions cannot be used for these metals: water's H⁺ ions would be discharged at the cathode instead, giving hydrogen.

The sequence of extraction: concentrate the ore → roast or calcine to the oxide → reduce the oxide → refine the metal (electrorefining, for reference only in this syllabus).

Roasting, Calcination and Reduction: formulas and key facts

Roasting of zinc blende: $2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$
Calcination of calamine: $Z n C O_{3} Δ Z n O + C O_{2}$
Calcination of bauxite: $A l_{2} O_{3} \cdot 2 H_{2} O Δ A l_{2} O_{3} + 2 H_{2} O$
Reduction by carbon and by carbon monoxide: $Z n O + C \to Z n + C O; F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$
Reduction by hydrogen: $C u O + H_{2} \to C u + H_{2} O$
Roasting versus calcination: Air excess in roasting; absent or limited in calcination
Ores sulphides are roasted; carbonates and hydrated ores are calcined
Gas given off sulphur dioxide in roasting; carbon dioxide or steam in calcination
Example zinc blende is roasted; calamine is calcined
How each oxide is reduced: Copper(II) oxide hydrogen, carbon or CO
Lead(II) oxide carbon or CO
Zinc oxide carbon (coke)
Iron(III) oxide carbon monoxide
Aluminium oxide and magnesium oxide not reduced; metals obtained by electrolysis of fused compounds

Air	excess in roasting; absent or limited in calcination
Ores	sulphides are roasted; carbonates and hydrated ores are calcined
Gas given off	sulphur dioxide in roasting; carbon dioxide or steam in calcination
Example	zinc blende is roasted; calamine is calcined

Copper(II) oxide	hydrogen, carbon or CO
Lead(II) oxide	carbon or CO
Zinc oxide	carbon (coke)
Iron(III) oxide	carbon monoxide
Aluminium oxide and magnesium oxide	not reduced; metals obtained by electrolysis of fused compounds

How to solve Roasting, Calcination and Reduction questions

Most questions ask you to name the step, give the condition and write a balanced equation. Decide first what kind of ore or oxide you have.

1Identify the ore type: sulphide (ends in -S, e.g. ZnS) means roasting; carbonate (CO₃) or hydrated (·xH₂O) means calcination.
2State the condition: roasting in excess air; calcination in absence or limited supply of air; both below the melting point.
3Write the equation and balance it (roasting of ZnS needs 3O₂ for 2ZnS).
4For reduction, place the metal in the activity series: zinc and below, use C, CO or H₂; aluminium and above, use electrolysis of the fused compound.
5For mass or volume questions, use the balanced equation with moles, as in any stoichiometry problem.

Roasting, Calcination and Reduction in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Sulphur dioxide from roasting

Suppose a zinc smelter roasts 97 kg of zinc blende (ZnS) in a day: $2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$ . (Zn = 65, S = 32, O = 16)

What mass and what volume at S.T.P. of sulphur dioxide is released, and why must it not be let into the air?

Try it first, then show the solutionSolution

Moles of ZnS $= 97 000/97 = 1000$ mol; ZnS : SO₂ = 1 : 1, so 1000 mol of SO₂.
Mass $= 1000 \times 64 = 64000$ g $= 64$ kg; volume $= 1000 \times 22.4 = 22400$ L $= 22.4$ m³ at S.T.P.
SO₂ is poisonous and forms acid rain, so smelters capture it (it can be used to make sulphuric acid).

Answer: 64 kg, about 22.4 m³ of SO₂ at S.T.P.; it must be trapped because it is toxic and causes acid rain.

Memory hook: Roast a sulphide, release SO₂: catch the gas, don't pollute.

Tricks to remember Roasting, Calcination and Reduction

Short memory aids. Say each one aloud once, then use it on a question the same day.

Roast in air, Calcine without care (of air)
Roasting = Rich supply of air; Calcination = Cut off the air.
Use it when: Not mixing up the two processes.
S for Sulphide, S for SO₂
Sulphide ores are roasted and give SO₂; Carbonate ores are Calcined and give CO₂.
Use it when: Matching ore type to process and gas.
Above zinc, use current
K, Na, Ca, Mg, Al: electrolysis. Zn, Fe, Pb, Cu: carbon, CO or hydrogen.
Use it when: Choosing the reduction method.

Roasting, Calcination and Reduction: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Gas from roasting · 1 mark · MCQBoard level

The gas given off when zinc blende is roasted is:
(a) carbon dioxide
(b) sulphur dioxide
(c) hydrogen sulphide
(d) oxygen

Try it first, then show the solutionSolution

Zinc blende is ZnS; roasting in excess air removes sulphur.
$2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$ .

Answer: (b) sulphur dioxide.

Example 2: Two differences · 2 marks · Short answerBoard level

State two differences between roasting and calcination.

Try it first, then show the solutionSolution

Roasting is done in excess air; calcination in the absence or limited supply of air.
Roasting is used for sulphide ores (gives SO₂); calcination for carbonate and hydrated ores (gives CO₂ or water).

Answer: Air supply (excess versus absent or limited) and type of ore and gas (sulphide, SO₂ versus carbonate or hydrate, CO₂ or H₂O).

Example 3: Choosing the reducing method · 3 marks · Short answerHarder board level

State how each oxide is converted to the metal, with a balanced equation where a chemical reducing agent is used: (a) iron(III) oxide, (b) zinc oxide, (c) aluminium oxide.

Try it first, then show the solutionSolution

(a) Heated with carbon monoxide: $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$ .
(b) Heated with coke: $Z n O + C \to Z n + C O$ .
(c) Not reduced by C, CO or H₂ because aluminium is highly reactive; aluminium is obtained by electrolysis of alumina dissolved in molten cryolite.

Answer: (a) CO reduction (b) carbon reduction (c) electrolysis of fused alumina.

Example 4: Why convert to the oxide? · 3 marks · Short answerHarder board level

Zinc blende is roasted before reduction instead of being heated directly with coke. (a) Why is the sulphide converted to the oxide? (b) Name one other substance removed during roasting. (c) Write the equations for roasting and for reduction of the product.

Try it first, then show the solutionSolution

(a) Carbon reduces metal oxides but not metal sulphides, so the sulphide must first become the oxide.
(b) Volatile impurities (for example arsenic as its oxide) and sulphur as SO₂.
(c) $2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$ ; $Z n O + C \to Z n + C O$ .

Answer: (a) Oxides are easily reduced by carbon; sulphides are not. (b) Volatile impurities. (c) Equations above.

Example 5: From calamine to zinc: masses · 4 marks · Long answerHOTS / competency

12.5 g of pure calamine (ZnCO₃) is calcined and the residue is completely reduced by coke. Calculate (a) the mass of zinc oxide formed, (b) the volume of CO₂ released at S.T.P., (c) the mass of zinc obtained. (Zn = 65, C = 12, O = 16)

Try it first, then show the solutionSolution

Molar mass of ZnCO₃ $= 65 + 12 + 48 = 125$ ; moles $= 12.5/125 = 0.1$ .
(a) $Z n C O_{3} \to Z n O + C O_{2}$ : 0.1 mol ZnO $= 0.1 \times 81 = 8.1$ g.
(b) 0.1 mol CO₂ $= 0.1 \times 22.4 = 2.24$ L.
(c) $Z n O + C \to Z n + C O$ : 0.1 mol Zn $= 6.5$ g.

Answer: (a) 8.1 g (b) 2.24 L (c) 6.5 g.

Example 6: Identifying the extraction route · 3 marks · Short answerHOTS / competency

The oxide of a metal M is not reduced when heated with coke, carbon monoxide or hydrogen. The metal is obtained by passing electricity through its molten chloride, MCl₂, and a greenish-yellow gas forms at the anode. (a) Where is M in the activity series? (b) Suggest an identity for M from the syllabus. (c) Why is the molten chloride used, not its aqueous solution?

Try it first, then show the solutionSolution

(a) Near the top: its oxide is too stable for chemical reducing agents.
(b) MCl₂ with a reactive Group 2 metal: calcium (or magnesium); $C a C l_{2} \to C a + C l_{2}$ on electrolysis.
(c) In water, H⁺ ions would be discharged at the cathode, giving hydrogen instead of the metal.

Answer: (a) High, very reactive. (b) Calcium (or magnesium). (c) Aqueous solution would give hydrogen at the cathode.

Where students lose marks in Roasting, Calcination and Reduction

The slips examiners see most often, and what to write instead.

Mistake: Mixing up roasting and calcination
Examiners reported this confusion in both 2024 (Q2(iii)) and 2025 (Q2(iii)).
Do this: Link the process to the ore: sulphide → roasting (air); carbonate or hydrate → calcination (no air).
Usually costs: 1 mark each time.
Mistake: Choosing Al₂O₃ as reducible by common reducing agents
In 2024 (Q2(iii)) and 2025 (Q1(iii)) candidates chose aluminium or sodium oxides as reducible by carbon or hydrogen.
Do this: Only oxides of metals from zinc downwards (Zn, Fe, Pb, Cu) are reduced by C, CO or H₂.
Usually costs: 1 mark.
Mistake: Unbalanced roasting equation
Students write ZnS + O₂ → ZnO + SO₂, which does not balance oxygen.
Wrong working
ZnS + O₂ → ZnO + SO₂
Right working
2ZnS + 3O₂ → 2ZnO + 2SO₂
Do this: Count oxygen atoms on both sides: 6 on each.
Usually costs: 1 mark.
Mistake: Forgetting the condition
Equations for roasting and calcination are written without 'excess air' or 'absence of air'.
Do this: Write the condition in words next to the equation; the syllabus asks for equations with conditions.
Usually costs: ½ to 1 mark.

Exam tips

When asked 'name the process', give one word (roasting, calcination, reduction, electrolysis) and then the equation if marks allow.
Use coke (carbon) for ZnO and PbO, carbon monoxide for Fe₂O₃, and hydrogen for CuO in your answers: these are standard examples.
Highlight 'not' in MCQ stems such as 'which oxide can NOT be reduced'.
Electrorefining is for reference only in Metallurgy; the full equations belong to Electrolysis.

Practice questions on Roasting, Calcination and Reduction

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of these oxides cannot be reduced to the metal by heating with hydrogen?
(a) CuO
(b) PbO
(c) MgO
(d) Fe₂O₃
Hint
Which metal is most reactive?
Answer
(c) MgO.
Q2 · 1 mark · Very short answerBoard level
Define calcination.
Hint
Mention the air supply and the type of ore.
Answer
Heating a carbonate or hydrated ore strongly, below its melting point, in the absence or limited supply of air, to convert it into the oxide.
Q3 · 2 marks · Short answerBoard level
Write a balanced equation for the reduction of iron(III) oxide by hydrogen and name the substance oxidised.
Hint
Hydrogen takes the oxygen.
Answer
$F e_{2} O_{3} + 3 H_{2} \to 2 F e + 3 H_{2} O$ ; hydrogen is oxidised (to water).
Q4 · 1 mark · Very short answerHarder board level
Why is roasting carried out below the melting point of the ore?
Hint
Air must reach the particles.
Answer
So that the ore stays solid and porous; air can then reach all the particles to oxidise them. A molten ore would expose only its surface.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Calcination is used to convert calamine into zinc oxide.
Reason (R): Carbonates decompose on heating, giving off carbon dioxide, without needing oxygen.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Is air needed to decompose ZnCO₃?
Answer
(a).
Q6 · 3 marks · Short answerHarder board level
9.7 g of zinc blende is roasted: $2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$ . Calculate the volume of oxygen used and of sulphur dioxide formed at S.T.P. (Zn = 65, S = 32)
Hint
Moles of ZnS first.
Answer
Moles of ZnS $= 9.7/97 = 0.1$ . O₂ $= 0.1 \times \frac{3}{2} = 0.15$ mol $= 3.36$ L. SO₂ $= 0.1$ mol $= 2.24$ L.
Q7 · 2 marks · Short answerHarder board level
Why are sodium and calcium obtained by electrolysis of their fused chlorides and not of their aqueous solutions?
Hint
Selective discharge at the cathode.
Answer
In aqueous solution H⁺ ions from water are discharged at the cathode in preference to Na⁺ or Ca²⁺, giving hydrogen, not the metal. In the fused chloride the metal ion is the only cation, so the metal is deposited.
Q8 · 3 marks · Short answerHOTS / competency
Suppose black copper(II) oxide and white magnesium oxide are heated separately in a stream of dry hydrogen. Describe what is seen in each tube and write the equation for any reaction.
Hint
Only one oxide is reduced.
Answer
Copper(II) oxide: the black solid turns reddish-brown and droplets of water collect on the cooler part of the tube; $C u O + H_{2} \to C u + H_{2} O$ . Magnesium oxide: no change, because magnesium is too reactive for hydrogen to remove its oxygen.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Gas released on calcining calamine?Show answer
Carbon dioxide.
2.Which process suits zinc blende?Show answer
Roasting.
3.Name two oxides that carbon cannot reduce.Show answer
Al₂O₃ and MgO.
4.How are sodium and potassium extracted?Show answer
By electrolysis of their fused chlorides.
5.Reducing agent for Fe₂O₃ in the equation Fe₂O₃ + 3CO → 2Fe + 3CO₂?Show answer
Carbon monoxide.

Roasting, Calcination and Reduction: frequently asked questions

What is the main difference between roasting and calcination?

Roasting heats a sulphide ore in excess air and gives off SO₂; calcination heats a carbonate or hydrated ore in the absence or limited supply of air and gives off CO₂ or water.

Why can carbon reduce zinc oxide but not aluminium oxide?

Aluminium is much higher in the activity series and holds oxygen more strongly than carbon can remove it. Zinc holds oxygen less strongly, so hot carbon takes the oxygen away.

Why is a sulphide ore converted to an oxide before reduction?

Carbon and carbon monoxide readily reduce oxides but not sulphides, and roasting also removes sulphur and volatile impurities.

Which metals are extracted by electrolysis?

Highly reactive metals such as sodium, potassium and calcium (from fused chlorides) and aluminium (from alumina in molten cryolite).