Metallurgy ICSE Class 10: Extraction of Metals, Aluminium and Alloys

What Metallurgy is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Metallurgy is in the 7. Metallurgy part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Minerals, ores and gangue

• Mineral: a naturally occurring compound of a metal, with other elements, found in the earth's crust.
• Ore: a mineral from which the metal can be extracted profitably and conveniently. All ores are minerals, but not all minerals are ores (clay contains aluminium but is not used as an aluminium ore).
• Gangue (matrix): the earthy impurities (sand, rock, clay) mixed with an ore.

Metal	Ores (formula)
Iron	haematite Fe₂O₃, magnetite Fe₃O₄
Aluminium	bauxite Al₂O₃·2H₂O, cryolite Na₃AlF₆, corundum Al₂O₃
Zinc	zinc blende ZnS, calamine ZnCO₃

The chief ore of aluminium is bauxite; of zinc, zinc blende; of iron, haematite.

Worked example

Why is bauxite, and not clay, used to extract aluminium?

Try it first, then show the solutionSolution

1. Both contain aluminium compounds, so both are minerals.
2. Bauxite has a high content of alumina and the metal can be obtained from it economically.
3. Extraction from clay is too difficult and costly.

Answer: Bauxite is an ore (economic to process); clay is only a mineral.

Pulverisation and concentration (dressing) of the ore

The ore is first crushed and ground to a fine powder (pulverisation), then the gangue is removed (concentration or dressing). The method depends on a property that differs between ore and gangue:

• Hydrolytic method (gravity separation): the powdered ore is washed in a stream of water; the heavier ore particles settle, the lighter gangue is washed away. Used for heavy oxide ores such as haematite.
• Magnetic separation: the powdered ore falls on a belt moving over a magnetic roller; the magnetic ore is held and drops nearer the roller, the non-magnetic gangue falls further away. Used for magnetite (Fe₃O₄).
• Froth flotation: the powdered ore is mixed with water and pine oil and air is blown through. Sulphide ore particles are wetted by the oil and rise with the froth; gangue is wetted by water and sinks. Used for zinc blende (ZnS).

Worked example

Name the concentration method for (a) zinc blende, (b) magnetite, (c) haematite.

Try it first, then show the solutionSolution

1. (a) Sulphide ore: froth flotation.
2. (b) Magnetic ore: magnetic separation.
3. (c) Heavy oxide ore: hydrolytic (gravity) method.

Answer: (a) Froth flotation (b) Magnetic separation (c) Hydrolytic method

Roasting and calcination

The concentrated ore is changed to the oxide, which is easier to reduce.

Roasting: heating the ore strongly in excess air, below its melting point. Used for sulphide ores; sulphur dioxide is released.

$2ZnS+3O_2\to 2ZnO+2S{O}_2$

Calcination: heating the ore strongly in the absence (or limited supply) of air. Used for carbonate and hydrated ores; carbon dioxide or water is driven off.

$ZnC{O}_3\to ZnO+C{O}_2$

$A{l}_2{O}_3\cdot 2H_{2}O\to A{l}_2{O}_3+2H_{2}O$

Both also remove volatile impurities and make the ore porous.

Worked example

Name the process and write the equation to convert (a) calamine, (b) zinc blende into zinc oxide.

Try it first, then show the solutionSolution

1. (a) Carbonate ore, heated without air: calcination, $ZnC{O}_3\to ZnO+C{O}_2$.
2. (b) Sulphide ore, heated in excess air: roasting, $2ZnS+3O_2\to 2ZnO+2S{O}_2$.

Answer: (a) Calcination (b) Roasting; equations as above.

Reduction of oxides and the activity series

How a metal oxide is reduced depends on the metal's reactivity:

• Moderately reactive metals (Zn, Fe, Pb, Cu): their oxides are reduced by carbon (coke), carbon monoxide or hydrogen on heating.

$ZnO+C\to Zn+CO$
$F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$
$PbO+C\to Pb+CO$
$CuO+H_2\to Cu+H_{2}O$

• Highly reactive metals (K, Na, Ca, Mg, Al): they hold oxygen very strongly, so oxides like Al₂O₃ and MgO cannot be reduced by carbon, CO or hydrogen. These metals are extracted by electrolysis of their fused compounds (Na, K and Ca from their fused chlorides; Al from fused alumina).

Electro-refining (reference only): the impure metal is made the anode, a thin sheet of pure metal the cathode, in a solution of a salt of the metal; pure metal builds up on the cathode.

Worked example

Which of these oxides can be reduced by heating with hydrogen: MgO, CuO, Al₂O₃? Write the equation.

Try it first, then show the solutionSolution

1. Mg and Al are highly reactive: their oxides are not reduced by hydrogen.
2. Copper is low in the series: $CuO+H_2\to Cu+H_{2}O$.

Answer: Only CuO.

Baeyer's process: purifying bauxite

Bauxite contains iron(III) oxide and silica as impurities. Baeyer's process uses the amphoteric nature of alumina:

1. Powdered bauxite is heated with concentrated sodium hydroxide solution under pressure. Alumina dissolves as sodium aluminate; iron(III) oxide does not dissolve:
$A{l}_2{O}_3\cdot 2H_{2}O+2NaOH\to 2NaAl{O}_2+3H_{2}O$
2. The insoluble impurities are filtered off.
3. The sodium aluminate solution is diluted with water, cooled and seeded with a little freshly made aluminium hydroxide, so aluminium hydroxide precipitates:
$NaAl{O}_2+2H_{2}O\to NaOH+Al(OH{)}_3$
4. The precipitate is filtered, washed and heated strongly (ignited) to give pure alumina:
$2Al(OH{)}_3\to A{l}_2{O}_3+3H_{2}O$

Worked example

In Baeyer's process, why does iron(III) oxide get separated from alumina?

Try it first, then show the solutionSolution

1. Alumina is amphoteric and dissolves in NaOH as sodium aluminate.
2. Iron(III) oxide is basic and does not react with NaOH.
3. So it stays as a solid and is filtered off.

Answer: Only the amphoteric alumina dissolves in NaOH.

Hall-Heroult process: electrolytic extraction of aluminium

• Electrolyte: pure alumina (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆), with some fluorspar (CaF₂).
• Cryolite lowers the fusion temperature (alumina alone melts at about 2050 °C; the mixture works at about 950 °C) and increases the conductivity.
• Fluorspar further increases the fluidity (and conductivity) of the melt.
• Cathode: the carbon (graphite) lining of the steel tank.
• Anode: thick carbon (graphite) rods dipping into the electrolyte.
• A layer of powdered coke is spread on top of the electrolyte to reduce heat loss and to stop the anodes burning in air.
• Ions: $A{l}_2{O}_3\to 2A{l}^{3+}+3O^{2-}$
• Cathode: $A{l}^{3+}+3e^{-}\to Al$; molten aluminium collects at the bottom and is tapped off.
• Anode: $O^{2-}-2e^{-}\to O$; $O+O\to O_2$.
• The oxygen formed reacts with the hot carbon anodes ($C+O_2\to C{O}_2$, also some CO), so the anodes burn away and must be replaced periodically.

Worked example

Give the purpose of (a) cryolite, (b) fluorspar, (c) powdered coke in the Hall-Heroult process.

Try it first, then show the solutionSolution

1. (a) Cryolite lowers the fusion temperature of the electrolyte and makes it a better conductor.
2. (b) Fluorspar increases the fluidity of the electrolyte.
3. (c) Coke reduces heat loss from the surface and protects the anodes from burning in air.

Answer: As above.

Alloys and amalgams

An alloy is a homogeneous mixture of two or more metals (or a metal and a non-metal such as carbon), made by melting them together. An amalgam is an alloy in which one metal is mercury (for example, the silver-tin amalgam once used to fill teeth). Alloys are harder, stronger or more corrosion-resistant than the pure metals, or have a lower melting point.

Alloy	Main components (approximate)	Uses
Stainless steel	iron with chromium and nickel (little carbon)	cutlery, utensils, surgical instruments
Magnalium	aluminium with magnesium	light balance beams, light instruments, aircraft parts
Duralumin	aluminium with copper (some Mg, Mn)	aircraft bodies, pressure cookers
Brass	copper with zinc	electrical fittings, decorative articles, utensils
Bronze	copper with tin (sometimes zinc)	statues, medals, coins
Fuse metal / solder	lead with tin	electric fuses (low melting point); joining wires

The main metal is the one in the largest proportion: iron in stainless steel, aluminium in duralumin and magnalium, copper in brass and bronze.

Worked example

Why is solder suitable for joining electrical wires, and why is duralumin used in aircraft?

Try it first, then show the solutionSolution

1. Solder (lead + tin) has a low melting point, so it melts easily and sets to join the wires.
2. Duralumin is light (mostly aluminium) and much stronger than pure aluminium.

Answer: Low melting point; light and strong.

Chemistry in real life: Metallurgy

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.

Topics in this chapter

• Roasting, Calcination and Reduction
• Extraction of Aluminium
• Ores and Concentration of Ores

Metallurgy formula sheet

Roasting of zinc blende

$2ZnS+3O_2\to 2ZnO+2S{O}_2$

Excess air.

Calcination of calamine

$ZnC{O}_3\to ZnO+C{O}_2$

Absence of air.

Reduction of zinc oxide

$ZnO+C\to Zn+CO$

Coke, high temperature.

Reduction of iron(III) oxide

$F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$

Carbon monoxide as reducing agent.

Baeyer's process

$A{l}_2{O}_3\cdot 2H_{2}O+2NaOH\to 2NaAl{O}_2+3H_{2}O$

Then NaAlO₂ + 2H₂O → NaOH + Al(OH)₃; 2Al(OH)₃ → Al₂O₃ + 3H₂O.

Hall-Heroult cathode

$A{l}^{3+}+3e^{-}\to Al$

Carbon lining is the cathode.

Hall-Heroult anode

$2O^{2-}-4e^{-}\to O_2$

Anodes burn: C + O₂ → CO₂.

Metallurgy: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Metallurgy

The question patterns to expect, and the method that works for each.

Tricks to remember Metallurgy

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

• Roast the Sulphide, Calcine the Carbonate

  R-S (roast-sulphide, needs air to burn S away), C-C (calcine-carbonate, no air needed).

  Use it when: Roasting vs calcination.

• Froth for sulphides, Magnet for magnetite

  Froth floats sulphides (zinc blende); magnets pick magnetite; water washes away light gangue from heavy haematite.

  Use it when: Concentration methods.

• BRASS has a Z, BRONZE has a T

  Brass: copper + zinc. Bronze: copper + tin ('bronze medals' are tin-ted).

  Use it when: Alloy composition MCQs.

• Cryolite Cools, Fluorspar Flows, Coke Covers

  Cryolite lowers the temperature; fluorspar improves flow; coke covers the surface.

  Use it when: Purpose of each substance in Hall-Heroult.

Metallurgy: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Metallurgy

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

• ✗Mistake: Confusing bronze and brass

  CISCE examiners' comments, 2024: for an alloy of copper, zinc and tin, candidates confused bronze with brass, and a few chose duralumin.

  Wrong working

  Copper + tin = brass

  Right working

  Copper + tin (sometimes zinc) = bronze; copper + zinc = brass.

  ✓Do this: Brass has a Z (zinc); bronze has a T (tin).

  Usually costs: 1 mark.

• ✗Mistake: Not knowing the electrolyte of Hall-Heroult

  CISCE examiners' comments, 2024: candidates overlooked 'not' in the question and were unfamiliar with the formula of cryolite and the other constituents.

  Wrong working

  Cryolite is not part of the electrolyte.

  Right working

  Electrolyte: Al₂O₃ + Na₃AlF₆ (cryolite) + CaF₂ (fluorspar).

  ✓Do this: Learn formulae: Na₃AlF₆, CaF₂; circle 'NOT'.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up roasting and calcination

  CISCE examiners' comments, 2024 and 2025: candidates mixed up roasting and calcination in complete-the-sentence questions.

  Wrong working

  Zinc carbonate is roasted to zinc oxide.

  Right working

  Zinc carbonate is calcined (no air) to zinc oxide.

  ✓Do this: Roast the Sulphide, Calcine the Carbonate.

  Usually costs: 1 mark.

• ✗Mistake: Choosing a reactive metal oxide as reducible

  CISCE examiners' comments, 2024 and 2025: candidates chose Al₂O₃, sodium or aluminium for the oxide reducible by common reducing agents.

  Wrong working

  Al₂O₃ is reduced by carbon monoxide.

  Right working

  ZnO, Fe₂O₃, PbO and CuO are reduced by C/CO/H₂; Al₂O₃ and MgO are not.

  ✓Do this: Link reducibility to the activity series.

  Usually costs: 1 mark.

• ✗Mistake: Confusing Baeyer's with Hall-Heroult

  CISCE examiners' comments, 2025: in questions on concentrating bauxite, candidates described the Hall-Heroult process and wrote several unbalanced reactions.

  Wrong working

  Bauxite is concentrated by electrolysis with cryolite.

  Right working

  Bauxite is purified (concentrated) by Baeyer's process with NaOH; Hall-Heroult is the electrolysis of the purified alumina.

  ✓Do this: Baeyer = chemical purification; Hall-Heroult = electrolytic extraction.

  Usually costs: 1-2 marks.

• ✗Mistake: Listing all components instead of the main metal

  CISCE examiners' comments, 2024: asked for the main metal in duralumin and stainless steel, candidates listed every component.

  Wrong working

  Duralumin: Al, Cu, Mg, Mn

  Right working

  Main metal: aluminium

  ✓Do this: Read 'main metal' and give one metal.

  Usually costs: 1 mark.

Practice questions on Metallurgy

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

1. Q1 · 1 mark · MCQBoard level

   The chief ore of zinc is:
   (a) calamine
   (b) zinc blende
   (c) haematite
   (d) cryolite

   Hint

   A sulphide.

   Answer

   (b)

2. Q2 · 1 mark · MCQBoard level

   Heating zinc carbonate in the absence of air is called:
   (a) roasting
   (b) smelting
   (c) calcination
   (d) refining

   Hint

   C-C.

   Answer

   (c)

3. Q3 · 1 mark · MCQHarder board level

   Statement I: In the Hall-Heroult process, the anodes are replaced from time to time.
   Statement II: Aluminium is deposited at the anode.
   (a) Both true
   (b) Both false
   (c) I true, II false
   (d) I false, II true

   Hint

   Cations go to the cathode.

   Answer

   (c)

4. Q4 · 3 marks · Very short answerBoard level

   Fill in: (a) The gangue in froth flotation is wetted by ______. (b) Fluorspar ______ the fluidity of the electrolyte. (c) Fe₂O₃ is reduced by ______ in its extraction.

   Hint

   Water; increases; CO.

   Answer

   (a) water (b) increases (c) carbon monoxide

5. Q5 · 3 marks · Short answerHarder board level

   Write balanced equations for: (a) roasting of zinc blende; (b) reduction of lead(II) oxide by carbon; (c) reduction of copper(II) oxide by hydrogen.

   Hint

   Balance oxygen in (a).

   Answer

   (a) $2ZnS+3O_2\to 2ZnO+2S{O}_2$ (b) $PbO+C\to Pb+CO$ (c) $CuO+H_2\to Cu+H_{2}O$

6. Q6 · 3 marks · Short answerHarder board level

   Give reasons: (a) Pure alumina is used in the Hall-Heroult process. (b) The electrolytic cell is covered with powdered coke. (c) Aluminium is extracted by electrolysis, not by reduction with carbon.

   Hint

   Impurities; heat; affinity for oxygen.

   Answer

   (a) Impurities like iron would also be discharged and contaminate the aluminium. (b) To reduce heat loss and prevent the anodes burning in air. (c) Aluminium has a strong affinity for oxygen; carbon cannot reduce Al₂O₃.

7. Q7 · 4 marks · Long answerHOTS / competency

   An ore X is heated strongly in air and gives a gas that turns acidified potassium dichromate green; the residue is reduced by coke to a bluish-white metal. Identify X, the gas, the process, and write both equations.

   Hint

   Sulphide ore of zinc.

   Answer

   X = zinc blende (ZnS); gas SO₂; process roasting. $2ZnS+3O_2\to 2ZnO+2S{O}_2$; $ZnO+C\to Zn+CO$.

8. Q8 · 2 marks · Very short answerBoard level

   Name the alloy: (a) used for electric fuses, (b) containing copper and zinc, (c) used for surgical instruments, (d) aluminium and magnesium.

   Hint

   Half a mark each.

   Answer

   (a) Fuse metal (lead-tin) (b) Brass (c) Stainless steel (d) Magnalium

9. Q9 · 3 marks · Short answerHarder board level

   Explain why alumina dissolves in sodium hydroxide but iron(III) oxide does not, and how this is used in purifying bauxite.

   Hint

   Amphoteric.

   Answer

   Alumina is amphoteric and reacts with NaOH to form soluble sodium aluminate; iron(III) oxide is basic and does not react. The insoluble Fe₂O₃ is filtered off, and alumina is recovered from the solution (Baeyer's process).

10. Q10 · 4 marks · Long answerHOTS / competency

    Arrange the steps of extracting aluminium from bauxite in order, naming the process at each stage, and state where each electrode reaction occurs.

    Hint

    Pulverise, purify, electrolyse.

    Answer

    Pulverise bauxite → Baeyer's process (NaOH, filter, dilute and seed, ignite to pure Al₂O₃) → dissolve in molten cryolite with fluorspar → Hall-Heroult electrolysis: Al³⁺ + 3e⁻ → Al at the carbon-lined cathode; 2O²⁻ - 4e⁻ → O₂ at the carbon anodes.

11. Q11 · 2 marks · Short answerHarder board level

    Why is magnetite but not zinc blende concentrated by magnetic separation? Suggest the correct method for zinc blende.

    Hint

    Property used.

    Answer

    Magnetite is magnetic and the gangue is not, so a magnet separates them; zinc blende is not magnetic. Zinc blende (a sulphide) is concentrated by froth flotation.

12. Q12 · 2 marks · Short answerHarder board level

    Write the equations for the calcination of bauxite and the decomposition of aluminium hydroxide.

    Hint

    Water is driven off.

    Answer

    $A{l}_2{O}_3\cdot 2H_{2}O\to A{l}_2{O}_3+2H_{2}O$; $2Al(OH{)}_3\to A{l}_2{O}_3+3H_{2}O$

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. 1.Define an ore.Show answer

   A mineral from which a metal can be extracted profitably and conveniently.

2. 2.Formula of cryolite?Show answer

   Na₃AlF₆.

3. 3.Chief ore of aluminium?Show answer

   Bauxite, Al₂O₃·2H₂O.

4. 4.Ore concentrated by froth flotation?Show answer

   Zinc blende (sulphide ores).

5. 5.Define calcination.Show answer

   Heating an ore strongly in the absence of air to remove CO₂ or water.

6. 6.Two oxides not reduced by carbon?Show answer

   Al₂O₃ and MgO.

7. 7.Components of brass?Show answer

   Copper and zinc.

8. 8.Cathode in the Hall-Heroult cell?Show answer

   Carbon lining of the steel tank.

9. 9.What is an amalgam?Show answer

   An alloy containing mercury.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give two ores each of iron, aluminium and zinc with formulae?
• ☐Can you match each concentration method to an ore?
• ☐Can you write roasting and calcination equations for zinc ores?
• ☐Can you explain why Al₂O₃ is not reduced by coke?
• ☐Can you draw and label the Hall-Heroult cell and give the purpose of cryolite, fluorspar and coke?
• ☐Can you give the composition and a use of six alloys?

Metallurgy: frequently asked questions