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ICSE Class 10 Chemistry Question 34 of 64

Metallurgy — Question 34

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Question 5.1(2015)

Describe the role played in the extraction of aluminium by each of the substances listed.

  1. Cryolite
  2. Sodium hydroxide
  3. Graphite
Answer
  1. Addition of Cryolite :
    (i) lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
    (ii) enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
    (iii) enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.
  2. Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of NaOH under pressure for 2 hrs as a first step in obtaining Al2O3. The impurities present in bauxite mainly Fe2O3 and SiO2 remain unaffected with conc. NaOH as impurities are not amphoteric. Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.
  3. Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.

Chapter Overview: Metallurgy

Metallurgy is the process of extracting metals from their ores and refining them for use. Metals occur in nature as free elements (gold, silver) or combined in ores (oxides, sulphides, carbonates). The extraction process depends on the metal's reactivity: highly reactive metals (Na, Al) are extracted by electrolysis, moderately reactive metals (Fe, Zn) by reduction with carbon, and least reactive metals (Cu, Hg) by simple heating. The general steps include mining, crushing and concentration of ore, extraction (reduction), and refining. Students must understand the extraction of aluminium (electrolysis of alumina in cryolite), iron (blast furnace), and zinc (reduction of ZnO with carbon). The reactivity series determines the method of extraction: metals above carbon require electrolysis; metals below carbon can be reduced by carbon. The chapter also covers alloys (mixtures of metals or metals with non-metals), their composition and uses, and the concept of corrosion and its prevention.

Key Concepts & Reactions

Term / Process Details
OreMineral from which a metal can be profitably extracted
GangueEarthy impurities present in the ore
CalcinationHeating ore strongly in limited air to remove moisture and CO2
RoastingHeating ore in excess air to convert sulphides to oxides
SmeltingReduction of metal oxide using carbon or CO in a furnace
FluxSubstance added to remove gangue by forming fusible slag
SlagFusible product of flux + gangue (e.g., CaSiO3)
AlloyHomogeneous mixture of two or more metals (or metal + non-metal)

Must-Know Concepts

  • Blast furnace reactions for iron extraction: Fe2O3 + 3CO → 2Fe + 3CO2
  • Coke burns: C + O2 → CO2; then CO2 + C → 2CO (reducing agent)
  • Limestone acts as flux: CaCO3 → CaO + CO2; then CaO + SiO2 → CaSiO3 (slag)
  • Aluminium extraction: Al2O3 dissolved in cryolite (Na3AlF6), electrolysed at ~950°C
  • Common alloys: Brass (Cu + Zn), Bronze (Cu + Sn), Steel (Fe + C), Stainless Steel (Fe + Cr + Ni + C)
  • Corrosion prevention: painting, galvanising, electroplating, alloying

Calcination vs Roasting

Feature Calcination Roasting
Air SupplyLimited or no airExcess air
Ore TypeCarbonates and hydrated oresSulphide ores
Gas EvolvedCO2 and water vapourSO2
ExampleZnCO3 → ZnO + CO22ZnS + 3O2 → 2ZnO + 2SO2

Important Diagrams to Practice

  • Labelled diagram of the blast furnace with temperature zones and reactions
  • Electrolytic cell for aluminium extraction (Hall-Heroult process)
  • Flowchart of general metallurgical steps: ore → concentration → extraction → refining

Common Mistakes

  • Confusing calcination (limited air, carbonates) with roasting (excess air, sulphides)
  • Saying "coke reduces iron oxide" directly (CO is the actual reducing agent, not C)
  • Writing wrong alloy compositions (brass is Cu+Zn, NOT Cu+Sn which is bronze)
  • Forgetting the role of cryolite in aluminium extraction (lowers melting point)

Scoring Tips

  • Draw and label the blast furnace diagram neatly - this is a very common question
  • Write balanced equations for each zone of the blast furnace
  • Memorise alloy compositions and uses as a table for quick revision
  • Link extraction method to position in the reactivity series

Frequently Asked Questions

Why can't aluminium be extracted by carbon reduction?

Aluminium is above carbon in the reactivity series, so carbon cannot reduce Al2O3. Electrolysis of molten alumina (with cryolite) is required to extract aluminium.

What is the role of limestone in the blast furnace?

Limestone (CaCO3) decomposes to CaO, which acts as a flux. It combines with silica (SiO2, the gangue) to form calcium silicate slag (CaSiO3), which floats on molten iron and is removed.

Why are alloys preferred over pure metals?

Alloys are harder, stronger, more resistant to corrosion, and have better properties than pure metals. For example, stainless steel resists rusting unlike pure iron, and brass is harder than pure copper.