General Principles of Metallurgy and s-Block Elements
This domain tests the sequence of metallurgical operations from concentration to refining, and the trends, anomalies and important compounds of groups 1 and 2.
Concentration and reduction
Concentration by gravity, magnetic separation, froth flotation (sulphide ores with pine oil) or leaching (bauxite with NaOH, gold with NaCN). Roasting heats a sulphide ore in air; calcination heats a carbonate or hydrated ore with limited air. Reduction uses carbon (ZnO), CO, aluminium (thermite, Cr₂O₃), self-reduction (Cu₂S) or electrolysis (Na, Al). Refining may be electrolytic (Cu), by zone melting (Si, Ge), or by vapour phase decomposition - Mond's process for nickel via Ni(CO)₄, van Arkel's for titanium.
s-Block trends
Down group 1, size increases, ionisation enthalpy falls and reactivity rises. Lithium is anomalous: it forms Li₃N, its carbonate decomposes on heating, and its fluoride, carbonate and hydroxide are poorly soluble. Alkali metals give oxides (Li), peroxides (Na) and superoxides (K, Rb, Cs). In group 2, beryllium is covalent and amphoteric; BeCl₂ is polymeric while the rest are ionic. Sulphate solubility falls from Ca to Ba, hydroxide solubility rises.
Important compounds
NaOH by Castner–Kellner brine electrolysis; Na₂CO₃ by the Solvay process, where sparingly soluble NaHCO₃ precipitates first; plaster of Paris, CaSO₄·½H₂O, from gypsum at 393 K, setting back to gypsum with expansion.
Worked example
Explain why Li₂CO₃ decomposes on heating while Na₂CO₃ does not. The very small Li⁺ ion strongly polarises the large CO₃²⁻ ion, destabilising the carbonate and favouring Li₂O + CO₂ on heating. Na⁺ is much larger and its polarising power is too low to distort the carbonate appreciably, so Na₂CO₃ survives ordinary heating.
Traps that cost marks
- Swapping roasting (sulphide, air) and calcination (carbonate, limited air).
- Attributing Mond's process to copper refining instead of nickel.
- Writing Na₂CO₃ → Na₂O + CO₂ on gentle heating.
- Giving plaster of Paris as CaSO₄·2H₂O.