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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2023 · Mains Paper I

Q1(a). Crystal structures and a point defect

Question

Match the following: (i) Frenkel defect; (ii) Perovskite; (iii) Spinel; (iv) Ilmenite; (v) Corundum. (A) Al₂O₃; (B) MgAl₂O₄; (C) FeTiO₃; (D) CaTiO₃; (E) AgCl.

Answer in brief

(i)-E; (ii)-D; (iii)-B; (iv)-C; (v)-A.

Key points

  • Frenkel defect: Ag⁺ can leave a lattice site and occupy an interstitial site in AgCl, producing a vacancy-interstitial pair.
  • Perovskite: CaTiO₃ is the prototype ABO₃ composition; actual CaTiO₃ is a distorted perovskite.
  • Spinel: MgAl₂O₄ has Mg²⁺ in tetrahedral and Al³⁺ in octahedral sites of the oxide framework.
  • Ilmenite: FeTiO₃ contains Fe²⁺ and Ti⁴⁺, satisfying charge neutrality with three O²⁻.
  • Corundum: α-Al₂O₃ has Al³⁺ occupying two-thirds of the octahedral holes in a close-packed oxide array.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(a), 2023 Mains Paper I: solution pages 1 and 2
Solution pages 1 and 2 of 26
Read the whole 2023 solution2023 question paper

Topic notes: defects, non-stoichiometry and semiconductors, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversFrenkel defectperovskite CaTiO3spinel MgAl2O4ilmenite FeTiO3corundum Al2O3

All parts of Question 1