Indian Forest Service (Mains)
Chemistry optional
- 2 papers
- 200 marks each
- 3 h per paper
Paper I covers physical and inorganic chemistry.
Solid state: every question, 2021 to 2025
14 parts worth 127 marks have been set on this unit in UPSC IFoS Chemistry Paper I from 2021 to 2025. Each one opens on its own page with the question, the answer in brief and the key points.
Forms of solids, law of constancy of interfacial angles, crystal systems and crystal classes (crystallographic groups). Designation of crystal faces, lattice structures and unit cell. Laws of rational indices. Bragg's law. X-ray diffraction by crystals. Close packing, radius ratio rules, calculation of some limiting radius ratio values. Structures of NaCl, ZnS, CsCl, CaF₂, CdI₂ and rutile. Imperfections in crystals, stoichiometric and nonstoichiometric defects, impurity defects, semi-conductors. Elementary study of liquid crystals.
Notes: Defects in solids, non-stoichiometry and semiconductors (full notes) · short notes
Notes: Crystal systems, Miller indices, Bragg's law and radius ratio (full notes) · short notes
Notes: Inorganic solids: perovskite, spinel, ReO₃, corundum and high-Tc superconductors (full notes) · short notes
Notes: Liquid state: surface tension, Kelvin equation, capillary action, viscosity and liquid crystals (full notes) · short notes
2025
- Q1(b) Formula from occupied crystal voids8 marks Answered
- Q3(a)(i) BCC edge length from diffraction5 marks Answered
- Q3(a)(ii) NaCl unit cell and Avogadro constant10 marks Answered
2024
- Q2(b) Equilibrium vacancy concentration15 marks Answered
- Q3(d) Cubic diffraction selection rules5 marks Question only
- Q8(d) Colour centres in sodium chloride10 marks Answered
2023
- Q1(c) Ions per unit cell of FeO8 marks Answered
- Q3(a) Ionic distances and radius-ratio predictions10 marks Question only
- Q3(b) Avogadro constant from diffraction10 marks Question only
2022
- Q1(c) MgO impurity in alumina8 marks Answered
- Q2(c)(i) Limiting radius ratio for trigonal coordination10 marks Answered
- Q2(c)(ii) Why zinc blende is face-centred cubic5 marks Answered
2021
- Q1(a) Frenkel disorder in silver bromide8 marks Answered
- Q2(c) Void fraction with one octahedral occupant15 marks Answered
UPSC Indian Forest Service (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Physical chemistry numericals worked with units; inorganic structures drawn
- Topic-wise weightage and a topic-wise index