Indian Forest Service (Mains)
Chemistry optional
- 2 papers
- 200 marks each
- 3 h per paper
Paper I covers physical and inorganic chemistry.
Phase equilibria and solutions: every question, 2021 to 2025
9 parts worth 86 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.
Phase equilibria in pure substances; Clausius-Clapeyron equation; phase diagram for a pure substance; phase equilibria in binary systems, partially miscible liquids: upper and lower critical solution temperatures; partial molar quantities, their significance and determination; excess thermodynamic functions and their determination.
Notes: Phase rule, phase diagrams, Clausius–Clapeyron equation, eutectics and partially miscible liquids (full notes) · short notes
Notes: Solutions: Raoult’s and Henry’s laws, colligative properties, van ’t Hoff factor and activity (full notes) · short notes
2025
- Q4(a)(i) Boiling point at reduced atmospheric pressure5 marks Question only
- Q4(a)(ii) Chemical potential of supercooled water5 marks Question only
- Q4(b)(i) Incongruent melting and azeotropy5 marks Question only
2024
- Q1(d) Components in ammonium chloride dissociation8 marks Answered
- Q4(a) Lead–silver eutectic and desilverisation15 marks Answered
2023
- Q4(b) Chemical potential versus temperature10 marks Answered
2022
- Q3(c) Sulfur phase diagram and metastability15 marks Question only
2021
- Q1(d) Desilverisation of lead8 marks Answered
- Q4(c) Clausius–Clapeyron equation15 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