UPSC Civil Services (Mains)
Chemistry optional
- 2 papers
- 250 marks each
- 3 h per paper
- 5 of 8 questions, Q1 and Q5 compulsory
Paper I covers physical and inorganic chemistry.
Phase equilibria and solutions: every question, 2021 to 2025
8 parts worth 105 marks have been set on this unit in UPSC CSE 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.
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
- Q3(c) Phase diagram of butane and two compressions10 marks Answered
2024
2023
- Q3(b) Reading a one-component phase diagram20 marks Answered
2022
- Q3(d) Phase diagram of water; critical point and triple point10 marks Answered
- Q4(b) Mean molar heat of vaporisation of water from two vapour pressures10 marks Answered
2021
- Q1(d) Vapour pressure of water at 100 °C from its value at 95 °C10 marks Answered
- Q3(c)(i) Enthalpy of vaporisation from the triple point to the critical point10 marks Answered
- Q4(c) Phenol–water system and the importance of tie lines20 marks Answered
UPSC Civil Services (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Question choice and time plan for each paper
- Topic-wise weightage and a topic-wise index