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.
The gaseous state and transport phenomenon: every question, 2021 to 2025
13 parts worth 115 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.
Equation of state for real gases, intermolecular interactions and critical phenomena and liquefaction of gases, Maxwell's distribution of speeds, intermolecular collisions, collisions on the wall and effusion; thermal conductivity and viscosity of ideal gases.
Notes: Kinetic theory of gases: Maxwell distribution, collisions, mean free path, effusion and transport (full notes) · short notes
Notes: Real gases: van der Waals equation, critical constants, Boyle temperature (full notes) · short notes
2025
- Q1(d) Heat conduction between two copper sheets through air5 marks Answered
- Q1(h) Sealed sample: ethane, neon or a mixture5 marks Answered
- Q3(a) Collisions of O₂ with the wall10 marks Answered
2024
- Q1(b)(i) Separation factor for ²³⁵UF₆ and ²³⁸UF₆ in one effusion stage5 marks Answered
- Q1(d)(i) Will CCl₂F₂ condense at 18 atm and 20 °C?5 marks Answered
- Q3(a)(i) Viscosity of air at 298 K and at 0 K10 marks Answered
- Q3(a)(ii) Boyle temperature of Ar, CH₄ and C₆H₆5 marks Answered
2023
- Q1(d) Root-mean-square speed of water molecules10 marks Answered
- Q4(a) van der Waals constant b, compression factor, interactions20 marks Answered
2022
- Q3(c) Speed distribution curves of argon and helium10 marks Answered
- Q4(a) The state of H₂ corresponding to N₂ at 126 K and 1 atm10 marks Answered
2021
- Q4(b)(i) Thermal conductivity of the gas in an ageing foam10 marks Answered
- Q4(b)(ii) van der Waals constants of NO and CCl₄; nearness to ideal behaviour10 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