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.
All years
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Paper I, physical and inorganic chemistry
Question paper, 2026
UPSC Civil Services (Mains), Chemistry optional. 12 pages, as issued by UPSC.
Every question of 2026, one page each53 parts in 8 questions, with the question as text and its full model answer
Question 1 50 marks
- 1(a)Attraction and repulsion parameters of the van der Waals equation5 marks
- 1(b)Liquid of higher surface tension: five properties5 marks
- 1(c)Two metal blocks in thermal contact5 marks
- 1(d)Kₚ and Δ G^° for N₂O₄ ⇌ 2NO₂ at 300 K5 marks
- 1(e)Components and phases when NH₄Cl is heated5 marks
- 1(f)Concentration cell of two silver electrodes5 marks
- 1(g)Why the cell potential is measured at zero current5 marks
- 1(h)Activation energy of food spoilage5 marks
- 1(i)Colour of a red flower in green light5 marks
- 1(j)Why low temperature favours adsorption of a gas5 marks
Question 2 50 marks
- 2(a)Maxwell distribution of speeds: effect of T and of M15 marks
- 2(b)(i)Interfacial tension between two immiscible liquids5 marks
- 2(b)(ii)Pressure difference across an ethanol droplet of radius 220 nm10 marks
- 2(c)(i)Enthalpy of formation of HI(g) from the gaseous elements at 225°C10 marks
- 2(c)(ii)Δ G^° of ammonia formation at 400 K10 marks
Question 3 50 marks
- 3(a)(i)Chemical potential from U, H, A and G10 marks
- 3(a)(ii)Cd–Bi simple eutectic: phase diagram and cooling curves10 marks
- 3(a)(iii)How an acetone–dry ice bath works5 marks
- 3(b)Stokes radius and diffusion coefficient of Zn²⁺10 marks
- 3(c)(i)Energy of one einstein of 800 nm radiation5 marks
- 3(c)(ii)Photodecomposition of HI: rate, quantum yield and order10 marks
Question 4 50 marks
Question 5 50 marks
- 5(a)de Broglie wavelength of a baseball5 marks
- 5(b)cis- and trans-dichloroethylene from dipole moments5 marks
- 5(c)Resonance structures of the nitrate ion5 marks
- 5(d)Spacing of the (111) planes of a simple cubic lattice5 marks
- 5(e)Ferredoxins and their classification5 marks
- 5(f)Chelate effect and chelate ring size5 marks
- 5(g)Ferrocene, the sandwich compound5 marks
- 5(h)Ionisation isomerism, and three pairs of isomers5 marks
- 5(i)Borazine, the “inorganic benzene”5 marks
- 5(j)Actinide contraction5 marks
Question 6 50 marks
- 6(a)(i)Lewis structures and VSEPR shapes of SO₂, I₃⁻, SF₄, BrF₅, XeF₄10 marks
- 6(a)(ii)MO diagram of F₂; bond order and magnetism10 marks
- 6(b)(i)Pale pink [Co(H₂O)₆]²+ and deep blue [CoCl₄]²-10 marks
- 6(b)(ii)EAN rule; EAN of four complexes10 marks
- 6(c)Quadruple bond and bond order 4 in [Re₂Cl₈]²-10 marks
Question 7 50 marks
- 7(a)(i)e^(α x) as an eigenfunction of dⁿ/dxⁿ5 marks
- 7(a)(ii)Why n=0 is not allowed for a particle in a box5 marks
- 7(b)(i)Uncertainty in momentum and speed of an electron, Δ x≈50 pm10 marks
- 7(b)(ii)Quantum numbers of the 2p_z orbital5 marks
- 7(c)(i)Packing fractions of SC, BCC and FCC20 marks
- 7(c)(ii)Bragg condition for constructive interference5 marks
Question 8 50 marks
- 8(a)T and R forms of haemoglobin10 marks
- 8(b)(i)Structure and bonding of diborane10 marks
- 8(b)(ii)Classification of silicates; SiO₄⁴⁻, Si₂O₇⁶⁻ and Si₃O₉⁶⁻10 marks
- 8(c)(i)Magnetic moments of Sm³⁺ and Eu³⁺ at 300 K5 marks
- 8(c)(ii)Metallic radii from La to Lu, and the exceptions Eu and Yb5 marks
- 8(c)(iii)Separation of the lanthanides by complex formation (ion exchange)5 marks
- 8(c)(iv)Why Ce³⁺ and Yb³⁺ are colourless but absorb in the UV5 marks