UPSC Civil Services (Main) Examination 2026 — Chemistry Optional Paper I Analysis, Trends & Comparison | OurEducation

Last Updated: Sep 8, 2026

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An analysis of Chemistry Optional Paper I of UPSC Civil Services (Main) Examination 2026 (held 2026-08-30) — difficulty, topic spread, and how it compares with previous years.

Official paper: official source.

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UPSC CSE (Main) 2026 – Chemistry Optional Paper I: Analysis

1. Overall Difficulty & Balance

The 2026 Chemistry Optional Paper I maintained a rigorous yet balanced standard, with a slight uptick in conceptual depth compared to recent years. The paper tested a mix of classical physical chemistry, advanced physical chemistry, and inorganic chemistry, with a notable emphasis on thermodynamics, kinetics, and quantum mechanics. While the questions were straightforward in formulation, their execution demanded precise understanding and multi-step reasoning. The balance between numerical problems, derivations, and conceptual explanations was well-maintained, though the thermodynamics-heavy section (Q1c) and quantum mechanics (Q6a) posed significant challenges.

There was a subtle shift toward more applied and interdisciplinary questions—especially in surface chemistry (Q1b) and biochemistry-related inorganic chemistry (Q7a, Q7c)—reflecting a broader trend in UPSC’s evolving expectations toward contextual understanding.

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2. Section & Topic Distribution

Physical Chemistry (≈45% weightage):

• Thermodynamics & Statistical Mechanics: Dominated by questions on Maxwell-Boltzmann distribution (Q1a), enthalpy calculations at different temperatures (Q1c), and phase equilibrium (Q2a).
• Kinetics & Photochemistry: Included complex reaction mechanisms (Q3c), diffusion and mobility of ions (Q2b), and photochemical decomposition (Q3c).
• Quantum Chemistry & Spectroscopy: Featured Schrödinger equation applications (Q6a), uncertainty principle (Q6b), and molecular orbital theory (Q5a).
• Surface & Colloid Chemistry: Interfacial tension (Q1b), polarography (Q3a), and diffusion coefficients (Q2b) were tested.

Inorganic Chemistry (≈35% weightage):

• Coordination Chemistry: Resonance structures (Q4c), isomerism (Q4d), EAN rule (Q5b), and MO diagrams (Q5a) were central.
• Solid State & Crystal Chemistry: Lattice types and packing efficiency (Q6c), Bragg’s law (Q6c), and silicates (Q7b) were included.
• Descriptive Inorganic: Ferredoxin (Q4e), actinide contraction (Q4e), and lanthanide separation (Q7c) tested advanced knowledge.

Interdisciplinary & Applied (≈20% weightage):

• Biological Inorganic: Hemoglobin conformations (Q7a) and magnetic behavior of lanthanides (Q7c).
• Structural Chemistry: VSEPR theory (Q5a), boranes (Q7b), and sandwich compounds (Q4e).

Observation: Physical chemistry retained its dominance, but inorganic chemistry saw a rise in conceptual and structural questions, moving beyond classical descriptive topics.

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3. Comparison with Last Year & Multi-Year Trend

• Consistency in Core Areas: Thermodynamics, kinetics, quantum mechanics, and coordination chemistry remain perennial favorites. Questions on Maxwell-Boltzmann distribution, phase diagrams, and Schrödinger equation have appeared in similar forms over the past 3–4 years.
• Shift Toward Application: While classical derivations (e.g., chemical potential in Q2a) remain, newer questions test real-world applications—e.g., ethanol droplet pressure (Q1b), diffusion in biological systems (Q2b), and hemoglobin states (Q7a).
• Increased Emphasis on Spectroscopy & MO Theory: The inclusion of MO diagrams for F₂ (Q5a) and electronic spectra of cobalt complexes (Q5b) reflects a growing focus on linking theory to observable phenomena.
• Reduction in Pure Descriptive Questions: Earlier papers often asked for definitions (e.g., “What is ferredoxin?”), but now such topics are embedded in analytical contexts (e.g., “Explain its structure and classification” in Q4e).

Notable Repeats: Phase diagrams (Q2a), isomerism in coordination compounds (Q4d), and silicates (Q7b) have appeared in multiple years, underscoring their importance.

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4. Notable & Unexpected Questions

• Maxwell-Boltzmann Plot Analysis (Q1a): Unusual in its request for comparative plots under varying conditions (temperature and molar mass). Tests deep understanding of the distribution function’s dependence on parameters—a concept often glossed over in coaching notes.
• Relaxation Kinetics (Q3b): A sophisticated application of chemical relaxation methods, rarely seen in UPSC. Requires knowledge of T-jump techniques and rate constant derivation from relaxation time—typically graduate-level material.
• De Broglie Wavelength of a Baseball (Q4a): A playful yet insightful question testing wave-particle duality at macroscopic scales. Challenges aspirants to reconcile quantum concepts with everyday objects—a trend toward interdisciplinary thinking.
• Lanthanide Magnetic Anomalies (Q7c): Focused on deviations from expected magnetic moments in Eu³⁺ and Sm³⁺, and their separation via complexation. Tests advanced inorganic chemistry beyond standard syllabus coverage.

Why They Matter: These questions signal UPSC’s intent to move beyond rote learning. They reward conceptual clarity, interdisciplinary connections, and the ability to apply theory to novel contexts—skills critical for higher-order civil service roles.

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5. Key Takeaways for Aspirants

• Master the Fundamentals, Then Go Deeper: While classical topics (e.g., thermodynamics, phase rule) are essential, aspirants must be prepared to derive expressions, interpret plots, and justify trends—not just state facts.
• Practice Multi-Step Problems: Many questions (e.g., Q1c, Q3b) require chaining multiple concepts—enthalpy calculations across temperatures, rate constant derivation from relaxation data. Build a habit of structured problem-solving.
• Develop Visual & Analytical Skills: Questions like plotting Maxwell-Boltzmann curves (Q1a) or sketching phase diagrams (Q2a) demand spatial and analytical reasoning. Use graph paper and sketch regularly during revision.
• Interlink Topics: Expect questions that bridge physical and inorganic chemistry (e.g., magnetic properties of lanthanides in Q7c) or quantum mechanics and spectroscopy (e.g., MO diagrams in Q5a). Build mind maps connecting these domains.
• Stay Updated on Interdisciplinary Trends: Topics like hemoglobin conformations (Q7a), diffusion in biological systems (Q2b), and photochemical reactions (Q3c) reflect UPSC’s interest in applied science. Supplement standard texts with journals or advanced modules.
• Focus on Derivations & Justifications: Many questions (e.g., chemical potential expressions in Q2a, Schrödinger equation solutions in Q6a) require derivations with clear justifications. Practice writing these under timed conditions.

Final Note: The 2026 paper reinforces that Chemistry Optional rewards depth, clarity, and the ability to think critically—not just memory. Aspirants should prioritize understanding over cramming, and cultivate a habit of connecting concepts across sub-disciplines.

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Questions asked

1. (a) $$dN_c = 4\pi N \left( \frac{M}{2\pi RT} \right)^{3/2} c^2 e^{-Mc^2/2RT} dc$$ The Maxwell distribution of molecular speed of a gaseous system is $$dN_c = 4\pi N \left( \frac{M}{2\pi RT} \right)^{3/2} c^2 e^{-Mc^2/2RT} dc$$ (Symbols have their usual meanings) Plot $$\left( \frac{1}{N} \times \frac{dN_c}{dc} \right)$$ against c— (i) at two different temperatures $T_1$ and $T_2$; $T_2 > T_1$; (ii) at two molar masses $M_1$ and $M_2$; $M_2 > M_1$. Justify your plots. 8+7=15 (b) Two immiscible liquids A and B have surface tension values $\gamma_A$ and $\gamma_B$, respectively. When both the liquids are taken in a container, then what would be the interfacial tension between the two liquid phases? (Given : $\gamma_A > \gamma_B$) Calculate the pressure differential across the surface of ethanol spherical droplet of radius 220 nm at 20 °C. The surface tension of ethanol at that temperature is 22.39 mN m$^{-1}$. [ P.T.O. (c) $$H_2(g); C_{p,m} = 29.08 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$I_2(g); C_{p,m} = 33.56 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$HI(g); C_{p,m} = 29.87 \text{ J K}^{-1} \text{ mol}^{-1}$$ At 25 °C, the latent heat of sublimation per mole of iodine is 62.3 kJ mol⁻¹ and the standard enthalpy of formation of HI(g) is 24.7 kJ mol⁻¹. Calculate the enthalpy change which occurs when HI(g) is formed from the gaseous elements at 225 °C. The mean molar heat capacities over the temperature range 25 °C to 225 °C are : $$H_2(g); C_{p,m} = 29.08 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$I_2(g); C_{p,m} = 33.56 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$HI(g); C_{p,m} = 29.87 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$\frac{1}{2}N_2(g) + \frac{3}{2}H_2(g) \rightarrow NH_3(g)$$ $$S^\circ_{NH_3} = 192.45 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$S^\circ_{N_2} = 191.61 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$S^\circ_{H_2} = 130.68 \text{ J K}^{-1} \text{ mol}^{-1})$$ Calculate ΔG° at 400 K temperature for the following reaction : $$\frac{1}{2}N_2(g) + \frac{3}{2}H_2(g) \rightarrow NH_3(g)$$ Assume that ΔS° is independent of temperature. (Given : ΔG°₂₉₈K = -16.496 kJ mol⁻¹ $$S^\circ_{NH_3} = 192.45 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$S^\circ_{N_2} = 191.61 \text{ J K}^{-1} \text{ mol}^{-1}$$ $$S^\circ_{H_2} = 130.68 \text{ J K}^{-1} \text{ mol}^{-1})$$
2. (a) Derive the expressions of chemical potential of a component in a mixture system in terms of extensive thermodynamic properties U, H, A and G. Cadmium (m. pt.=321 °C) and bismuth (m. pt.=271 °C) do not form solid solutions or compounds with one another. Their eutectic point lies at 61 weight percent of bismuth and 146 °C. Sketch their phase diagram and label each region to show what phases are present. Explain the cooling curves of this eutectic mixture. How does acetone-dry ice freezing mixture work? (b) In a homogeneous electric field E = 5 V cm⁻¹, the speed ν of Zn²⁺ ions in aqueous solution at 25 °C is 2.74 × 10⁻⁵ m s⁻¹. (i) Estimate the radius r of the hydrated Zn²⁺ ion. The coefficient of viscosity η of water at the given temperature is 0.890 mPa s. (ii) Calculate the diffusion coefficient D of the ion. (c) Calculate the energy of 800 nm electromagnetic radiation per einstein in SI system. $$\begin{array}{l} \mathrm{HI(g)} + h\nu \longrightarrow \mathrm{H(g)} + \mathrm{I(g)} \\ \mathrm{H(g)} + \mathrm{HI(g)} \xrightarrow{k_2} \mathrm{H_2(g)} + \mathrm{I(g)} \\ \mathrm{I(g)} + \mathrm{I(g)} \xrightarrow{k_3} \mathrm{I_2(g)} \end{array}$$ The photodecomposition of HI(g) takes place following three elementary steps : $$\begin{array}{l} \mathrm{HI(g)} + h\nu \longrightarrow \mathrm{H(g)} + \mathrm{I(g)} \\ \mathrm{H(g)} + \mathrm{HI(g)} \xrightarrow{k_2} \mathrm{H_2(g)} + \mathrm{I(g)} \\ \mathrm{I(g)} + \mathrm{I(g)} \xrightarrow{k_3} \mathrm{I_2(g)} \end{array}$$ Find the expression of $-\frac{d[\mathrm{HI(g)}]}{dt}$, quantum yield (φ) and kinetic order of the reaction.
3. (a) Draw and discuss the polarographic wave obtained in the case of a dropping mercury electrode in polarography. (b) The equilibrium $A \rightleftharpoons B + C$ at 25 °C is subjected to a sudden temperature increase that slightly increases the concentrations of $B$ and $C$. The measured relaxation time is 3.0 μs. The equilibrium constant for the system is $2.0 \times 10^{-16} \text{ mol dm}^{-3}$ at 25 °C, and the equilibrium concentrations of $B$ and $C$ at 25 °C are both $2.0 \times 10^{-4} \text{ mol dm}^{-3}$. Calculate the rate constants involved in the system. A substance $B$ converts simultaneously to the products $D$ and $D'$ by two parallel elementary reactions $D \xleftarrow{k_1} B \xrightarrow{k_2} D'$. The initial concentration of $B$ is $C_{B,0} = 0.5 \text{ kmol m}^{-3}$. After 40 minutes, the concentration of $B$ has decreased to $0.05 \text{ kmol m}^{-3}$; in the same time, the product $D'$ with a concentration $C_{D'} = 0.1 \text{ kmol m}^{-3}$ is formed. Calculate the rate coefficients $k_1$ and $k_2$. (c) The kinetic order of the solid-surface catalyzed gas-phase decomposition reaction varies with the pressure of the gaseous reactant. Explain with plausible mechanism.
4. (a) What is the de Broglie wavelength for a baseball (140 g) moving at 40 m s$^{-1}$? Comment on your finding. (b) Distinguish between cis- and trans-dichloroethylene by using dipole moments. (c) Provide the resonance structures with formal charges for the nitrate ion. (d) What is the shortest distance between two successive 111 planes of simple cubic lattice of edge length 200 pm? Explain. (e) What is ferredoxin? How is it classified? What is chelate effect? How is it affected by the ring size? Why is ferrocene termed as a sandwich compound? Explain its structure. (i) $[\text{Cr}(\text{H}_2\text{O})_6\text{Cl}_3]$ तथा $[\text{CrCl}(\text{H}_2\text{O})_5]\text{Cl}_2 \cdot \text{H}_2\text{O}$ (ii) $[\text{Co}(\text{NH}_3)_5\text{SO}_4]\text{NO}_3$ तथा $[\text{Co}(\text{NH}_3)_5\text{NO}_3]\text{SO}_4$ (iii) $[\text{Pt}(\text{NH}_3)_4][\text{PtCl}_6]$ तथा $[\text{Pt}(\text{NH}_3)_4\text{Cl}_2][\text{PtCl}_4]$ What is ionization isomerism? Mention the type of isomerism in the given pairs of complexes : (i) $[\text{Cr}(\text{H}_2\text{O})_6\text{Cl}_3]$ and $[\text{CrCl}(\text{H}_2\text{O})_5]\text{Cl}_2 \cdot \text{H}_2\text{O}$ (ii) $[\text{Co}(\text{NH}_3)_5\text{SO}_4]\text{NO}_3$ and $[\text{Co}(\text{NH}_3)_5\text{NO}_3]\text{SO}_4$ (iii) $[\text{Pt}(\text{NH}_3)_4] [\text{PtCl}_6]$ and $[\text{Pt}(\text{NH}_3)_4\text{Cl}_2] [\text{PtCl}_4]$ What is inorganic benzene? Explain it with structure. What is actinide contraction? Explain.
5. (a) (1) $\text{SO}_2$ , (2) $\text{I}_3^-$ , (3) $\text{SF}_4$ , (4) $\text{BrF}_5$ , (5) $\text{XeF}_4$ Predict the Lewis structures and geometry of the following as per VSEPR theory : (1) $\text{SO}_2$ , (2) $\text{I}_3^-$ , (3) $\text{SF}_4$ , (4) $\text{BrF}_5$ , (5) $\text{XeF}_4$ Draw the molecular orbital diagram for $\text{F}_2$ molecule, and mention its bond order and magnetic behaviour. (b) Why is the electronic spectrum of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ pale pink, whereas the spectrum of $[\text{CoCl}_4]^{2-}$ is deep blue? Explain. (1) $[\text{Cr}(\text{CO})_6]$ (2) $[\text{Fe}(\text{CN})_6]^{4-}$ (3) $[\text{Co}(\text{NH}_3)_6]^{3+}$ (4) $[\text{Ni}(\text{NH}_3)_6]^{2+}$ What is EAN rule? Calculate the EAN value of the following : (1) $[\text{Cr}(\text{CO})_6]$ (2) $[\text{Fe}(\text{CN})_6]^{4-}$ (3) $[\text{Co}(\text{NH}_3)_6]^{3+}$ (4) $[\text{Ni}(\text{NH}_3)_6]^{2+}$ (c) Describe the bond order and quadruple bonding in $[\text{Re}_2\text{Cl}_8]^{2-}$ ion.
6. (a) Show that $\psi(x) = e^{\alpha x}$ is an eigenfunction for the operator $\hat{A} = \frac{d^n}{dx^n}$ . What is the eigenvalue? $$\frac{d^2\psi}{dx^2} + \frac{2mE}{\hbar^2}\psi(x) = 0;\ 0 \leq x \leq L$$ The time-independent Schrödinger equation for a free particle (of mass $m$ ) in a box of length $L$ (inside the box potential energy is zero, but outside the box is infinity) is $$\frac{d^2\psi}{dx^2} + \frac{2mE}{\hbar^2}\psi(x) = 0;\ 0 \leq x \leq L$$ The solution of the above equation is $\psi_n(x) = \sqrt{\frac{2}{L}}\sin\left(\frac{n\pi x}{L}\right)$ ; where $n = 1, 2, 3, \dots$ . Why is $n = 0$ not allowed? Explain. (b) Find the minimum uncertainty in momentum and speed if we wish to locate an electron within an atom so that $\Delta x \approx 50$ pm, using $\Delta p_x \times \Delta x \geq h$ . Comment on your two findings. What are the $n, l$ and $m_l$ values of $2p_z$ orbital? Explain. (c) Out of three lattice structures SC, BCC and FCC, which one is most economically packed and which one least? Give your answer, calculating packing fraction of each lattice structure. For Bragg X-ray diffraction studies of crystalline solid, what is the Bragg condition for constructive interference? Explain.
7. (a) Differentiate between T and R conformation of hemoglobin. (b) Describe the structure and bonding in diborane. (1) $\text{SiO}_4^{4-}$ आयन, (2) $\text{Si}_2\text{O}_7^{6-}$ आयन, (3) $\text{Si}_3\text{O}_9^{6-}$ आयन How are silicates classified? Predict the names and structures of the following silicates : (1) $\text{SiO}_4^{4-}$ ion, (2) $\text{Si}_2\text{O}_7^{6-}$ ion, (3) $\text{Si}_3\text{O}_9^{6-}$ ion (c) Why do $\text{Eu}^{3+}$ and $\text{Sm}^{3+}$ not follow the trend of magnetic moment of lanthanides at 300 K? Explain. Why do the metallic radii decrease from La to Lu except at Eu and Yb? $3+2=5$ How are lanthanides separated by a complex formation method? Give description. Why do $\text{Ce}^{3+}$ and $\text{Yb}^{3+}$ not absorb in the visible region, but show sharp absorption in the ultraviolet region? ★★★ SB27—420

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