CSS past paper

CSS Chemistry Paper I Past Paper 2026

CSS Chemistry Paper I 2026 carries 100 marks over 3 hours. It has 7 questions.

Marks: 100
Time: 3 hours
Questions: 7

Years

How to attempt: Attempt ONLY FOUR questions from PART-II. ALL questions carry EQUAL marks.

Questions

Question 1 (Part I) is the paper's MCQs, which are not reproduced here.

Q. 2
(20)
- (a)
  Explain the difference between physisorption and chemisorption in terms of energetics and kinetics. Provide typical experimental evidence to distinguish them.
  (6)
- (b)
  Define and explain the concepts of internal energy (E) and enthalpy (H). Show the mathematical relationship between them.
  (7)
- (c)
  Discuss the physical meaning of eigenfunctions and eigenvalues, and explain why operators must be Hermitian in quantum mechanics.
  (7)
Q. 3
(20)
- (a)
  Outline selection criteria for stationary and mobile phases in column chromatography for separating polar organic molecules.
  (6)
- (b)
  Propose a corrosion prevention strategy for a steel water pipeline system exposed to chloride-rich environments.
  (7)
- (c)
  Derive the integrated rate equation for a second-order reaction where two reactants of equal concentration react (2A → Products).
  (7)
Q. 4
(20)
- (a)
  Given small sample data, compute 95% confidence interval for the mean and interpret the result.
  (6)
- (b)
  Predict and justify which complex will undergo a stronger Jahn–Teller distortion:
  (7)
  $\ce{[Cu(H2O)6]^2+} \text{ or } \ce{[Mn(H2O)6]^3+}$
  Support your reasoning with electronic configurations and degeneracy arguments.
- (c)
  Explain how transport numbers are determined experimentally and derive their relationship with ionic mobilities.
  (7)
Q. 5
(20)
- (a)
  Explain the photoelectric effect and how it supports the particle nature of light. Discuss the role of work function and threshold frequency.
  (6)
- (b)
  Explain the common ion effect and its application in controlling the solubility of sparingly soluble salts. Use an example to illustrate.
  (7)
- (c)
  Explain the mechanism of enzyme catalysis, highlighting the role of the active site and the factors that influence enzyme activity.
  (7)
Q. 6
(20)
- (a)
  Use VSEPR to predict the shape and bond angles of $\ce{SF4}$ and explain the concept of stereochemically active lone pairs.
  (6)
- (b)
  Using the Van der Waals equation, explain qualitatively how critical constants (Tc, Pc, Vc) relate to a, b parameters.
  (7)
- (c)
  Explain Debye–Hückel theory and derive the limiting law for the mean ionic activity coefficient. Describe its region of validity.
  (7)
Q. 7
(20)
- (a)
  Discuss isomerism in coordination complexes (geometric, optical, linkage) with three illustrative examples and draw structures.
  (6)
- (b)
  A buffer is to be made at pH 5.00 using sodium phosphate salts. Which pair would you choose and how would you prepare it? Show calculations.
  (7)
- (c)
  Explain valence bond theory and hybridization for methane ( $\ce{CH4}$ ), and contrast it with an MO description of bonding in the same molecule.
  (7)
Q. 8
(20)
- (a)
  Discuss the principle of electrophoresis and its application in the separation and characterization of proteins.
  (6)
- (b)
  Describe common statistical tests (t-test, chi-square) and their applications in analytical chemistry. Provide decision criteria.
  (7)
- (c)
  Using initial rate data (suppose), determine reaction order and rate constant by method of initial rates. (Provide a short worked example).
  (7)

Part-II is to be attempted on the separate Answer Book.
Attempt ONLY FOUR questions from PART-II. ALL questions carry EQUAL marks.
All the parts (if any) of each Question must be attempted at one place instead of at different places.
Write Q. No. in the Answer Book in accordance with Q. No. in the Q.Paper.
No Page/Space be left blank between the answers. All the blank pages of Answer Book must be crossed.
Extra attempt of any question or any part of the question will not be considered.
Use of Calculator is allowed.