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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.

UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q1(e). Components and phases when NH₄Cl is heated

  • 5 marks
  • Answered
Question

NH₄Cl(s) decomposes when heated.
(i) How many components and phases are present when the salt is heated in a closed container?
(ii) Now, suppose additional NH₃ is also present in the container. How many components and phases will be present?

Answer in brief

(i) C = 1, P = 2. (ii) C = 2, P = 2.

Key points

  • Species S=3 (NH₄Cl, NH₃, HCl); one equilibrium, R=1. All the gas comes from the salt, so p_(NH₃)=p_(HCl): one concentration restriction.
  • P=2; F=1-2+2=1 (fixing T fixes the dissociation pressure).
  • The gas now has more NH₃ than HCl, so p_(NH₃)=p_(HCl) no longer holds: C=3-1= 2; phases are still solid + gas, P=2. F=2-2+2=2 (both T and the gas composition can be varied).

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

NH₄Cl(s) ⇌ NH₃(g) + HCl(g). Components: the fewest independent species that fix the composition of every phase.

(i) NH₄Cl alone in a closed vessel

  1. Species S=3 (NH₄Cl, NH₃, HCl); one equilibrium, R=1. All the gas comes from the salt, so p_(NH₃)=p_(HCl): one concentration restriction.
  2. C=3-1-1= 1; solid + gas: P=2; F=1-2+2=1 (fixing T fixes the dissociation pressure).

(ii) Extra NH₃ present

  1. The gas now has more NH₃ than HCl, so p_(NH₃)=p_(HCl) no longer holds: C=3-1= 2; phases are still solid + gas, P=2. F=2-2+2=2 (both T and the gas composition can be varied).

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Model answer to Q1(e), 2026 Paper I: solution page 5
Solution page 5 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 1