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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(b). Liquid of higher surface tension: five properties

  • 5 marks
  • Answered
Question

The surface tension of liquid A is greater than that of liquid B. Which liquid will show larger (i) vapour pressure, (ii) cohesive force, (iii) boiling point, (iv) capillary action and (v) wetting property? Explain.

Answer in brief

B has the larger vapour pressure and the better wetting. A has the larger cohesive force, boiling point and capillary rise.

Key points

  • B. Molecules of B escape more easily from the surface, so B is more volatile.
  • A. This is the same attraction that produces the larger γ.
  • A. Its lower vapour pressure reaches 1 atm only at a higher temperature.
  • A, taking the two liquids to have the same density and to wet the glass fully (θ≈0, as most clean liquids do in glass), in the same tube. From h=2γθ/ρ g r, a larger γ gives a larger rise.

The full model answer

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

Surface molecules lose part of their attractions, so a higher γ means stronger intermolecular forces in A.

  1. Vapour pressure: B. Molecules of B escape more easily from the surface, so B is more volatile.
  2. Cohesive force: A. This is the same attraction that produces the larger γ.
  3. Boiling point: A. Its lower vapour pressure reaches 1 atm only at a higher temperature.
  4. Capillary action: A, taking the two liquids to have the same density and to wet the glass fully (θ≈0, as most clean liquids do in glass), in the same tube. From h=2γθ/ρ g r, a larger γ gives a larger rise.
  5. Wetting: B. From θ=(γ_(sg)-γₛₗ)/γ_(lg), a smaller γ_(lg) gives a smaller contact angle, so B spreads over the solid while A, held together by stronger cohesion, tends to form beads.

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Model answer to Q1(b), 2026 Paper I: solution page 3
Solution page 3 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 1