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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper II

Q1(a). Compressibility factor and its significance

  • 5 marks
  • Topic: Kinetic theory and the gaseous state
  • Answered
Question

Define compressibility factor (Z). What is its significance: when its value is either (i) 1 or (ii) <1 or (iii) >1?

Answer in brief

Z=(PVₘ)/(RT). (i) Z=1: ideal behaviour. (ii) Z<1: attractive forces dominate, the gas is more compressible than ideal. (iii) Z>1: repulsive forces dominate, the gas is less compressible.

Key points

  • the ratio of the actual molar volume to RT/P.
  • Z=1 for an ideal gas at all P and T, and for any real gas when P→0 or at the Boyle temperature (low P).
  • intermolecular attraction pulls molecules together, Vₘ<RT/P. Z>1 (high P or high T): the finite size of molecules, Vₘ>RT/P.

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  • Z=(Vₘ(real))/(Vₘ(ideal)): the ratio of the actual molar volume to RT/P.
  • Z=1 for an ideal gas at all P and T, and for any real gas when P→0 or at the Boyle temperature (low P).
  • Z<1 (moderate P, low T): intermolecular attraction pulls molecules together, Vₘ<RT/P. Z>1 (high P or high T): the finite size of molecules, Vₘ>RT/P.
Model answer to Q1(a), 2026 Mains Paper II: solution page 2
Solution page 2 of 22
Read the whole 2026 solution2026 question paper

Coversreal gasesCompressibility factor and its significance

All parts of Question 1