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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 · 2018 · Paper I

Q4(c)(ii). Vapour and gas from the critical temperature

  • 5 marks
  • Answered
Question

Account for the difference between vapour and gas on the basis of critical temperature.

Answer in brief

Above its critical temperature a substance cannot be liquefied by pressure alone; in that state it is a gas. Below T_c it can be liquefied by compression at constant temperature and can coexist with its liquid; in that state it is called a vapour.

Key points

  • The critical temperature T_c is the temperature above which the liquid phase cannot be formed however high the pressure. On the isotherms, the horizontal (liquid + vapour) region shrinks as T rises and vanishes at the critical…
  • Below T_c compression at constant T eventually condenses the substance (the flat part of the isotherm): it is a vapour, in equilibrium with its liquid at the vapour pressure.
  • Above T_c compression only gives a denser fluid, with no meniscus and no condensation: it is a gas (a supercritical fluid at high pressure).
  • CO₂ (T_c=304.2 K, 31.0°C) is a vapour at room temperature and is stored as a liquid in cylinders; N₂, O₂, H₂ and He have T_c far below room temperature, so they are gases and must be cooled below T_c before pressure can…

The full model answer

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

  1. The critical temperature T_c is the temperature above which the liquid phase cannot be formed however high the pressure. On the isotherms, the horizontal (liquid + vapour) region shrinks as T rises and vanishes at the critical point.
  2. Below T_c compression at constant T eventually condenses the substance (the flat part of the isotherm): it is a vapour, in equilibrium with its liquid at the vapour pressure.
  3. Above T_c compression only gives a denser fluid, with no meniscus and no condensation: it is a gas (a supercritical fluid at high pressure).
  4. Examples: CO₂ (T_c=304.2 K, 31.0°C) is a vapour at room temperature and is stored as a liquid in cylinders; N₂, O₂, H₂ and He have T_c far below room temperature, so they are gases and must be cooled below T_c before pressure can liquefy them (Linde process).

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Model answer to Q4(c)(ii), 2018 Paper I: solution page 27
Solution page 27 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 4