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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 · 2021 · Paper I

Q3(c)(i). Enthalpy of vaporisation from the triple point to the critical point

Question

Draw and explain the graph of enthalpy of vapourization from the triple point (Tp) to the critical point (Tc).

Answer in brief

ΔHvap is largest at the triple point, falls slowly at first and then more and more steeply as temperature rises, and becomes zero at Tc, where the curve meets the temperature axis vertically.

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Topic notes: the phase rule, phase diagrams, the Clausius–Clapeyron equation, eutectics and partially miscible liquids, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversenthalpy of vaporisationcritical pointtriple pointWatson relationKirchhoff equation

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