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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(d)(ii). Three liquid–liquid phase diagrams

  • 5 marks
  • Answered
Question

Label and compare the phase diagrams given above: q4dii_q

Answer in brief

They are temperature–composition diagrams of partially miscible liquid pairs. (1) Dome: upper critical solution temperature (phenol–water, about 66°C). (2) U: lower critical solution temperature (triethylamine–water, about 19°C). (3) Closed loop: both (nicotine–water, 61°C and 210°C). Inside each curve two liquid layers coexist; outside it there is one liquid phase.

Key points

  • (1) Upper CST (phenol–water, also hexane–nitrobenzene): mutual solubility rises on heating; the two branches meet at the top, the upper critical solution temperature T_(uc), above which the liquids mix in all proportions.…
  • (2) Lower CST (triethylamine–water): the liquids are miscible in all proportions below T_(lc)≈ 19°C (292 K) and separate above it. At low T they form weak (hydrogen-bonded) complexes; heating breaks them.
  • (3) Closed loop (nicotine–water): both effects, two layers only between T_(lc)=61°C and T_(uc)=210°C (under pressure). Complexes break above 61°C; above 210°C thermal motion makes them miscible again.
  • in all three, a tie line across the two-phase region links the compositions of the two conjugate layers, and the lever rule gives their amounts. They differ in where the miscibility gap lies: below T_(uc) in (1), above T_(lc)…

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. (1) Upper CST (phenol–water, also hexane–nitrobenzene): mutual solubility rises on heating; the two branches meet at the top, the upper critical solution temperature T_(uc), above which the liquids mix in all proportions. Mixing is endothermic; heating overcomes the like–like attractions.
  2. (2) Lower CST (triethylamine–water): the liquids are miscible in all proportions below T_(lc)≈ 19°C (292 K) and separate above it. At low T they form weak (hydrogen-bonded) complexes; heating breaks them.
  3. (3) Closed loop (nicotine–water): both effects, two layers only between T_(lc)=61°C and T_(uc)=210°C (under pressure). Complexes break above 61°C; above 210°C thermal motion makes them miscible again.
  4. Comparison: in all three, a tie line across the two-phase region links the compositions of the two conjugate layers, and the lever rule gives their amounts. They differ in where the miscibility gap lies: below T_(uc) in (1), above T_(lc) in (2), between the two in (3).

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Model answer to Q4(d)(ii), 2018 Paper I: solution page 29
Solution page 29 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 4