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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 · 2017 · Paper I

Q1(d)(ii). On heating in an open vessel sulfur melts, iodine sublimes

  • 5 marks
  • Answered
Question

(ii) In open container, on heating, solid sulphur melts, but solid iodine sublimes. Explain.

Answer in brief

Solid iodine has a high vapour pressure, so it evaporates fast well below its melting point (114°C) and the vapour escapes; solid sulfur has a very low vapour pressure up to its melting point, so it melts.

Key points

  • A solid passes into vapour wherever its vapour pressure is appreciable; in an open vessel the vapour is carried away, so the solid keeps evaporating and the partial pressure above it stays low.
  • I₂ molecules are held by weak dispersion forces, so the solid has a high vapour pressure and a high triple-point pressure. On heating, the solid–vapour curve reaches a large vapour pressure before the triple point (m.p.…
  • its vapour pressure stays very small up to its melting point (about 119°C). The solid reaches the solid–liquid line first and melts.
  • In a closed vessel iodine also melts (m.p. 114°C, b.p. 184°C); it "sublimes" only because the vapour is removed.

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. A solid passes into vapour wherever its vapour pressure is appreciable; in an open vessel the vapour is carried away, so the solid keeps evaporating and the partial pressure above it stays low.
  2. Iodine: I₂ molecules are held by weak dispersion forces, so the solid has a high vapour pressure and a high triple-point pressure. On heating, the solid–vapour curve reaches a large vapour pressure before the triple point (m.p. 114°C): the solid sublimes to violet vapour.
  3. Sulfur: its vapour pressure stays very small up to its melting point (about 119°C). The solid reaches the solid–liquid line first and melts.
  4. In a closed vessel iodine also melts (m.p. 114°C, b.p. 184°C); it "sublimes" only because the vapour is removed.

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Model answer to Q1(d)(ii), 2017 Paper I: solution page 5
Solution page 5 of 38
Read the whole 2017 solution2017 question paper

All parts of Question 1