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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 · 2026 · Paper I

Q3(a)(iii). How an acetone–dry ice bath works

  • 5 marks
  • Answered
Question

How does acetone-dry ice freezing mixture work?

Answer in brief

Solid CO₂ sublimes at 1 atm at a fixed temperature, about -78°C (194.6 K). Heat leaking in is used up as enthalpy of sublimation, so while dry ice remains the bath stays at this temperature. Acetone stays liquid and carries the heat from the flask to the dry ice.

Key points

  • The triple point of CO₂ lies at 5.11 atm, above atmospheric pressure. At 1 atm solid CO₂ cannot melt; it sublimes (hence “dry” ice).
  • Solid and gas in equilibrium at a fixed pressure: C=1, P=2, so F'=C-P+1=0. The temperature is fixed at the sublimation point, 194.6 K, about -78°C.
  • Heat flowing in from the room or from the flask does not warm the bath. It sublimes more CO₂, about 57 kJ for every 100 g of dry ice (Δ_(sub)H=25.2 kJ mol⁻¹), and the gas bubbles away. The bath stays at -78°C as long as some…
  • Lumps of dry ice alone touch a flask only at a few points. Acetone does not freeze at -78°C (m.p. -95°C), stays mobile, wets the flask and the lumps, and gives good thermal contact all round.

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 triple point of CO₂ lies at 5.11 atm, above atmospheric pressure. At 1 atm solid CO₂ cannot melt; it sublimes (hence “dry” ice).
  2. Solid and gas in equilibrium at a fixed pressure: C=1, P=2, so F'=C-P+1=0. The temperature is fixed at the sublimation point, 194.6 K, about -78°C.
  3. Heat flowing in from the room or from the flask does not warm the bath. It sublimes more CO₂, about 57 kJ for every 100 g of dry ice (Δ_(sub)H=25.2 kJ mol⁻¹), and the gas bubbles away. The bath stays at -78°C as long as some solid is left, as an ice–water bath stays at 0°C.
  4. Lumps of dry ice alone touch a flask only at a few points. Acetone does not freeze at -78°C (m.p. -95°C), stays mobile, wets the flask and the lumps, and gives good thermal contact all round.
  5. The temperature is set by CO₂, not by the liquid. Unlike an ice–salt mixture, it is not a freezing-point depression. Dry ice is added a little at a time, because the warm acetone first boils it off vigorously.

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Model answer to Q3(a)(iii), 2026 Paper I: solution page 18
Solution page 18 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 3