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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

Q3(a)(i). First law and the cooling of rising clouds

  • 5 marks
  • Answered
Question

Using the statement of First law of thermodynamics, argue upon the various effects on rising clouds as it goes to higher altitudes.

Answer in brief

Rising air meets lower pressure and expands; it does work on the surroundings with no time to take in heat (q≈0). By the first law Δ U=w<0, so the air cools. Cooling brings it to the dew point: water condenses, the cloud grows, and the latent heat released slows further cooling.

Key points

  • Δ U=q+w. For a perfect gas U depends only on T (dU=nC_(V,m)dT), so a fall in U means a fall in temperature.
  • A rising parcel of moist air moves into regions of lower pressure, so it expands and does work against the surrounding air: w=-pₑₓdV<0.
  • Air is a poor conductor and the rise is fast, so almost no heat enters: the change is adiabatic, q≈0. Then dU=w<0: the air cools, by about 9.8 K for every km (dry adiabatic rate, g/cₚ).
  • When the air cools to its dew point, water vapour condenses on dust and salt nuclei: the cloud forms or grows. Condensation releases the enthalpy of vaporisation (q>0 now), so saturated air cools more slowly.

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. First law: Δ U=q+w. For a perfect gas U depends only on T (dU=nC_(V,m)dT), so a fall in U means a fall in temperature.
  2. A rising parcel of moist air moves into regions of lower pressure, so it expands and does work against the surrounding air: w=-pₑₓdV<0.
  3. Air is a poor conductor and the rise is fast, so almost no heat enters: the change is adiabatic, q≈0. Then dU=w<0: the air cools, by about 9.8 K for every km (dry adiabatic rate, g/cₚ).
  4. When the air cools to its dew point, water vapour condenses on dust and salt nuclei: the cloud forms or grows. Condensation releases the enthalpy of vaporisation (q>0 now), so saturated air cools more slowly.
  5. Higher still, droplets grow, freeze to ice crystals and fall as rain, snow or hail. Expansion cooling is the energy source of cloud formation; the same first-law argument explains why mountain tops are cold.

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Model answer to Q3(a)(i), 2018 Paper I: solution page 16
Solution page 16 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 3