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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 · 2025 · Paper I

Q3(b). Droplets made from one mole of water

Question

Suppose that 10.0 J of work is required to create droplets of uniform size from a mole of water in bulk at 25 °C and 1 atm pressure. (i) Assuming that surface tension is independent of area, calculate the radius of the droplets. (ii) Calculate the number of water molecules in a droplet. Given: Surface tension of water = 0.072 J/m².

Answer in brief

(i) r = 3.9 × 10⁻⁷ m (0.39 µm); (ii) 8.2 × 10⁹ molecules per droplet.

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Topic notes: the liquid state: surface tension, the Kelvin equation, capillary action, viscosity and liquid crystals, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverssurface tensionsurface worksurface Gibbs energydroplet radiusarea to volume ratio

All parts of Question 3