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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper II

Q1(c). Surface energy of cube and sphere

Question

Show that the surface energy of a cube-shaped liquid is about 1.24 times that of its spherical shape.

Answer in brief

For equal liquid volume and surface tension, E_(cube)/Eₛₚₕₑᵣₑ=6/(36π)1/3=1.2407.

Key points

  • Let the common volume be V. A cube has side V1/3 and area Ac=6V2/3.
  • A sphere has radius (3V/4π)1/3 and area Aₛ=4π(3V/4π)2/3=(36π)1/3V2/3.
  • With constant surface tension, surface free energy is E=γ A.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(c), 2019 Mains Paper II: solution pages 1 and 2
Solution pages 1 and 2 of 17
Read the whole 2019 solution2019 question paper

CoversSurface energy of cubesphereSurface chemistry

All parts of Question 1