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UPSC Combined Geo-Scientist

Chemist

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

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III

Q1(d). Flame absorption profiles of magnesium and silver

Question

Show the flame absorption profiles for magnesium and silver with respect to flame height. Justify.

Answer in brief

Mg: absorbance rises, passes through a maximum near the middle of the flame, then falls (MgO forms). Ag: absorbance rises continuously from the base to the top of the flame (no stable oxide).

Key points

  • Rise (both metals): higher up, longer residence time in the hot flame gives more complete atomisation and more ground-state atoms.
  • Mg: in the outer, oxidant-rich zone atoms are oxidised to stable MgO, which does not absorb; the opposing effects give a maximum near the middle.
  • Ag: not easily oxidised; the number of metal atoms, and the absorbance, rises continuously to the top of the flame.
  • Cr (contrast) forms very stable oxides at once, so its absorbance falls continuously from the base.
  • The burner (observation) height must be optimised for each element and kept the same for standards and samples.

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(d), 2025 Mains Paper III: solution pages 5 and 6
Solution pages 5 and 6 of 37
Read the whole 2025 solution2025 question paper

Topic notes: spectroscopic instruments (Beer–Lambert law, UV–vis and IR instruments, Job's method, AAS and flame emission), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversflame absorption profileatomic absorption spectroscopyAASobservation heightmagnesium oxideatomisation

All parts of Question 1