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UPSC Combined Geo-Scientist

Chemist

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

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I

Q4(b). Homogeneous precipitation

  • 10 marks
  • Topic: Gravimetric analysis
  • Answered
Question

What is homogeneous precipitation? Discuss its advantages over direct precipitation.

Answer in brief

In homogeneous precipitation the precipitating agent is generated slowly inside the solution by a chemical reaction, so no part of the solution is ever strongly supersaturated. The crystals are larger, denser and purer than those from direct addition of reagent.

Key points

  • the reagent is added from a burette; at the point of mixing Q≫ S, so nucleation dominates and many tiny, impure particles form (colloids, occlusion).
  • the reagent is produced throughout the solution at a slow, uniform rate on heating, so Q exceeds S only slightly; crystal growth dominates over nucleation.
  • easily filtered and washed; less surface area, less adsorbed impurity.
  • pure precipitate, as the lattice grows slowly and rejects foreign ions.

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  1. Direct precipitation: the reagent is added from a burette; at the point of mixing Q≫ S, so nucleation dominates and many tiny, impure particles form (colloids, occlusion).
  2. Homogeneous method: the reagent is produced throughout the solution at a slow, uniform rate on heating, so Q exceeds S only slightly; crystal growth dominates over nucleation.
Reagent sourceReactionUsed for
urea (OH⁻)(NH₂)₂CO + 3H₂O → CO₂ + 2NH₄⁺ + 2OH⁻Al³⁺, Fe³⁺, Th⁴⁺ as hydroxides or basic salts
thioacetamide (H₂S)CH₃CSNH₂ + 2H₂O → CH₃COO⁻ + NH₄⁺ + H₂Ssulphides of Cu, Cd, Zn, As
sulphamic acid (SO₄²⁻)NH₂SO₃H + H₂O → NH₄⁺ + H⁺ + SO₄²⁻Ba²⁺, Pb²⁺ as sulphates
trimethyl phosphate (PO₄³⁻)(CH₃O)₃PO + 3H₂O → 3CH₃OH + H₃PO₄Zr⁴⁺, Hf⁴⁺ phosphates
urea + oxalic acid (C₂O₄²⁻)pH rises, HC₂O₄⁻ → C₂O₄²⁻Ca²⁺ as CaC₂O₄

Advantages over direct precipitation

  • Larger, denser crystals: easily filtered and washed; less surface area, less adsorbed impurity.
  • Less co-precipitation and occlusion: pure precipitate, as the lattice grows slowly and rejects foreign ions.
  • Sharp separation: for pairs close in solubility (Al³⁺ from Mg²⁺) because the pH rises uniformly.
  • Gelatinous hydroxides (Al(OH)₃, Fe(OH)₃) become dense and filterable.
  • Digestion at low supersaturation gives a constant composition, so the gravimetric factor is reliable.

the drawback is time (an hour of heating) and, sometimes, the precipitate sticking to the glass walls; a Teflon beaker avoids this.

Model answer to Q4(b), 2026 Mains Paper I: solution page 13
Solution page 13 of 26
Read the whole 2026 solution2026 question paper

Coversprecipitation methodsHomogeneous precipitation

All parts of Question 4