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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

Q8(b). Silicones, hexamethyldisiloxane and chain stopping

Question

What are silicones? Mention some of their uses. How will you prepare hexamethyldisiloxane? Draw its structure. What happens if some (CH₃)₃SiCl is mixed with (CH₃)₂SiCl₂ and hydrolysed?

Answer in brief

Silicones are polymers with an –Si–O–Si– backbone and organic groups on silicon, (R₂SiO)n; (CH₃)₃SiCl added to (CH₃)₂SiCl₂ acts as a chain stopper and gives shorter linear chains with (CH₃)₃Si ends.

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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.

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Topic notes: main group chemistry: boranes, borazine, phosphazenes, silicates, silicones, interhalogens, xenon and sulfur–nitrogen compounds, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverssiliconeshexamethyldisiloxanechain stopperdimethyldichlorosilanesiloxane polymersilicone oil

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