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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 · 2026 · Mains Paper II

Q1(h). Soap and detergent

  • 5 marks
  • Topic: Surface chemistry
  • Answered
Question

What are the basic differences between soap and detergent though both are used for cleaning purpose of clothes?

Answer in brief

Soap is the sodium or potassium salt of a long-chain carboxylic (fatty) acid; a detergent is a synthetic salt of a sulphonic or sulphuric acid (or a quaternary ammonium or non-ionic compound). Detergents work in hard water and acid; soaps do not.

Key points

  • Both lower the surface tension, form micelles and take oily dirt into the hydrophobic core, which is washed away.

The full model answer

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

SoapDetergent
Chemical typeRCOO⁻Na⁺, R = C₁₂–C₁₈RC₆H₄SO₃⁻Na⁺ or ROSO₃⁻Na⁺
Sourcefats and oils + NaOH (saponification)petroleum products
Hard waterforms insoluble Ca²⁺/Mg²⁺ salts (scum)Ca and Mg salts are soluble: no scum
Acidic watergives fatty acid, loses actionstill active (strong acid anion)
Biodegradabilityfully biodegradablelinear-chain: yes; branched-chain: poorly

Both lower the surface tension, form micelles and take oily dirt into the hydrophobic core, which is washed away.

Model answer to Q1(h), 2026 Mains Paper II: solution page 6
Solution page 6 of 22
Read the whole 2026 solution2026 question paper

CoverssurfactantsSoap and detergent

All parts of Question 1