MPPSC 2020 Prelims Paper-I · Question 62 of 100

Correct — D, Caustic soda. Caustic soda (sodium hydroxide, NaOH) is a product of the chlor-alkali industry and is used in soap, paper, textiles, alumina and detergents — not as a feedstock for fertilizers. The other three are all genuine fertilizer raw materials: naphtha is a feedstock for making ammonia (and hence nitrogenous fertilizers like urea), sulphur is turned into sulphuric acid to make phosphatic fertilizers, and gypsum is used in producing ammonium sulphate and as a source of calcium and sulphur. So caustic soda is the odd one out.

• (a)Naphtha — A petroleum fraction used as feedstock to produce ammonia — the building block of nitrogenous fertilizers such as urea; it IS a fertilizer raw material.
• (b)Gypsum — Used in the manufacture of ammonium sulphate and as a soil amendment supplying calcium and sulphur; it IS a fertilizer raw material.
• (c)Sulphur — Converted to sulphuric acid, which is essential for making phosphatic fertilizers (single superphosphate, DAP); it IS a fertilizer raw material.

The fertilizer industry draws on distinct raw materials for each nutrient: nitrogen from ammonia (made from natural gas or naphtha), phosphorus from rock phosphate treated with sulphuric acid (from sulphur), and potassium from potash. Gypsum and sulphur feed the phosphatic/ammonium-sulphate lines. Caustic soda belongs to a different family — the chlor-alkali industry — so it is not a fertilizer input.

The odd-one-out is the chemical that belongs to another industry. Three options tie directly to nitrogen or phosphate fertilizer manufacture; only caustic soda serves soap/paper/textiles instead. Spotting the 'wrong industry' member gives the answer.

• Naphtha / natural gas → ammonia → urea and other nitrogenous fertilizers.
• Sulphur → sulphuric acid → phosphatic fertilizers (SSP, DAP).
• Gypsum → ammonium sulphate; also a soil amendment (calcium + sulphur).
• Caustic soda (NaOH) is a chlor-alkali product (soap, paper, alumina) — not a fertilizer feedstock.

• Assuming any common industrial chemical is a fertilizer input — caustic soda belongs to chlor-alkali.
• Forgetting that sulphur's role is indirect (via sulphuric acid), yet it is still a raw material.

Usually an odd-one-out ('which is NOT a raw material') or a match-the-industry item. Fix the chains: naphtha/gas→N fertilizers, sulphur/rock phosphate→P fertilizers, caustic soda→chlor-alkali (not fertilizer).

• practice — not a real PYQ

  Sulphur is used in the fertilizer industry mainly to manufacture:

  • (a)Sulphuric acid for phosphatic fertilizers
  • (b)Ammonia
  • (c)Potash
  • (d)Urea directly

  Answer(a) Sulphuric acid — used to make phosphatic fertilizers.

• practice — not a real PYQ

  Caustic soda (sodium hydroxide) is chiefly a product of which industry?

  • (a)Fertilizer
  • (b)Chlor-alkali
  • (c)Sugar
  • (d)Cement

  Answer(b) Chlor-alkali industry.