UPPSC 2021 Prelims GS Paper-I · Question 86 of 150

Correct — C, 'Souring of milk - Nitric acid' is the pair that is NOT correctly matched. When milk turns sour it is because bacteria (chiefly Lactobacillus/Lactococcus) ferment the milk sugar lactose into LACTIC ACID; the rising acidity coagulates the casein protein and curdles the milk. Nitric acid (HNO3) is a strong industrial mineral acid and has nothing to do with milk — so this pairing is false. Since the question asks for the WRONGLY matched pair, option (c) is the answer. The other three pairs are all correct.

• (a)Acid present in vinegar - Acetic acid — Correctly matched (so not the answer) — vinegar is a dilute solution of acetic acid (ethanoic acid, CH3COOH), which gives it its sour taste and pungent smell.
• (b)Compound present in bones - Calcium phosphate — Correctly matched (so not the answer) — the hard mineral of bone and teeth is largely calcium phosphate (as hydroxyapatite), which gives the skeleton its rigidity.
• (d)Acid present in gastric juice - Hydrochloric acid — Correctly matched (so not the answer) — gastric juice contains hydrochloric acid (HCl) secreted by the stomach's parietal cells, which activates pepsin and kills ingested microbes.

This is a 'common substance ↔ characteristic acid/compound' recall item from everyday chemistry. Vinegar owes its sourness to acetic acid; the stomach digests food in hydrochloric acid; bone is mineralised with calcium phosphate; and the souring/curdling of milk is caused by lactic acid produced when bacteria ferment lactose. The one deliberately false pairing here substitutes nitric acid — a strong mineral acid used in fertilisers and explosives — for the lactic acid that actually sours milk.

Read the stem carefully: it asks which pair is NOT correct, so you must hunt for the single false statement, not confirm the true ones. Three of the four are textbook-standard pairings you should recognise instantly; the odd one out is milk-souring, where the everyday, biologically produced acid is LACTIC acid, never nitric acid. Spotting that mismatch gives option (c).

• Souring/curdling of milk is due to lactic acid formed by bacterial fermentation of lactose — not nitric acid.
• Vinegar = dilute acetic acid (CH3COOH).
• Human gastric juice contains hydrochloric acid (HCl).
• Bone and teeth are mineralised chiefly with calcium phosphate (hydroxyapatite).

• In a 'NOT correctly matched' item, ticking a correctly matched pair instead of the false one
• Confusing the biologically produced lactic acid of soured milk with a strong mineral acid like nitric acid

UPPSC and UPSC repeatedly test 'substance ↔ its acid/chemical' pairs — memorise acetic-vinegar, lactic-sour milk, citric-citrus, oxalic-tomato/spinach, tartaric-tamarind, formic-ant sting, HCl-gastric juice.

• practice — not a real PYQ

  The souring of milk is chiefly caused by the formation of which acid?

  • (a)Citric acid
  • (b)Lactic acid
  • (c)Tartaric acid
  • (d)Acetic acid

  Answer(b) Lactic acid — bacteria ferment the lactose in milk into lactic acid, whose acidity coagulates casein and curdles the milk.

• practice — not a real PYQ

  Which one of the following pairs (Source – Acid) is correctly matched?

  • (a)Tamarind – Oxalic acid
  • (b)Ant sting – Formic acid
  • (c)Vinegar – Citric acid
  • (d)Curd – Nitric acid

  Answer(b) Ant sting – Formic acid — ants inject formic acid; tamarind has tartaric acid, vinegar has acetic acid, and curd has lactic acid.