Which of the following is NOT correctly matched?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2021 Q86 — Science & Technology PYQ with Answer

State PCS 2021 Q86 — Science & Technology PYQ with Answer