Which one of the following is NOT correctly matched ?
(Substance) — (Use)

Question

UnlockIAS · Accepted Answer

Correct — A, 'Silver bromide — artificial photosynthesis' is the pair that is NOT correctly matched. Silver bromide is a light-sensitive silver halide: on exposure to light it is reduced to metallic silver (2AgBr → 2Ag + Br2), and it is exactly this property that makes AgBr crystals suspended in gelatin the emulsion of black-and-white photographic film and printing paper. Its correct partner in the second column is therefore photography, not photosynthesis. Artificial photosynthesis is something else entirely — laboratory systems (semiconductor photocatalysts, photoelectrochemical cells, molecular water-oxidation catalysts) that use sunlight to split water or reduce carbon dioxide into fuels — and silver bromide is not the agent behind it. The remaining three pairs are matched as the paper intends, so (a) is the odd one out.

Why the other options are wrong — (b) Correctly matched, so it is not the answer. Silver iodide's crystal lattice closely resembles that of ice, so its particles act as ice nuclei around which supercooled cloud droplets freeze and grow — which is why AgI is the classic cloud-seeding chemical used for rain-making.  (c) Treated as correctly matched by the paper, so it is not the answer. Lithium salts have a long documented history in gout and rheumatic complaints — lithium urate is comparatively soluble, which was the original rationale for prescribing them. (In modern medicine lithium carbonate's principal use is in bipolar disorder, so read this pairing as the exam's traditional textbook pairing.)  (d) Correctly matched, so it is not the answer. Milk of magnesia is an aqueous suspension of magnesium hydroxide, Mg(OH)2 — a weak, sparingly soluble base that neutralises excess hydrochloric acid in the stomach (and acts as an osmotic laxative in larger doses).

Concept: This is a substance-to-use mapping question built on three separate bits of chemistry: the photosensitivity of silver halides (AgBr, AgCl decompose to silver in light — the basis of film photography), ice nucleation (silver iodide is seeded into clouds because its lattice mimics ice), and acid–base neutralisation (a weak base such as Mg(OH)2 relieves acidity). Only one pairing has to be broken for the question to work.

State PCS 2019 Q18 — Science & Technology PYQ with Answer

State PCS 2019 Q18 — Science & Technology PYQ with Answer