UPPSC 2019 Prelims GS Paper-I · Question 18 of 150

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.

• (b)Silver iodide — artificial rain — 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)Lithium bicarbonate — treatment of rheumatism — 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)Milk of magnesia — antacid — 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).

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.

The trap is a single word. 'Photography' and 'photosynthesis' both open with photo-, and a hurried reader accepts option (a) as familiar. Slow down on the pairing that looks scientific but is subtly swapped. Also note the negative stem — the question wants the pair that is wrong, so mark the odd one rather than the most convincing one.

• Silver bromide and silver chloride are light-sensitive silver halides; AgBr is the principal salt in black-and-white photographic emulsions
• Silver iodide is used in cloud seeding because its crystal lattice resembles ice and nucleates ice crystals
• Milk of magnesia = a suspension of magnesium hydroxide, Mg(OH)2 — antacid and osmotic laxative
• Sodium thiosulphate ('hypo') is the photographic fixer — it dissolves the unexposed silver halide from the film
• Artificial photosynthesis means sunlight-driven water splitting or CO2 reduction using photocatalysts — not a silver-halide process

• 'Photography' versus 'photosynthesis' — the entire question turns on that one word
• Forgetting that the stem asks for the pair that is NOT correctly matched
• Swapping silver iodide (rain-making) with silver bromide (photography)

Both UPPSC and UPSC favour 'which pair is NOT correctly matched' and List-I/List-II substance–use grids. Keep a running two-column list of everyday chemicals against one signature use each.

• practice — not a real PYQ

  The chemical most commonly used for cloud seeding to induce artificial rain is

  • (a)Silver bromide
  • (b)Silver iodide
  • (c)Sodium thiosulphate
  • (d)Potassium bromide

  Answer(b) Silver iodide — its crystal lattice mimics ice, so it nucleates ice crystals in supercooled clouds.

• practice — not a real PYQ

  Photographic films are coated with an emulsion of

  • (a)Silver bromide
  • (b)Silver nitrate
  • (c)Sodium thiosulphate
  • (d)Magnesium hydroxide

  Answer(a) Silver bromide — light reduces AgBr to metallic silver, forming the latent image; sodium thiosulphate is the fixer, not the emulsion.