Environment · Air Pollution Control (Air Act, NCAP, Stubble Burning)
The concept being tested: Cloud seeding for artificial rainfall and air pollution control.
Artificial way of causing rainfall to reduce air pollution makes use of
Cloud seeding works by dispersing chemical nuclei into moisture-bearing clouds so that water vapour condenses and falls as rain faster than it would naturally, and the standard nucleating agents used for this, including in India's own trial seeding flights over Delhi-NCR aimed at washing out particulate pollution, are silver iodide and potassium iodide. Silver iodide has a crystal structure very close to that of ice, so it gives supercooled droplets a surface to freeze and grow around; potassium iodide works on the same hygroscopic-nucleation principle and is commonly paired with it in seeding flares released from aircraft. These are the actual compounds loaded into operational seeding cartridges, not the superficially similar-sounding nitrate or chloride salts of silver and potassium that chemistry students often confuse them with, which is why silver iodide and potassium iodide is the answer UPSC has fixed here.
What UPSC was testing
UPSC is testing precise recall of the specific chemical agents used in cloud seeding, silver iodide and potassium iodide, against distractor options built by swapping in chemically familiar but functionally wrong salts such as silver nitrate, potassium nitrate and potassium chloride; the trap is assuming any silver or potassium salt will do.
How often this comes up
This concept has appeared 10 times in UPSC Prelims General Studies Paper I between 2015 and 2025 — in 2016, 2019, 2020, 2021, 2022, 2024 and 2025.
The option-by-option autopsy for this question — why each of the other three is wrong, and which neighbouring concept each one was built from — is in the app.
Other past-year questions touching Air Pollution Control (Air Act, NCAP, Stubble Burning).