UPSC Prelims 2025 Q85: Science & Technology – Chemistry
- (a) silver iodide and potassium iodide
- (b) silver nitrate and potassium iodide
- (c) silver iodide and potassium nitrate
- (d) silver nitrate and potassium chloride
Answer: (a) silver iodide and potassium iodide
Explanation
Cloud Seeding (Artificial Rainfall) — Chemicals Used
Cloud seeding is a form of weather modification that introduces particles into clouds to trigger precipitation (rain/snow). Used to reduce air pollution, fight droughts, and enhance rainfall.
The Two Key Chemicals:
| Chemical | Role | Why Used |
|---|---|---|
| Silver Iodide (AgI) | Primary cloud seeding agent | Its crystal structure is very similar to ice crystals → acts as an ice nucleus → supercooled water droplets freeze around it → ice crystals grow → fall as precipitation |
| Potassium Iodide (KI) | Secondary/supplementary agent | Also used as an ice nucleus in cloud seeding, often alongside silver iodide |
Answer: (a) silver iodide and potassium iodide.
Other Agents Used in Cloud Seeding:
| Agent | Use |
|---|---|
| Silver iodide | Most common - mimics ice crystal structure |
| Potassium iodide | Also an ice nucleus agent |
| Dry ice (solid CO₂) | Rapid cooling to create ice nuclei |
| Liquid propane | Expands and cools to create ice nuclei |
| Salt (NaCl) | Warm cloud seeding - promotes droplet coalescence |
Why NOT silver nitrate or potassium chloride/nitrate?
These do not have ice-nucleating properties and are NOT used in standard cloud seeding operations.
Why each option is right or wrong
- (a) CORRECT. Silver iodide (AgI) + potassium iodide (KI) are the standard chemicals used in cloud seeding. Silver iodide's crystal lattice mimics ice, making it the world's most widely used cloud seeding agent since 1946.
- (b) Incorrect. Silver NITRATE is not used for cloud seeding — it does not have ice-nucleating properties. Cloud seeding requires chemicals that mimic ice crystal structure.
- (c) Incorrect. Potassium NITRATE is not a cloud seeding agent. The combination of silver iodide + potassium nitrate is not used in artificial rainfall operations.
- (d) Incorrect. Silver NITRATE + potassium CHLORIDE — neither combination is used for cloud seeding. These salts do not serve as effective ice nuclei.
Key point
Cloud seeding = Silver Iodide (primary) + Potassium Iodide. Silver iodide mimics ice crystal structure → supercooled water freezes around it → precipitation forms.
Remember
Cloud seeding for artificial rainfall uses Silver Iodide + Potassium Iodide. Silver iodide is the primary agent — its crystal structure mimics ice, triggering precipitation in clouds.
Common mistakes
- Confusing silver iodide with silver nitrate — silver IODIDE (AgI) is the specific compound used; silver nitrate (AgNO₃) is different and not used in cloud seeding.
- Thinking any silver or potassium compound works — the iodide anion (I⁻) specifically enables ice nucleation because of its lattice structure match with ice.
- Not knowing that cloud seeding was first developed in 1946 — Vincent Schaefer and Irving Langmuir demonstrated dry ice and silver iodide cloud seeding.
Exam tip
Silver IODIDE is the cloud seeding agent — remember 'I' for 'Ice nucleation' and 'I' for 'Iodide.' The iodide's crystal structure = ice crystal structure = triggers freezing. Used in India's cloud seeding experiments over Maharashtra (Mahabeej project) and other water-stressed states.
Sources
- NCERT Class XI — Chemistry (Silver and its compounds)
- IMD (India Meteorological Department) — Cloud Seeding Operations in India
- WMO (World Meteorological Organization) — Weather Modification Techniques