Question 23→ 2025 IAS Prelims GS I : Genius Classroom Explanation
📘 IAS Prelims 2025 — GS-I Q.23 | Classroom Explanation
📌 Question
Q.23) Consider the following water bodies:
I. Lake Tanganyika
II. Lake Tonlé Sap
III. Patos Lagoon
Through how many of them does the Equator pass?
(a) Only one
(b) Only two
(c) All the three
(d) None
✅ Correct Answer: (d)
🧑🏫 Classroom Explanation
This question tests latitudinal awareness combined with world geography, a classic UPSC elimination area involving Equator, Tropics, and major water bodies.
🔍 Key Concept: Equator (0° Latitude)
• The Equator (0°) divides Earth into Northern and Southern Hemispheres
• Passes through:
- South America
- Africa
- Southeast Asia (Indonesia region)
- Major oceans (Atlantic, Pacific, Indian)
UPSC often frames trap questions where a feature lies near the equator but not on it.
🔍 Water Body–wise Analysis
❌ Lake Tanganyika
• Located in East Africa
• Extends roughly between 3°S to 9°S latitude
• Lies entirely south of the Equator
• Part of the Western Rift Valley of the East African Rift System
Extra info:
- Longest freshwater lake in the world (≈660 km)
- Second deepest lake after Lake Baikal
- Borders Tanzania, DRC, Burundi, Zambia
➡️ Equator does NOT pass through Lake Tanganyika
❌ Lake Tonlé Sap
• Located in central Cambodia
• Lies around 13°N latitude
• Connected to the Mekong River via the Tonlé Sap River
Extra info:
- Famous for seasonal flow reversal
- Expands up to 5 times during monsoon
- UNESCO Biosphere Reserve
➡️ Far north of the Equator
❌ Patos Lagoon
• Located in southern Brazil
• Lies around 30°S latitude
• Situated well within the Southern Hemisphere
Extra info:
- Largest lagoon in Brazil
- Second largest lagoon in South America
- Separated from Atlantic Ocean by sand barriers
➡️ Far south of the Equator
🧮 Logical Elimination (UPSC Style)
• Lake Tanganyika → South of Equator
• Lake Tonlé Sap → North of Equator
• Patos Lagoon → Far South of Equator
➡️ Equator passes through none of the given water bodies
🎯 Final Answer
✅ Correct Answer: (d) None
🔗 Extra Info Points (Prelims Gold)
• Equator passes through:
- Lake Victoria (Africa)
- Amazon Basin (South America)
- Indonesia (island chains)
• UPSC often confuses:
- Lake Tanganyika ❌ (south of Equator)
- Lake Victoria ✅ (crossed by Equator)
🗺️ Map-Based Visual Reinforcement
📍 GS Mapping
GS Paper I
Subject: Geography
Theme: Latitude, Physical Geography
Nature: Conceptual + Map-based Elimination
🗺️ Single Annotated Equator World Map — Revision-Ready (UPSC Prelims)
🔎 How to read this map in 20–30 seconds
Trace the red line (0° latitude) once, left to right.
✅ Must-remember crossings (High-yield)
South America:
Ecuador (country) → Amazon Basin / Amazon River
Africa:
Gulf of Guinea → Congo Basin / Congo River → Lake Victoria
Southeast Asia:
Indonesia (Sumatra, Borneo/Kalimantan, Sulawesi)
Oceans:
Atlantic → Indian → Pacific
❌ Classic traps (near but NOT on Equator)
Lake Tanganyika (entirely south)
Tonlé Sap (Cambodia, ~13°N)
Patos Lagoon (Brazil, ~30°S)
🧠 Kill-switch rules (UPSC elimination)
Rivers & basins love the Equator; long rift lakes usually don’t.
Brazil often signals Tropic of Capricorn, not the Equator.
If Indonesia appears, suspect Equator unless specified otherwise.
📌 Ultra-short mantra
EQUATOR = Amazon + Congo + Indonesia (+ Lake Victoria)
🧠 Final Memory Lock
Equator loves rivers and basins, not long rift lakes.