Consider the following statements : Statement-I: Rainfall is one of the reasons for weathering of rocks. Statement II: Rain water contains carbon dioxide in solution. Statement-III: Rain water contains atmospheric oxygen. Which one of the following is correct in respect of the above statements ?
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- ABoth Statement-II and Statement-III are correct and both of them explain Statement-I
- BBoth Statement-II and Statement-III are correct, but only one of them explains Statement-I
- COnly one of the Statements II and III is correct and that explains Statement-I
- DNeither Statement-II nor Statement-Ill is correct
Show answer
Answer: (A) Both Statement-II and Statement-III are correct and both of them explain Statement-I
Correct Answer: (a) Both Statement-II and Statement-III are correct, and BOTH explain Statement-I.
Statement I: Rainfall causes weathering of rocks — ✓ CORRECT.
Statement II: Rainwater contains dissolved CO₂ — ✓ CORRECT.
This makes it slightly acidic (carbonic acid), which dissolves rocks like limestone.
This is called 'solution' weathering.
Statement III: Rainwater contains dissolved oxygen — ✓ CORRECT.
This causes 'oxidation' weathering — for example, iron in rocks reacts with oxygen to form rust (iron oxide), which crumbles easily.
Both CO₂ (causing chemical dissolution) and oxygen (causing oxidation) in rainwater contribute to weathering, so both statements explain why rain weathers rocks.
Chemical weathering through rainwater creates major landforms like limestone caves, laterite soils, and contributes to rock decomposition across India's diverse climate zones.
UPSC is testing whether students understand the specific chemistry behind weathering - that rainwater is not pure H2O but contains dissolved gases that actively break down rocks.
The question checks if students can distinguish between correlation and causation - both CO2 and oxygen in rainwater directly cause weathering through different chemical processes.
Chemical Weathering Processes
Geography weathering carbon dioxide atmospheric oxygen
Chemical Weathering: Solution, Oxidation & Key Processes
Chemical weathering breaks down rocks through chemical reactions, not physical force
Solution weathering occurs when rainwater + CO₂ forms carbonic acid that dissolves rocks
Oxidation weathering happens when oxygen in rainwater rusts iron-bearing minerals
Both processes work together during rainfall to decompose rock structures
Why Rain Weathers Rocks
Rainwater is not pure H₂O — it contains dissolved gases from the atmosphere that make it chemically reactive. When rain falls, it acts like a weak acid that slowly dissolves rocks and causes metal minerals to rust and crumble.
Chemical Weathering Types
Process | Chemical Agent | How It Works | Rock Types Affected | End Product |
|---|---|---|---|---|
Solution | Carbonic acid (H₂CO₃) | Dissolves rock minerals directly | Limestone, marble, chalk | Dissolved ions in water |
Oxidation | Atmospheric oxygen (O₂) | Forms oxides that weaken rock structure | Iron-rich rocks, basalt | Iron oxide (rust), crumbling |
Hydrolysis | Water molecules | Breaks chemical bonds in minerals | Feldspar, granite | Clay minerals |
Carbonation | CO₂ + water | Forms weak carbonic acid | Calcareous rocks | Soluble carbonates |
Solution Weathering Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**CO₂ + Rainwater**
Atmospheric **carbon dioxide** dissolves in rainwater`"]
s2["`**Carbonic Acid Formation**
Creates **H₂CO₃** — a weak but effective acid`"]
s3["`**Rock Contact**
Acidic rainwater contacts **limestone/marble** surfaces`"]
s4["`**Chemical Dissolution**
Carbonic acid **dissolves calcium carbonate** in rock`"]
s5["`**Weathered Landscape**
Creates caves, **karst topography**, dissolved rock features`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5UPSC Testing Pattern
Questions often test which specific agents (CO₂, O₂, water) cause which type of weathering
Statement-based questions check if you know both gases in rainwater contribute to weathering
Oxidation vs Solution distinction is frequently tested — oxidation creates rust, solution dissolves completely
Rock-specific weathering — limestone weathers by solution, iron-rich rocks by oxidation
Trap: Thinking only CO₂ causes chemical weathering — oxygen also causes oxidation weathering
Trap: Confusing physical vs chemical weathering — rain's chemical action is different from freeze-thaw
Trap: Missing that both Statement II and III explain Statement I — students often pick only one mechanism
Trap: Assuming pure rainwater weathers rocks — it's the dissolved atmospheric gases that matter
Rainwater & Atmospheric Gases
Geography rain water carbon dioxide atmospheric oxygen
Rainwater Composition: CO₂, Oxygen & Chemical Properties
Rainwater contains dissolved CO₂ from atmosphere — makes it slightly acidic
Rainwater contains dissolved oxygen — enables oxidation reactions
Natural rainwater pH is ~5.6 due to carbonic acid formation
Why Rainwater Isn't Pure
As water evaporates and condenses in the atmosphere, it dissolves atmospheric gases. Carbon dioxide and oxygen are the most important dissolved gases that give rainwater its chemical weathering power.
Dissolved Gases in Rainwater
Gas | Source | Chemical Form | Effect on Rainwater | Weathering Role |
|---|---|---|---|---|
Carbon Dioxide | Atmospheric CO₂ | H₂CO₃ (carbonic acid) | Makes water slightly acidic | Solution weathering |
Oxygen | Atmospheric O₂ | Dissolved O₂ | Provides oxidizing agent | Oxidation weathering |
Nitrogen | Atmospheric N₂ | Dissolved N₂ | Largely inert in water | Minimal weathering effect |
Chemical Reactions
CO₂ + H₂O → H₂CO₃ — forms carbonic acid that dissolves carbonates
O₂ + iron minerals → iron oxides — causes rusting and rock breakdown
Natural pH of rainwater is around 5.6 (slightly acidic) due to CO₂
Acid rain (pH < 5.6) occurs when industrial pollutants further acidify rainwater
Trap: Thinking rainwater is chemically pure — it always contains dissolved atmospheric gases
Trap: Forgetting that natural rainwater is already acidic — CO₂ makes it pH ~5.6
Trap: Missing that both CO₂ and O₂ are present — questions test if you know both contribute
Physical vs Chemical Weathering
Geography weathering
Weathering Types: Physical vs Chemical Processes
Physical weathering breaks rocks without changing chemical composition
Chemical weathering changes rock's chemical structure through reactions
Both processes often work together — physical cracks expose more surface for chemical attack
Weathering Comparison
Type | Process | Rock Changes | Key Agents | Climate Preference | Examples |
|---|---|---|---|---|---|
Physical | Mechanical breakdown | Size reduces, composition same | Temperature, frost, salt | Cold/dry climates | Freeze-thaw, thermal expansion |
Chemical | Chemical reactions | Composition changes | Water, acids, oxygen | Warm/humid climates | Solution, oxidation, hydrolysis |
Biological | Living organisms | Both physical + chemical | Roots, acids from organisms | All climates | Root wedging, lichen acids |
Weathering Classification
# Rock Weathering
## Physical/Mechanical
- Freeze-thaw
- Thermal expansion
- Salt crystallization
- Pressure release
## Chemical
- Solution
- Oxidation
- Hydrolysis
- Carbonation
## Biological
- Root wedging
- Burrowing animals
- Organic acids
- Lichen activityTrap: Confusing rain-caused chemical weathering with physical freeze-thaw action
Trap: Thinking weathering = erosion — weathering breaks down in place, erosion moves material
Trap: Missing that tropical regions favor chemical weathering, polar regions favor physical