CAPF AC 2021 Q61: Geography – Physical Geography & Geomorphology
Which one of the following is not a type of chemical weathering?
- (a) Carbonation solution
- (b) Salt crystal growth
- (c) Hydrolysis
- (d) Oxidation
Answer: (b) Salt crystal growth
Explanation
Chemical and physical weathering
How to think: in chemical weathering the rock's minerals change into new substances.
In physical weathering the rock only breaks into pieces.
Growing salt crystals push the rock apart; nothing changes chemically.
| Process | Type | What happens |
|---|---|---|
| Solution | Chemical | Minerals dissolve in water |
| Carbonation | Chemical | Carbonic acid reacts with limestone |
| Hydrolysis and hydration | Chemical | Minerals react with or take up water |
| Oxidation and reduction | Chemical | Minerals combine with or lose oxygen, as in rusting |
| Salt crystal growth | Physical | Crystals grow in cracks and prise the rock apart |
| Frost action, unloading, thermal expansion | Physical | Rock breaks by stress |
Answer: (b) Salt crystal growth.
Why each option is right or wrong
- (a) Incorrect. Carbonation and solution are chemical weathering processes.
- (b) CORRECT. Salt crystal growth breaks the rock by pressure, so it is physical weathering.
- (c) Incorrect. Hydrolysis is a chemical reaction of minerals with water.
- (d) Incorrect. Oxidation is chemical; it is the rusting of iron-bearing minerals.
Key point
Chemical weathering: solution, carbonation, hydration, hydrolysis, oxidation and reduction. Physical weathering: unloading, temperature change, freeze-thaw, salt weathering. Biological: roots, burrowing, humans.
Remember
Salt crystal growth = physical weathering.
Common mistakes
- Thinking anything involving salt must be a chemical process.
- Mixing up hydration or hydrolysis with physical swelling of rocks.
- Missing the word not and marking a true chemical process.
Exam tip
Educated guess: carbonation, hydrolysis and oxidation all sound like chemistry lab reactions. Crystal growth is a mechanical push.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography, Chapter 6: Geomorphic Processes (weathering)
- NCERT Class VII Geography — Our Changing Earth