Which one among the following processes is NOT part of chemical weathering?
- (a)Solution
- (b)Hydration
- (c)Reduction
- (d)Thawing
Correct — D, Thawing. Thawing is one half of the freeze-thaw (frost) cycle, a physical/mechanical weathering process: water freezing in rock cracks expands by about 9% and prises the rock apart, with no change to its chemical composition. Solution, hydration and reduction, by contrast, all alter the rock chemically — so thawing is the odd one out.
- (a)Solution — Solution is chemical weathering — soluble minerals (rock salt, gypsum, and limestone in carbonic-acid rainwater) dissolve into water, chemically removing material from the rock.
- (b)Hydration — Hydration is chemical weathering — minerals chemically take up water into their structure (e.g. anhydrite → gypsum), swelling and weakening the rock.
- (c)Reduction — Reduction is chemical weathering — a gain of electrons / loss of oxygen that alters minerals (the reverse of oxidation), typical of waterlogged, oxygen-poor ground.
Weathering is the in-situ breakdown of rock. Chemical weathering changes the rock's mineral composition through processes such as solution, carbonation, hydration, hydrolysis and oxidation-reduction. Physical (mechanical) weathering breaks rock without chemical change — by freeze-thaw (frost action), exfoliation, thermal expansion and salt crystallisation.
The trap is that thawing involves water, so it 'feels' chemical. But freezing-and-thawing only exerts mechanical stress; the rock's chemistry is unchanged. The other three options each transform minerals chemically.
- Chemical weathering: solution, carbonation, hydration, hydrolysis, oxidation and reduction.
- Physical/mechanical weathering: freeze-thaw (frost), exfoliation, thermal expansion, salt weathering.
- In freeze-thaw, water expands ~9% on freezing and levers rock apart on repeated cycles.
- Chemical weathering dominates in warm, humid climates; frost action in cold, high-altitude climates.
Only thawing is mechanical, not chemical — so the answer is (d) Thawing.
- Thinking 'thawing involves water, so it must be chemical' — it is mechanical.
- Confusing reduction (a chemical process) with a physical one.
'Which is NOT chemical weathering' or matching a named process to the chemical/physical category.
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 ?
- (a) Both Statement-II and Statement-III are correct and both of them explain Statement-I
- (b) Both Statement-II and Statement-III are correct, but only one of them explains Statement-I
- (c) Only one of the Statements II and III is correct and that explains Statement-I
- (d) Neither Statement-II nor Statement-III is correct
Answer(a) Both Statement-II and Statement-III are correct and both of them explain Statement-I
Same concept — chemical weathering. UPSC's answer turns on rainwater carrying dissolved CO2 (carbonation) and O2 (oxidation), the chemical weathering agents; this NDA item asks which listed process is NOT chemical weathering (thawing).
- practice — not a real PYQ
Which one of the following is a process of physical (mechanical) weathering?
- (a)Carbonation
- (b)Hydrolysis
- (c)Frost action
- (d)Oxidation
Answer(c) Frost action — freeze-thaw breaks rock mechanically, without chemical change.
- practice — not a real PYQ
The chemical weathering of limestone by rainwater containing carbonic acid is called:
- (a)Hydration
- (b)Carbonation
- (c)Exfoliation
- (d)Reduction
Answer(b) Carbonation — carbonic acid dissolves limestone, a chemical weathering process.