The process whereby certain minerals absorb water, expand and change is called as
- (a)Hydration.
- (b)Oxidation.
- (c)Hydrolysis.
- (d)Carbonation.
Correct — A, Hydration. Hydration is the chemical-weathering process in which minerals take up (absorb) water molecules into their crystal structure, causing them to swell and change form — for example, anhydrite (CaSO4) takes up water to become gypsum (CaSO4·2H2O), and the accompanying expansion sets up stresses that help break the rock apart. The definition given (absorb water, expand and change) is the textbook description of hydration.
- (b)Oxidation — Oxidation is the addition of oxygen to minerals (e.g., iron-bearing minerals 'rusting' to reddish oxides); it involves oxygen, not the absorption of water and swelling.
- (c)Hydrolysis — Hydrolysis is a chemical reaction between minerals and water in which the mineral is decomposed into new substances (e.g., feldspar altering to clay) — it breaks the mineral down rather than simply absorbing water and expanding.
- (d)Carbonation — Carbonation is the reaction of carbonic acid (from CO2 dissolved in rainwater) with carbonate minerals, dissolving rocks such as limestone; it is dissolution by acid, not water absorption and expansion.
Chemical weathering alters rocks through reactions with water, air and dissolved substances. Its main processes are hydration (absorption of water with swelling), hydrolysis (mineral decomposition by reaction with water), oxidation (combination with oxygen), carbonation (attack by carbonic acid) and solution (dissolving in water). Each acts on different minerals and produces different results.
All four options are chemical-weathering processes, so the choice turns on precise definitions. The clue words 'absorb water, expand and change' point specifically to hydration — the only process defined by water uptake and swelling rather than chemical breakdown or oxidation.
- Hydration: minerals absorb water into their structure and expand (anhydrite → gypsum).
- Hydrolysis: water reacts with and decomposes minerals (feldspar → clay minerals).
- Oxidation: minerals combine with oxygen (iron minerals rust to oxides).
- Carbonation: carbonic acid dissolves carbonate rocks such as limestone (forming karst).
The 'absorb water and expand' definition matches hydration.
- Confusing hydration (water absorbed, swelling) with hydrolysis (water reacts and decomposes the mineral).
- Treating all water-related processes as the same — the definition's key words decide which one.
Asked as 'name the process that does X' or by matching a weathering process to its example (limestone–carbonation, iron–oxidation).
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 II and III are correct and both explain I — dissolved CO2 (carbonation) and O2 (oxidation) drive chemical weathering.
Same topic — chemical weathering via rainwater's dissolved gases (carbonation, oxidation), the family of processes to which hydration belongs.
Which one among the following processes is NOT part of chemical weathering?
- (a) Solution
- (b) Hydration
- (c) Reduction
- (d) Thawing
Answer(d) Thawing
Directly related — it confirms hydration is a chemical-weathering process and separates it from physical processes like thawing, the exact classification tested here.
- practice — not a real PYQ
The chemical weathering of limestone by carbonic acid to form karst landforms is called
- (a)Hydration
- (b)Oxidation
- (c)Carbonation
- (d)Exfoliation
Answer(c) Carbonation — carbonic acid in rainwater dissolves carbonate rocks like limestone.
- practice — not a real PYQ
The reddish-brown colour of many iron-rich rocks on weathering is mainly due to
- (a)Hydrolysis
- (b)Oxidation
- (c)Carbonation
- (d)Frost action
Answer(b) Oxidation — iron minerals combine with oxygen to form reddish iron oxides.