CDS (II) 2022 GK Q91: Which one of the following processes leads to expansion of

Q: What is the answer to CDS (II) 2022 GK Q91?

B. Correct — B, Hydration. Hydration is the chemical weathering process in which a mineral takes water molecules into its own crystal structure, and the hydrated mineral that results occupies more room than the mineral it replaced. That swelling pushes outward from inside the rock, opening cracks and setting up the additional stress the stem describes, and repeated wetting and drying makes the damage cumulative until the rock granulates and flakes away. The textbook case is anhydrite, a calcium sulphate, which takes up water and becomes gypsum with a marked increase in volume; clay minerals swell the same way, which is why a clay-rich rock breaks down quickly under an alternating wet and dry climate.

CDS (II) 2022 General Knowledge · Question 91 of 120

Question text verified against the official CDS question paper.

Which one of the following processes leads to expansion of certain minerals as they take up water, causing additional stress in the rock?

(a)Hydrolysis
(b)Hydration
(c)Oxidation
(d)Carbonation

Answer

Why

Correct — B, Hydration. Hydration is the chemical weathering process in which a mineral takes water molecules into its own crystal structure, and the hydrated mineral that results occupies more room than the mineral it replaced. That swelling pushes outward from inside the rock, opening cracks and setting up the additional stress the stem describes, and repeated wetting and drying makes the damage cumulative until the rock granulates and flakes away. The textbook case is anhydrite, a calcium sulphate, which takes up water and becomes gypsum with a marked increase in volume; clay minerals swell the same way, which is why a clay-rich rock breaks down quickly under an alternating wet and dry climate.

Why the others are wrong

(a)Hydrolysis — Hydrolysis is a decomposition rather than a swelling. The water itself is split, and its hydrogen ions displace cations out of the mineral lattice, so the mineral is chemically converted — feldspar to clay is the standard example. The rock is weakened by being changed into something else, not by being pushed apart from within.
(c)Oxidation — Oxidation is the combination of oxygen with iron-bearing minerals, and it shows itself in the rusty brown and red staining on weathered rock faces. Oxygen and not water is the reagent, so it fails the stem's condition that the minerals take up water.
(d)Carbonation — Rainwater dissolves carbon dioxide to make a weak carbonic acid, which attacks limestone and turns it into soluble calcium bicarbonate that is carried away. That removes rock rather than expanding it — it is the process behind caves, sinkholes and the whole karst landscape.

Concept

Weathering is the breakdown of rock in place, and it is sorted into physical, chemical and biological groups. The chemical group has four members worth knowing by name. Hydration is the taking up of water into a mineral, with expansion. Hydrolysis is the splitting of water so that its ions enter the lattice and convert the mineral. Oxidation is the addition of oxygen, chiefly to iron. Carbonation is attack by carbonic acid, which dissolves carbonates outright. Hydration straddles the boundary between the chemical and the physical, because the chemical change produces a mechanical stress.

Every word in the stem is placed to select one process. Take up water rules out oxidation and carbonation; expansion of minerals rules out hydrolysis, which converts rather than swells; and additional stress in the rock names the mechanical consequence that only hydration produces. An NDA paper of 2021 puts the same definition almost word for word and reaches the same answer, which is a useful sign that this is a definition worth memorising exactly rather than paraphrasing.

Key facts

Hydration is the absorption of water into a mineral's structure, and the hydrated mineral occupies a larger volume.
Anhydrite taking up water to become gypsum is the standard example, and swelling clays behave the same way.
Hydrolysis splits the water molecule and converts the mineral, as when feldspar alters to clay.
Oxidation adds oxygen, chiefly to iron-bearing minerals, and leaves rusty staining.
Carbonation uses the weak carbonic acid of rainwater to dissolve limestone, producing karst features.

Four chemical weathering processes, and what each one actually does

Hydration→Water is taken into the mineral structure; the mineral swells and stresses the rock from inside. Anhydrite to gypsum.

Hydrolysis→Water is split and its ions enter the lattice; the mineral is converted. Feldspar to clay.

Oxidation→Oxygen combines with iron-bearing minerals. Rusty brown and red staining on the rock face.

Carbonation→Carbonic acid dissolves limestone into soluble bicarbonate. Caves, sinkholes and karst.

Only one of the four both takes in water and increases the volume, which is what the stem asks for.

Study next

Physical weathering — frost shattering, salt crystallisation, exfoliation and insolationKarst landforms produced by carbonation, above ground and belowHow climate controls which kind of weathering dominatesWeathering as the first step in soil formation, and the soil profile that resultsMass wasting and erosion, and how they differ from weathering

Common traps

Confusing hydration with hydrolysis; one swells the mineral, the other converts it.
Calling every water-related process chemical; frost shattering uses water but is physical.
Assuming chemical weathering is slow everywhere; it is fastest in the warm, wet tropics.

As a name-the-process item like this one, or as a which-of-these-is-not-chemical-weathering question.

Related PYQs

UPSC_2024_GS1_Q72024Open & attempt →

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

The same family of processes reached from the rain. Dissolved carbon dioxide is what drives carbonation and dissolved oxygen is what drives oxidation, so that item covers two of the three wrong options here while this one isolates the fourth.

NDA_GAT_2021_I_Q1112021Open & attempt →

The process whereby certain minerals absorb water, expand and change is called as

(a) Hydration.
(b) Oxidation.
(c) Hydrolysis.
(d) Carbonation.

Answer(a) Hydration.

The same definition with the same four options, a year earlier. Absorbing water, expanding and changing is hydration in both papers, and seeing the wording repeated almost verbatim is a strong hint to learn the definition as it stands.

Practice

practice — not a real PYQ
The conversion of feldspar into clay minerals by the action of water is an example of which one of the following?
- (a)Hydration
- (b)Hydrolysis
- (c)Oxidation
- (d)Carbonation
Answer(b) Hydrolysis — the water molecule is split and its ions replace cations in the lattice, converting the mineral.
practice — not a real PYQ
Which one of the following is NOT a process of chemical weathering?
- (a)Carbonation
- (b)Oxidation
- (c)Frost shattering
- (d)Hydration
Answer(c) Frost shattering — water freezing in a crack expands and prises the rock apart mechanically, with no chemical change.