---
title: "Physical and chemical weathering"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:physical-and-chemical-weathering"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/physical-and-chemical-weathering
site: BCT Engineering
---

# Physical and chemical weathering

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2081 Kartik (new course), 3 marks; 2081 Bhadra, 3 marks; 2076 Chaitra, 1.5 marks; 2078 Bhadra, 4 marks; 2076 Asoj, 4 marks (asked 5 times)
- Video: [Physical (Mechanical) Weathering vs. Chemical Weathering](https://www.youtube.com/watch?v=xKzYk4Siq68) — chemistryinaminute
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/physical-and-chemical-weathering
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

Discuss in brief the physical and chemical weathering processes (differentiate between chemical and physical weathering).

## Short answer

Physical weathering mechanically breaks rock (frost wedging, exfoliation, thermal stress) without mineral change. Chemical weathering alters minerals through solution, hydrolysis, oxidation, and carbonation, forming clays and soluble salts; it dominates in warm, wet climates.

## Answer

**Weathering** is the breakdown of rocks in place by physical and chemical processes.

**Physical (mechanical) weathering** breaks rock into smaller pieces without changing its mineral composition. Main processes:

- **Frost wedging:** water freezes in joints and expands about 9%, prying the rock apart. Common in the high Himalaya.
- **Thermal expansion and contraction (insolation):** repeated day-night heating and cooling stresses the surface minerals.
- **Exfoliation (unloading):** removal of overlying rock releases pressure, so sheets peel off massive rocks such as granite.
- **Salt crystallisation:** salts growing in pores push the grains apart (arid and coastal areas).
- **Biological action:** plant roots widen cracks; burrowing animals expose fresh rock.

**Chemical weathering** changes the minerals of the rock into new, more stable minerals (mostly clay) and soluble salts. It is fastest in warm, wet climates. Main processes:

- **Solution:** soluble minerals such as rock salt and gypsum dissolve in water.
- **Carbonation:** CO$_2$ in rain forms carbonic acid, which dissolves limestone: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$. This produces karst.
- **Hydrolysis:** feldspar reacts with water to form clay: $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_2 + 2K^+$ (orthoclase to kaolinite).
- **Oxidation:** iron minerals combine with oxygen and turn into rust-coloured oxides, e.g. $4FeO + O_2 \rightarrow 2Fe_2O_3$.
- **Hydration:** minerals take up water and swell, e.g. anhydrite to gypsum, hematite to limonite.

#### Differences
| Point | Physical weathering | Chemical weathering |
|---|---|---|
| Change | Only size and shape | Mineral composition also changes |
| Agent | Temperature, frost, pressure, roots | Water, CO$_2$, O$_2$, acids |
| Climate | Cold and dry, or hot desert | Warm and humid |
| Product | Angular fragments, gravel, sand | Clay, soluble salts, oxides |
| Rock affected | Massive rocks with joints | Limestone, feldspar rich rocks |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the main difference between physical and chemical weathering?
   A. Physical changes minerals; chemical only breaks rock
   B. Physical needs water; chemical needs none
   C. Chemical weathering is fastest in cold dry climates
   D. Physical only breaks rock; chemical changes its mineral composition
   Answer: D. Physical weathering changes only size and shape; chemical weathering forms new minerals such as clay. (Key point: Key difference)

2. Why does frost wedging break rock?
   A. Water freezes in joints and expands about 9%, prying the rock apart
   B. Ice dissolves the minerals
   C. Ice reacts with iron to form rust
   D. Frozen water shrinks and leaves gaps
   Answer: A. Expansion of freezing water in cracks forces the joint wider. (Key point: Frost wedging)

3. What does carbonation do to limestone?
   A. Oxygen turns limestone into rust-coloured oxide
   B. CO2 in rain forms carbonic acid that dissolves CaCO3 into Ca(HCO3)2, producing karst
   C. Feldspar changes to kaolinite
   D. Limestone swells by taking up water into gypsum
   Answer: B. Carbonic acid dissolves limestone into soluble calcium bicarbonate. (Key point: Carbonation)

4. What is the product of hydrolysis of orthoclase feldspar?
   A. Fe2O3
   B. Gypsum
   C. Kaolinite clay, silica and K+ ions
   D. Angular gravel of unchanged feldspar
   Answer: C. Feldspar reacts with water and H+ to give clay (kaolinite) with silica and potassium ions. (Key point: Hydrolysis)

## Related in Engineering Geology I

- [Chemical versus physical weathering; factors](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Achemical-versus-physical-weathering-factors) — Engineering Geology I, chapter 6
- [Volcanism; chemical weathering](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Avolcanism-chemical-weathering) — Engineering Geology I, chapter 6
- [Weathering versus erosion](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aweathering-versus-erosion) — Engineering Geology I, chapter 6, asked 5×

## Related in other subjects

- [Physical and chemical causes of deterioration](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aphysical-chemical-causes-deterioration) — Concrete Technology and Masonry Structure, chapter 6, asked 5×
- [Physical and chemical disintegration](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aphysical-chemical-disintegration) — Soil Mechanics, chapter 1
- [Physical and chemical characteristics of solid waste](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Asolid-waste-characteristics) — Environment and Pollution Control, chapter 5