UPSC Prelims 2024 Q7: Rainwater, CO₂, Oxygen and Rock Weathering
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?
- Both Statement-II and Statement-III are correct and both of them explain Statement-I
- Both Statement-II and Statement-III are correct, but only one of them explains Statement-I
- Only one of the Statements II and III is correct and that explains Statement-I
- 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
Statement-wise explanation
- Statement-I: correct — Rain is a main agent of chemical weathering. NCERT's Class XI chapter 'Geomorphic Processes' says water and air (oxygen and carbon dioxide), helped by heat, must be present to speed up the chemical reactions that weather rock. Rainwater also soaks into cracks and pores, where it carries out solution, carbonation, hydration and oxidation.
- Statement-II: correct — As rain falls, it dissolves carbon dioxide from the air, and more from soil air, which forms weak carbonic acid (H₂O + CO₂ → H₂CO₃). This acid dissolves calcium and magnesium carbonates in limestone and dolomite. This process is called carbonation, and over time it forms caves and karst landscapes. Statement-II therefore explains Statement-I.
- Statement-III: correct — Rainwater also holds dissolved atmospheric oxygen. In weathering, oxidation means a mineral combining with oxygen to form oxides or hydroxides. NCERT notes that it happens where rock is exposed to the atmosphere and to oxygenated water. Iron is the classic example: iron-bearing minerals rust into red-brown ferric oxides, which weakens the rock. Statement-III therefore also explains Statement-I.
How to eliminate
Statement-I is basic geography: weathering by rain is common knowledge, so the question is only about II and III. Both are simple chemistry. Air contains both CO₂ and O₂, and water in contact with air dissolves some of each gas. If you know that, both statements are true, which rules out the two options that call one or both false. Then ask whether each dissolved gas does anything to rock. CO₂ gives carbonation (carbonic acid dissolves limestone) and O₂ gives oxidation (iron minerals rust). Those are two named processes of chemical weathering, so both statements explain Statement-I. Facts to remember: carbonation needs dissolved CO₂, oxidation needs dissolved O₂, and rainwater carries both.
Know more
Weathering is the breakdown of rock in place, with little or no movement. NCERT groups it into three types: physical (mechanical), chemical and biological. Chemical weathering changes the minerals themselves. It works fastest where there is plenty of water and heat, so it dominates in hot, humid regions like India's Western Ghats and the North-East.
Rainwater drives most chemical weathering because it carries dissolved gases and acids into rock.
| Process | What reacts | Typical result |
|---|---|---|
| Solution | Water dissolves soluble minerals | Rock salt and gypsum dissolve; nitrates and chlorides are leached |
| Carbonation | CO₂ + water → carbonic acid acting on carbonates | Limestone and dolomite dissolve; caves, sinkholes, karst |
| Oxidation | Dissolved O₂ + iron, manganese, sulphur minerals | Red-brown oxides; rock crumbles (laterite colour) |
| Reduction | Oxygen removed, e.g. below the water table | Iron turns grey-green (ferrous) |
| Hydration | Minerals take up water | Minerals swell; anhydrite becomes gypsum |
| Hydrolysis | Water reacts with silicates | Feldspar turns into clay (kaolinite) |
Links worth remembering
- Karst topography comes from carbonation. Examples are limestone caves with stalactites and stalagmites, such as the Borra Caves (Andhra Pradesh) and the caves of Meghalaya.
- Laterite soils form under heavy rain and high temperature. Silica is leached out and iron and aluminium oxides stay behind, which gives the red colour.
- Acid rain is rain made more acidic by sulphur dioxide and nitrogen oxides. It speeds up carbonation-type attack on marble and limestone, which is why it threatens monuments like the Taj Mahal.
- Climate link: silicate weathering by carbonic acid takes CO₂ out of the atmosphere over geological time. Oxidation of rock carbon and sulphide minerals can release CO₂.
NCERT Concepts to Revise
- Chemical weathering: The question asks whether gases dissolved in rainwater can chemically break down rock, which is the core idea of chemical weathering.
- Carbonation: Carbonation is the weathering process tied to carbon dioxide in rainwater, the gas named in Statement-II.
- Oxidation: Oxidation is the weathering process tied to oxygen in rainwater, the gas named in Statement-III.