Which event is an example of chemical weathering?
A Rocks falling off the face of a steep cliff
B. Feldspar in granite being crushed into clay-sized particles
C. Water freezing in cracks in a roadside outcrop
D. Acid rain reacting with limestone bedrock
The Correct Answer and Explanation is :
The correct answer is D. Acid rain reacting with limestone bedrock.
Explanation:
Chemical weathering refers to the process by which rocks and minerals undergo chemical reactions that change their composition and structure. This type of weathering typically involves interactions with water, air, or acidic substances that lead to the breakdown or alteration of minerals within the rock.
Option D describes a classic example of chemical weathering. Acid rain, which is rainwater that has been contaminated with sulfuric and nitric acids from pollutants in the air, reacts with the minerals in limestone, particularly calcium carbonate (CaCO₃). When acid rain comes into contact with limestone, a chemical reaction occurs, where the calcium carbonate reacts with the acids, breaking down into calcium ions, carbon dioxide, and water. This reaction results in the dissolution of the limestone, which can lead to the rock’s weakening and eventual breakdown over time.
The chemical reaction for this process is as follows:
[
\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{CO}_2 + \text{H}_2\text{O}
]
This is a form of chemical weathering because the limestone is chemically altered and dissolved by the acids in the rain, rather than being physically broken apart.
Why the other options are incorrect:
- Option A (Rocks falling off the face of a steep cliff): This describes physical weathering (specifically mechanical weathering) where rocks break off due to gravity, not a chemical reaction.
- Option B (Feldspar in granite being crushed into clay-sized particles): This describes a physical breakdown of the feldspar mineral into smaller particles. While feldspar may later undergo chemical weathering, this process itself is physical.
- Option C (Water freezing in cracks in a roadside outcrop): This is an example of freeze-thaw weathering, a type of mechanical weathering where water seeps into cracks in rocks and freezes, causing the rock to fracture. It does not involve a chemical change.
In conclusion, acid rain reacting with limestone is an example of chemical weathering because it involves the dissolution and chemical alteration of the rock through a reaction with acids.