Acid rain is no threat to lakes in areas where the rock is limestone (calcium carbonate), which can neutralize the acid. Where the rock is granite, however, no neutralization occurs. How does limestone neutralize acid?
The correct answer and explanation is:
Correct Answer:
Limestone neutralizes acid because it contains calcium carbonate (CaCO₃), which reacts with acids to form carbon dioxide, water, and a neutral salt.
Explanation:
Limestone is composed mainly of calcium carbonate (CaCO₃), a compound that acts as a natural buffer against acidic substances. When acid rain, which contains sulfuric acid (H₂SO₄) or nitric acid (HNO₃), falls on a region with limestone bedrock, a chemical reaction occurs between the acid and the calcium carbonate. The carbonate reacts with the hydrogen ions (H⁺) from the acid, neutralizing it and forming carbon dioxide gas (CO₂), water (H₂O), and a salt such as calcium sulfate (CaSO₄) or calcium nitrate (Ca(NO₃)₂).
The neutralization reaction for sulfuric acid is:
CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O
This reaction removes the acidic hydrogen ions from the water, which helps maintain a stable pH level in lakes and streams. The resulting water is less acidic and therefore less harmful to aquatic organisms. This buffering capacity of limestone is what protects lakes and aquatic life in areas where limestone is present in the soil or bedrock.
In contrast, granite does not contain calcium carbonate or any other effective neutralizing minerals. As a result, in granite-rich areas, acid rain is not neutralized and leads to a drop in the pH of lakes and rivers. The lower pH can harm or kill fish and other aquatic life, disrupt reproductive processes, and dissolve toxic metals like aluminum from surrounding rocks, making the water even more dangerous.
Therefore, regions with limestone are better protected from the harmful effects of acid rain, while granite regions are more vulnerable due to the lack of natural neutralization.