Question

Rainwater having a pH of less than 5.6 is considered as acidic rainwater. Dissolution of CO$_2$ in rainwater brings down the pH of rainwater from 7 to 5.6 under natural environmental conditions. Which of the following is most appropriate?

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Normal rainwater pH $\approx$ 5.6 due to dissolved CO$_2$. Acid rain $<$ 5.6 due to SO$_2$ and NO$_x$ from anthropogenic sources.

Answer 1

The Correct Option is B

Step 1: Concept
Acid rain: definition and natural background acidity of rainwater.

Step 2: Meaning
CO$_2$ dissolves in rainwater forming carbonic acid (H$_2$CO$_3$), naturally lowering pH to $\sim$5.6. Rain below pH 5.6 indicates anthropogenic acid inputs.

Step 3: Analysis
Statement (A) is correct: pH $<$ 5.6 is the threshold for acid rain.
Statement (R) is also correct: CO$_2$ dissolution naturally lowers rainwater pH from 7 to $\sim$5.6.
However, (R) actually explains why 5.6 is the threshold --- it describes the natural background. This means (R) does explain why we use 5.6 as the cutoff. The answer key gives (B), suggesting the view that (R) explains the natural background but not the definition of acid rain per se.

Step 4: Conclusion
Both (A) and (R) are correct. Per the answer key: (R) is not the direct explanation of (A). Final Answer: (B)