Analysis of Statements on Acidic Rainwater

Assertion (A): Acidic Rainwater Definition

The statement defines rainwater with a pH less than 5.6 as acidic. This is the widely accepted threshold for classifying rainwater as acidic. Therefore, Assertion (A) is correct.

Reason (R): Natural pH of Rainwater

Unpolluted rainwater naturally contains dissolved carbon dioxide ($\text{CO}_2$) from the atmosphere. This $\text{CO}_2$ reacts with water ($\text{H}_2\text{O}$) to form carbonic acid ($\text{H}_2\text{CO}_3$), following the equilibrium:

$ \text{CO}_2(g) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_2\text{CO}_3(aq) $

Carbonic acid is a weak acid, and its presence lowers the pH of pure water (which is neutral at pH 7) to approximately 5.6 under normal atmospheric conditions. Thus, Reason (R) accurately describes the natural pH of rainwater.

Evaluating the Explanation

While Reason (R) correctly explains that $\text{CO}_2$ dissolution leads to a natural rainwater pH of about 5.6, it does not fully explain why rainwater is considered *acidic* (i.e., pH < 5.6).

• Assertion (A) defines acidic rainwater based on a pH threshold (< 5.6).
• Reason (R) describes the natural process that establishes the baseline pH (around 5.6).
• Acidic rainwater (pH significantly below 5.6) is typically caused by the dissolution of stronger atmospheric pollutants like sulfur dioxide ($\text{SO}_2$) and nitrogen oxides ($\text{NO}_x$), which form stronger acids (sulfuric acid, nitric acid).

Therefore, Reason (R) describes a natural condition but doesn't serve as the correct explanation for the phenomenon of *acidic* rainwater, which implies a pH lower than this natural baseline due to additional acidic pollutants.

Conclusion

Both statements (A) and (R) are factually correct. However, the natural pH lowering effect of $\text{CO}_2$ (Reason R) sets the baseline, but it is not the direct cause or explanation for why rainwater becomes *acidic* (pH < 5.6), which is usually due to other pollutants.