Question

In areas with low air pollution, rain water generally has a pH of $\sim$5–6 because of

• A. $H_2SO_4$
• B. $H_2CO_3$
• C. $HNO_3$
• D. $HNO_2$

Hint:

- Normal rainwater pH $\approx 5.6$ (due to $CO_2$) - Acid rain (polluted areas) → pH $< 5$ due to $H_2SO_4$, $HNO_3$

Answer 1

The Correct Option is B

Concept: Even in unpolluted air, rainwater is slightly acidic due to the dissolution of carbon dioxide ($CO_2$) from the atmosphere, forming carbonic acid.

Step 1: Formation of carbonic acid.
\[ CO_2 + H_2O \rightarrow H_2CO_3 \]

Step 2: Effect on pH.
Carbonic acid is a weak acid, which lowers the pH of rainwater to around 5–6.

Step 3: Analyze other options.

• $H_2SO_4$ and $HNO_3$ → Strong acids formed due to pollution (acid rain)
• $HNO_2$ → Also due to pollutants

Step 4: Final conclusion.
Thus, the slight acidity of natural rainwater is due to: \[ H_2CO_3 \]