Question

The normal rain water is slightly acidic and its $pH$ value is $56$ because of which one of the following ?

• A. $CO _2+ H _2 O \rightarrow H _2 CO _3$
• B. $N _2 O _5+ H _2 O \rightarrow 2 HNO _3$
• C. $2 SO _2+ O _2+2 H _2 O \rightarrow 2 H _2 SO _4$
• D. $4 NO _2+ O _2+2 H _2 O \rightarrow 4 HNO _3$

Hint:

The acidity of rainwater is primarily due to carbon dioxide, not industrial pollutants.

Answer 1

The Correct Option is A

We are aware that normally rain water has a $pH$ of $5.6$ due to the presence of $H^{+}$ions formed by the reactions of rain water with carbon dioxide present in the atmosphere.
$H_{2}O(l)+C{O}_2(g)\leftrightharpoons H_{2}C{O}_3(aq)$
$H_{2}C{O}_3(aq)\leftrightharpoons H^{+}(aq)+HC{O}_3^{-}(aq)$
So , the correct option is (A) : $CO _2+ H _2 O \rightarrow H _2 CO _3$

Answer 2

Rainwater is naturally slightly acidic due to the dissolution of carbon dioxide (\( \text{CO}_2 \)) in water, forming carbonic acid (\( \text{H}_2\text{CO}_3 \)):

\[ \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \]

This weak acid dissociates to give a small concentration of \( \text{H}^+ \) ions, lowering the pH of the rainwater to about 5.6.

The other options are related to pollutants like NO\(_2\), SO\(_2\), and N\(_2\)O\(_5\), which are not the primary cause of the natural acidity of rainwater.