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

The question asks about the reason why normal rainwater is slightly acidic and its pH value is indicated as 5.6. Let's evaluate the options and understand why the correct option is due to the dissolution of carbon dioxide in water, forming a weak acid.

1. Normal rainwater typically has a pH of around 5.6 due to the presence of carbonic acid (H₂CO₃) which is formed when carbon dioxide (CO₂) dissolves in water. This can be represented by the chemical equation: \(CO_2 + H_2O \rightarrow H_2CO_3\). The carbonic acid dissociates slightly, making the rainwater slightly acidic.
2. Let's evaluate the other reactions:
   • The reaction \(N_2O_5 + H_2O \rightarrow 2 HNO_3\) and \(4 NO_2 + O_2 + 2 H_2O \rightarrow 4 HNO_3\) result in the formation of nitric acid (HNO₃), which is a strong acid and would significantly lower the pH below 5.6 if it were the primary cause.
   • The reaction \(2 SO_2 + O_2 + 2 H_2O \rightarrow 2 H_2SO_4\) forms sulfuric acid (H₂SO₄), another strong acid that contributes to acid rain but not typical rainwater. This reaction also results in a pH much lower than 5.6.
3. Since normal rainwater has a pH of around 5.6, the slight acidity can be explained by the presence of carbonic acid alone, which forms from the dissolution of atmospheric CO₂ in raindrops.

Therefore, the correct answer is \(CO_2 + H_2O \rightarrow H_2CO_3\), making the correct option:

$CO_2 + H_2O \rightarrow H_2CO_3$

This shows how natural atmospheric processes can impact the chemical nature of rainwater.