Given below are two statements.
Statement I The pH of rain water is normally ~ 5.6.
Statement II If

Q: Why isn't normal rain water perfectly neutral with a pH of 7?

Even in unpolluted air, carbon dioxide is naturally present. It dissolves in rain water to form carbonic acid, a weak acid, which lowers the pH to about 5.6.

Q: Does a pH of 5.6 mean the rain is harmful?

Not at all! A pH of 5.6 is perfectly natural and safe. It's only when the pH drops significantly below this level that it becomes harmful to the environment.

Q: What gases are responsible for acid rain?

The primary culprits are sulfur dioxide ($\mathrm{SO}_2$) and nitrogen oxides ($\mathrm{NO}_x$), which are released from burning fossil fuels in factories and vehicles.

Q: How do these gases lower the pH?

They react with water and oxygen in the atmosphere to form strong acids, specifically sulfuric acid ($\mathrm{H}_2\mathrm{SO}_4$) and nitric acid ($\mathrm{HNO}_3$). These strong acids dissociate completely, releasing many $\mathrm{H}^+$ ions and dropping the pH.

Q: Can acid rain have a pH of 6?

No. By definition, acid rain has a pH lower than the natural baseline of 5.6. A pH of 6 is actually less acidic than normal rain!

Question

Given below are two statements.
Statement I The pH of rain water is normally ~ 5.6.
Statement II If the pH of rain water drops below 5.6, it is called acid rain.
In the light of the above statements, choose the correct answer from the options given below.

Options:

(A) Both statements I and II are true.

(B) Both statements I and II are false.

(C) Statement I is true but statement II is false.

(D) Statement I is false but statement II is true.

Sigma Percentile · Accepted Answer

### The Myth of Neutral Rain

When we think of pure rain falling from the sky, it is tempting to imagine it as perfectly neutral water with a pH of exactly 7.0. However, nature has a different plan.

Even in the most pristine, unpolluted environments on Earth, rain is naturally slightly acidic.

Why does this happen? The answer lies in the composition of our atmosphere.

### The Role of Carbon Dioxide

Our atmosphere naturally contains carbon dioxide ($\mathrm{CO}_2$). As raindrops fall through the air, they absorb some of this gas.

When carbon dioxide dissolves in water, it undergoes a chemical reaction to form **carbonic acid** ($\mathrm{H}_2\mathrm{CO}_3$).

This reaction can be written as:
$$ \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3 $$

Carbonic acid is a weak acid, meaning it only partially dissociates in water. However, it releases enough hydrogen ions ($\mathrm{H}^+$) to lower the pH of the rain from a neutral 7.0 down to approximately **5.6**.

Therefore, a pH of 5.6 is considered the natural baseline for normal rain. This confirms that **Statement I is absolutely true**.

### The Threat of Acid Rain

The story changes when human activities enter the picture. Industrial processes, power plants, and vehicles release large amounts of pollutants into the air.

The primary culprits are **sulfur dioxide** ($\mathrm{SO}_2$) and **nitrogen oxides** ($\mathrm{NO}_x$).

Unlike carbon dioxide, when these gases mix with water and oxygen in the atmosphere, they form strong acids: **sulfuric acid** ($\mathrm{H}_2\mathrm{SO}_4$) and **nitric acid** ($\mathrm{HNO}_3$).

Because these are strong acids, they dissociate completely, flooding the rainwater with a high concentration of hydrogen ions.

### Crossing the Threshold

This massive influx of $\mathrm{H}^+$ ions pushes the pH of the rainwater significantly lower than the natural baseline.

When the pH of rain drops **below 5.6**, it crosses the threshold from normal rain into what we officially classify as **acid rain**.

Acid rain can have devastating effects on ecosystems, corroding buildings, and harming aquatic life. This confirms that **Statement II is also perfectly true**.

Since both statements are factually correct, the right choice is option (a).

Select Answer:

Visualized Solution

Normal Rain pH

Why is Normal Rain Acidic?

Acid Rain

pH Drop

Conclusion

The Sigma Insight: Atmospheric Pollution

The Myth of Neutral Rain

The Role of Carbon Dioxide

The Threat of Acid Rain

Crossing the Threshold

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