Question

What is the pH of acid rain? A) less than \(5.6\) B) between \(5.6\) and \(7.0\) C) greater than \(7.0\) D) greater than \(9.5\)

Short answer

A) less than 5.6

Step-by-step solution

Understand What pH Represents

The pH scale is a measure of how acidic or basic a solution is. It ranges from 0 to 14, with values below 7 indicating an acidic solution, values exactly at 7 indicating a neutral solution, and values above 7 indicating a basic solution.

Recall Typical pH Values for Rainwater

Normal, unpolluted rain has a pH of about 5.6 due to the natural presence of carbon dioxide in the atmosphere, which forms carbonic acid. Thus, normal rain is slightly acidic.

Define Acid Rain

Acid rain is defined as rainwater that has a pH value lower than that of typical rainwater (i.e., less than 5.6). This occurs when rainwater mixes with pollutants like sulfur dioxide and nitrogen oxides, forming stronger acids.

Determine the Correct Answer

Since acid rain has a pH of less than 5.6, the correct answer to the question 'What is the pH of acid rain?' is option A: less than 5.6.

Key concepts

Understanding the pH Scale

The pH scale is essential for understanding the acidity or basicity of any solution, including rainwater. It is a numerical scale that ranges from 0 to 14. On this scale, a value:

• Below 7 indicates that the solution is acidic.
• Exactly at 7 suggests a neutral solution, like pure water.
• Above 7 means the solution is basic or alkaline.

Acids have more hydrogen ions (\( ext{H}^+ \)) compared to bases, which have more hydroxide ions (\( ext{OH}^- \)). For instance, if we measure acid rain and find a pH below 5.6, it confirms its acidic nature, as normal rainwater itself is slightly acidic.

The Nature of Rainwater

Rainwater is naturally slightly acidic, with a pH around 5.6. This slight acidity is due to the interaction between atmospheric carbon dioxide and water, creating carbonic acid:

• Rainwater absorbs carbon dioxide while falling through the atmosphere.
• This reaction leads to the formation of weak carbonic acid (\( ext{H}_2 ext{CO}_3 \)), which lowers the pH to around 5.6.

Thus, unpolluted rain is mildly acidic but considered normal and not harmful in small amounts. However, when the pH drops significantly below this point, it indicates the presence of acid rain.

Impact of Sulfur Dioxide on Rain

Sulfur dioxide (\( ext{SO}_2 \)) is a significant contributor to the formation of acid rain. It is released into the atmosphere through various activities:

• Burning of fossil fuels like coal and oil in power plants and industrial factories.
• Volcanic eruptions and some natural sources also release \( ext{SO}_2 \) into the air.

Once in the atmosphere, sulfur dioxide reacts with water vapor to form sulfuric acid (\( ext{H}_2 ext{SO}_4 \)), a much stronger acid than carbonic acid. This dramatically lowers the pH of rainwater, especially when this acid is well-mixed in rain clouds.

Role of Nitrogen Oxides

Nitrogen oxides (\( ext{NO}_x \)), including nitrogen dioxide (\( ext{NO}_2 \)), also play a pivotal role in acid rain formation. These compounds are emitted from:

• Vehicle exhausts and industrial combustion processes.
• Natural sources such as lightning strikes contributing to nitrogen oxides.

When nitrogen oxides interact with atmospheric moisture, they form nitric acid (\( ext{HNO}_3 \)). The addition of nitric acid further decreases the pH value of rain, making it more acidic. This process not only affects rainfall but also impacts soil and water bodies, leading to more extensive ecological harm.

Formation and Impact of Carbonic Acid

Carbonic acid (\( ext{H}_2 ext{CO}_3 \)) forms when water (\( ext{H}_2 ext{O} \)) absorbs carbon dioxide (\( ext{CO}_2 \)) from the atmosphere. This process is a natural part of the water cycle:

• \( ext{CO}_2 \) dissolves in rainwater forming \( ext{H}_2 ext{CO}_3 \).
• This weak acid naturally lowers the pH to about 5.6, characterizing normal rain as slightly acidic.

However, when excess anthropogenic activities increase \( ext{CO}_2 \) through emissions, it can lead to slight increases in overall rain acidity. Despite being weak, when combined with stronger acids like sulfuric and nitric, the resulting rain becomes harmful to the environment and structures.