Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

When rain is accompanied by a thunderstorm, the collected rain water will have a pH value (a) which depends upon the amount of dust in the air (b) slightly lower than that of rain water without the thunderstorm (c) slightly higher than that when the thunderstorm is not here (d) uninfluenced by occurrence of thunderstorm

Step-by-Step Solution

Answer

(b) slightly lower than that of rainwater without the thunderstorm.

Step 1: Understanding pH and Acid Rain

pH is a measure of acidity or alkalinity of a solution, with lower pH values indicating higher acidity. Rainwater is naturally slightly acidic due to dissolved carbon dioxide forming weak carbonic acid. However, certain conditions can make it even more acidic, known as 'acid rain.'

Step 2: Analyzing the Effect of a Thunderstorm

Thunderstorms can introduce additional substances into the air, such as nitrogen oxides. Lightning in thunderstorms contributes significantly to forming nitrogen oxides, which then dissolve in rainwater and form nitric acid. This can lower the pH of rainwater further than regular rain.

Step 3: Comparing Scenarios with and Without Thunderstorms

Regular rain without thunderstorms already has a slightly acidic nature. In comparison, thunderstorm-induced rain has a greater chance of having additional acidic compounds due to the nitrogen oxides forming acids in the rainwater, potentially making the rain more acidic.

Step 4: Choosing the Correct Answer

The thunderstorm contributes to an increased amount of acidic content (like nitric acid) in the rainwater, which lowers its pH further than normal rainwater without a thunderstorm. Thus, the correct choice is that when rain is accompanied by a thunderstorm, the pH value is 'slightly lower than that of rainwater without the thunderstorm.' Option (b) is the correct answer.

Key Concepts

The pH Scale and Its Importance

The pH scale is a measurement system that ranges from 0 to 14, used to indicate how acidic or basic a substance is. A pH value below 7 is considered acidic, while a value above 7 indicates a basic or alkaline solution. A pH of 7 is neutral, representing pure water.
Acid rain typically has a pH less than normal rainwater, which might naturally have a pH of around 5.6 due to carbon dioxide forming carbonic acid. Lower pH values in rainwater mean higher acidity, which can occur when pollutants are present.
The scale helps in identifying the changes in rainwater's acidity due to pollutants such as nitrogen oxides, often accelerated by thunderstorms. Understanding this scale allows us to grasp how even slight shifts in pH can mean significant changes in environmental conditions and ecosystem health.

How Thunderstorms Influence Rain

Thunderstorms are natural weather phenomena characterized by lightning and heavy rain. They play a crucial role in influencing the properties of rainwater. One significant effect thunderstorms have is their ability to alter the chemical makeup of the atmosphere.
Lightning in thunderstorms can break down nitrogen molecules in the air. This leads to the formation of nitrogen oxides. These are highly reactive gases, including nitric oxide and nitrogen dioxide, that can dissolve in rainwater and lower its pH.

• Lightning generates enough energy to split nitrogen molecules, creating nitrogen oxides.
• Nitrogen oxides dissolve in atmospheric moisture, forming nitric acid.
• This nitric acid then becomes part of the rain, further lowering the pH of the rainwater compared to normal rain.

This process demonstrates how thunderstorms can lead to more acidic rain through the formation of additional acidic compounds.

Role of Nitrogen Oxides in Acid Rain

Nitrogen oxides are a group of gases essential in many atmospheric processes, often contributed to by human activities and naturally through thunderstorms. These gases are a mix of nitric oxide (NO) and nitrogen dioxide (NO₂), which contribute significantly to air pollution.
During thunderstorms, especially, these oxides are naturally generated due to the high energy from lightning. Once formed, nitrogen oxides can dissolve in water droplets in the atmosphere, forming nitric acid—a key component of acid rain.

• As nitrogen oxides react with water, they form acids like nitric acid.
• These acids lower the pH of water droplets, resulting in acid rain.
• Increased acidity due to these oxides affects not just rainwater but can harm ecosystems, aquatic life, and infrastructure.

Understanding the role of nitrogen oxides highlights their importance in environmental science, showcasing the need for accurate monitoring and mitigation strategies to manage air quality and environmental health.