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 rain water without the thunderstorm.

Step 1: Understanding pH and Thunderstorms

The pH value measures how acidic or basic a solution is. Normal rainwater is slightly acidic due to dissolved carbon dioxide forming carbonic acid. Thunderstorms introduce more gases, such as nitrogen oxides, which can form stronger acids.

Step 2: Analyzing Rainwater During Thunderstorms

When rain is accompanied by a thunderstorm, the production of nitrogen oxides from lightning can lead to the formation of nitric acid in the rainwater. This acid formation tends to lower the pH of the rainwater compared to regular rainwater.

Step 3: Choosing the Best Option

Given the effect of thunderstorms on rainwater composition, the pH is typically lowered due to the presence of stronger acids. Therefore, the correct answer is (b) slightly lower than that of rain water without the thunderstorm.

Key Concepts

Understanding pH Value

The concept of pH is essential when talking about the acidity or basicity of solutions, like rainwater. The pH scale ranges from 0 to 14, where:

• A pH of 7 is considered neutral.
• A pH below 7 indicates acidity.
• A pH above 7 indicates basicity.

Measuring the pH value of rain helps us determine how acidic the raindrops are. Normal rainwater typically has a slightly acidic pH value of about 5.6. This slight acidity comes from carbon dioxide in the atmosphere dissolving into the raindrops to form carbonic acid, a weak acid.
Understanding the role of pH in environmental contexts is crucial. It tells us how human activities or natural phenomena, like thunderstorms, can influence the rain's acidity.

Nitrogen Oxides in Air

Nitrogen oxides, often referred to as NOx, are significant contributors to air pollution and acid rain. These gases include nitric oxide (NO) and nitrogen dioxide (NO₂). They are products of combustion processes in vehicles and industries and can be naturally formed during thunderstorms from the immense heat of lightning.
When nitrogen oxides are in the atmosphere, they can react with other chemicals to form nitric acid (HNO₃). This nitric acid can dissolve in rain, increasing its acidity and consequently lowering its pH value.

• Nitric oxide is relatively harmless by itself.
• It becomes hazardous when converted into more reactive species like nitrogen dioxide.

The presence of nitrogen oxides is thus a crucial factor in determining the chemical composition of rainwater.

Impact of Thunderstorms

Thunderstorms are intense local weather events coupled with lightning and thunder. They play a significant role in affecting the atmospheric chemistry and altering rainwater composition. Due to lightning, there is a significant amount of nitrogen oxides produced during a thunderstorm.
The energy from lightning breaks nitrogen and oxygen molecules apart in the atmosphere. These atoms recombine to form nitrogen oxides.

• The NOx gases can lead to the formation of nitric acid.
• This acid formation contributes to making rainwater more acidic.

As a result, the presence of a thunderstorm often lowers the pH of rainwater, distinguishing it from normal rain that does not accompany such storms.

Rainwater Acidity

Rainwater acidity is influenced by various factors, from both natural and human-made sources. Typically, rain is slightly acidic, but its acidity can increase in particular scenarios like thunderstorms. The dissolved gases in raindrops are the primary determinant of rainwater's pH.
When stronger acids like nitric acid form in the atmosphere, they dissolve in raindrops and increase acidity. Consequently, the pH value lowers, making the rainwater more acidic than usual.

• Acid rain can have significant environmental impacts.
• It can harm ecosystems by altering soil pH and affecting plant life.

Thus, monitoring rainwater acidity gives insights into how atmospheric conditions and pollution affect our environment.