Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements: (1) PAN is secondary air pollutant (2) BOD and COD values are indicators of water pollution level. Which of the statements given above is/are correct? (a) both 1 and 2 (b) 1 only (c) 2 only (d) neither 1 nor 2

Step-by-Step Solution

Answer

(a) both 1 and 2

Step 1: Understand the Definition of Secondary Air Pollutant

A secondary air pollutant is not emitted directly. Instead, it forms in the atmosphere when primary pollutants react or interact. Peroxyacyl Nitrate (PAN) is an example, as it forms from the reaction of nitrogen oxides and volatile organic compounds (VOCs) in the presence of sunlight.

Step 2: Analyze Statement 1

Statement 1 claims that PAN is a secondary air pollutant. Based on our understanding from Step 1, this statement is correct because PAN forms from reactions between primary pollutants.

Step 3: Understand Indicators of Water Pollution

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are measures used to assess the level of organic pollutants in water. Higher values indicate more pollution.

Step 4: Analyze Statement 2

Statement 2 claims that BOD and COD values are indicators of water pollution level. Given our understanding from Step 3, this statement is correct because both BOD and COD values directly correlate with water pollution levels.

Step 5: Evaluate the Given Options

Now, based on our analysis, both statements are correct. Thus, the correct option is (a) both 1 and 2.

Key Concepts

Secondary Air Pollutants

When we talk about air pollution, it's essential to understand that not all pollutants are directly emitted into the air. Some pollutants are formed as a result of chemical reactions that occur in the atmosphere. These are known as secondary air pollutants.

Primary air pollutants, like sulfur dioxide or nitrogen oxides, are released directly into the atmosphere from sources such as car exhaust or industrial emissions. Secondary pollutants, on the other hand, are not directly emitted but are formed when primary pollutants react with other substances in the atmosphere.

• This can involve reactions with sunlight, as in the case of photochemical smog.
• Peroxyacyl Nitrate (PAN) is a classic example of a secondary pollutant. It forms when nitrogen oxides react with volatile organic compounds (VOCs) in the presence of sunlight.

Understanding secondary air pollutants is essential as they often contribute significantly to air quality issues and can have harmful effects on health and the environment.

Water Pollution Indicators

Water quality is a critical concern worldwide, and determining the level of pollution in water bodies is vital for environmental protection and public health.

There are several indicators used to measure water pollution, two of the most important being:

• Biochemical Oxygen Demand (BOD): This measures the amount of oxygen required by aerobic microorganisms to break down organic material in the water. High BOD levels suggest a high level of organic pollution, indicating that the water body is heavily contaminated.
• Chemical Oxygen Demand (COD): Unlike BOD, COD measures the total quantity of oxygen needed to chemically oxidize both biodegradable and non-biodegradable substances in a water sample. COD is beneficial because it provides a more comprehensive indication of pollution levels.

These indicators are crucial for assessing the quality of water and determining necessary measures for pollution control.

Biochemical Oxygen Demand (BOD)

BOD is a widely used indicator for evaluating the degree of organic pollution in water. It is measured over a period of five days at a specified temperature, usually 20°C.

The significance of BOD lies in its ability to reflect the amount of degradable organic matter in water, which directly correlates to the level of pollution.

Some key points about BOD include:

• BOD is utilized in wastewater treatment facilities to assess the effectiveness of treatment processes.
• High BOD values indicate more organic matter present in the water, requiring more oxygen to break it down, which can lead to oxygen depletion.
• This measure helps in understanding the kind of stress aquatic ecosystems might be under due to human activity.

Monitoring BOD is thus an essential part of maintaining water quality and ensuring aquatic life can thrive.

Chemical Oxygen Demand (COD)

COD provides a more rapid and complete measure of water's overall level of pollution by organic chemicals than BOD.

Unlike BOD, COD does not differentiate between biodegradable and non-biodegradable organic matter, allowing it to represent the total demand placed on oxygen. This characteristic makes COD particularly useful in industrial applications where water may contain substances not easily broken down by microbes.

Important aspects of COD include:

• COD testing is faster than BOD, taking only a few hours to yield results.
• It is often used in conjunction with BOD for a more comprehensive assessment of water quality.
• Like BOD, high COD values suggest a high level of pollution, potentially harming aquatic life.

By providing insights into the complete profile of organic pollutants in water, COD is an invaluable tool in monitoring and managing water resources.