CBSE Class 11 Chemistry Chapter 14: Environmental Chemistry NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 11 PDF

CBSE Class 11 Chemistry Chapter 14, Environmental Chemistry, introduces students to the vital aspects of our environment. This chapter explores critical topics like the greenhouse effect and its link to global warming, detailing the gases involved and their impact. It further explains the formation and composition of both photochemical and classical smog, common air pollutants that affect urban areas. The solutions also highlight the importance of Biochemical Oxygen Demand (BOD) as a key indicator of water pollution and discuss the role of the ozone layer in protecting life on Earth, along with the factors contributing to its depletion. These NCERT solutions offer clear, concise explanations, making complex environmental chemistry concepts accessible and aiding students in their preparation for examinations.

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Quick info

Board	CBSE
Class	Class 11
Subject	Chemistry Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 14

Chapter summary

Chapter 14 of the CBSE Class 11 Chemistry syllabus focuses on Environmental Chemistry. This NCERT Solutions set addresses multiple-choice questions related to air pollution, including greenhouse gases, photochemical smog, and classical smog. It also covers water pollution, explaining the concept of Biochemical Oxygen Demand (BOD) and its implications for water quality. The solutions clarify the role of the ozone layer and its depletion, providing a solid foundation for understanding environmental challenges.

Learning outcomes

Identify greenhouse gases and their impact.
Differentiate between photochemical and classical smog.
Understand the concept and significance of BOD in water quality assessment.
Explain the formation and effects of air and water pollution.
Describe the role of the ozone layer and the causes of its depletion.

Topics covered

Paper topics

Environmental Chemistry
Greenhouse Gases
Global Warming
Photochemical Smog
Classical Smog
Air Pollution
Biochemical Oxygen Demand (BOD)
Water Pollution
Ozone Layer
Ozone Depletion
Stratosphere
Troposphere

Important topics

Greenhouse Effect and Gases
Photochemical vs. Classical Smog
Biochemical Oxygen Demand (BOD)
Ozone Layer Depletion
Components of Air Pollution

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Questions and Solutions

Q. 1

Which of the following gases is not a green house gas?

(a) CO (b) $O_3$

Solution: Greenhouse gases are those that absorb and re-emit infrared radiation, trapping heat near the Earth's surface and contributing to the greenhouse effect. Common greenhouse gases include carbon dioxide (

CO_2

), water vapour (

H_2O

), methane (

CH_4

), ozone (

O_3

), and oxides of nitrogen. Carbon monoxide (CO) is not typically classified as a primary greenhouse gas, although it can indirectly affect atmospheric composition. Therefore, CO is the correct answer.

Q. 2

Photochemical smog occurs in warm, dry and sunny climate. One of the following is not amongst the components of photochemical smog, identify it.

(a) $NO_2$ (b) $O_3$

Solution: Photochemical smog, also known as Los Angeles smog, is formed in warm, dry, and sunny conditions. It results from photochemical reactions involving nitrogen oxides (

NO_x

) and volatile organic compounds (VOCs), including unsaturated hydrocarbons, in the presence of sunlight. These reactions produce pollutants such as ozone (

O_3

), peroxyacetyl nitrate (PAN), and aldehydes. Sulphur dioxide (

SO_2

) is a major component of classical smog, not photochemical smog. Thus,

SO_2

is not among the components of photochemical smog.

Q. 3

Which of the following statements is not true about classical smog?

Its main components are produced by the action of sunlight on emissions of automobiles and factories
Produced in cold and humid climate
It contains compounds of reducing nature
It contains smoke, fog and sulphur dioxide

Solution: Classical smog, also known as London smog or reducing smog, occurs in cold and humid climates. It is a mixture of smoke, fog, and sulphur dioxide (

SO_2

), and it contains compounds of a reducing nature. The statement that its main components are produced by the action of sunlight on emissions of automobiles and factories is incorrect; this description pertains to photochemical smog. Therefore, the statement that is not true about classical smog is the first one.

Q. 4

Biochemical Oxygen Demand, (BOD) is a measure of organic material present in water. BOD value less than 5 ppm indicates a water sample to be

(a) rich in dissolved oxygen

(b) poor in dissolved oxygen

(d) not suitable for aquatic life

Solution: Biochemical Oxygen Demand (BOD) is the measure of the amount of oxygen consumed by microorganisms (like bacteria) to decompose the organic matter present in a sample of water over a specific period, typically five days at 20°C. A low BOD value, such as less than 5 ppm, indicates that there is a relatively small amount of organic matter in the water. Consequently, the microorganisms consume less oxygen, meaning the water sample is rich in dissolved oxygen and is considered to be relatively pure and suitable for aquatic life.

Q. 5

Which of the following statement(s) is/are wrong?

(a) Ozone is not responsible for green house effect

(b) Ozone can oxidise sulphur dioxide present in the atmosphere to sulphur trioxide

(d) Ozone is produced in upper stratosphere by the action of UV rays on oxygen

Solution: Let's analyze each statement:

Ozone is not responsible for green house effect: This statement is wrong. Ozone ( $O_3$ ) is a greenhouse gas and contributes to the warming of the Earth, although its contribution is less significant than that of $CO_2$ or $H_2O$ vapor.
Ozone can oxidise sulphur dioxide present in the atmosphere to sulphur trioxide: This statement is correct. Ozone is a strong oxidizing agent and can react with $SO_2$ to form $SO_3$ , which can then form sulfuric acid aerosols.
Ozone hole is thinning of ozone layer present in stratosphere: This statement is correct. The 'ozone hole' refers to the significant depletion of the ozone layer, particularly over the polar regions, which occurs in the stratosphere.
Ozone is produced in upper stratosphere by the action of UV rays on oxygen: This statement is correct. In the upper stratosphere, UV radiation splits oxygen molecules ( $O_2$ ) into free oxygen atoms (O), which then combine with other oxygen molecules to form ozone ( $O_3$ ).

Therefore, the wrong statement is (a).

Q. 6

Sewage containing organic waste should not be disposed in water bodies because it causes major water pollution. Fishes in such a polluted water die because of

(a) large number of mosquitoes

(b) increase in the amount of dissolved oxygen

(d) clogging of gills by mud

Solution: When sewage containing organic waste is discharged into water bodies, microorganisms, primarily bacteria, begin to decompose this organic matter. This decomposition process requires a significant amount of dissolved oxygen present in the water. As the microorganisms consume the dissolved oxygen, its concentration in the water decreases drastically. Aquatic life, especially fish, requires dissolved oxygen for respiration. A severe depletion of dissolved oxygen, often measured by a high BOD, makes the water unsuitable for fish and other aquatic organisms, leading to their death.

Q. 7

Which of the following statements about photochemical smog is wrong?

(a) It has high concentration of oxidising agents

(b) It has low concentration of oxidising agent

(d) Plantation of some plants like pinus helps in controlling photochemical smog

Solution: Photochemical smog is characterized by the presence of high concentrations of oxidizing agents. These are formed through photochemical reactions involving nitrogen oxides (

NO_x

) and volatile organic compounds (VOCs) in the presence of sunlight. Key oxidants include ozone (

O_3

) and peroxyacetyl nitrate (PAN). Therefore, the statement that photochemical smog has a low concentration of oxidizing agents is incorrect. Controlling the release of

NO_2

and hydrocarbons is a primary method for its control, and certain plants can help mitigate its effects.

Common mistakes

Confusing the components of photochemical and classical smog.
Misinterpreting the meaning of BOD values.
Incorrectly identifying gases that contribute to the greenhouse effect.
Not distinguishing between the causes of classical smog and emissions from vehicles/factories.

Revision tips

Focus on the definitions and differences between various types of smog.
Memorize the key components of the greenhouse effect and the gases involved.
Understand the practical implications of BOD levels for water bodies.
Review the role of the ozone layer and the chemical processes involved in its depletion.

Practice MCQs

Q1. Which of the following gases is NOT considered a greenhouse gas?

Q2. Which of the following is NOT a component of photochemical smog?

Q3. Classical smog is typically formed in:

Q4. A water sample with a BOD value less than 5 ppm is generally considered:

Q5. Which statement about ozone is incorrect?

Q6. Fishes in water bodies polluted with sewage die mainly due to:

Q7. Photochemical smog is characterized by:

Frequently asked questions

What is Environmental Chemistry?

Environmental Chemistry studies the chemical phenomena occurring in the environment, focusing on pollution, its causes, effects, and control measures related to air, water, and soil.

What are the main types of smog discussed in this chapter?

The chapter discusses two main types of smog: photochemical smog, which forms in warm, sunny climates due to reactions involving sunlight, and classical smog (reducing smog), which forms in cold, humid climates and contains smoke, fog, and sulfur dioxide.

How is water pollution measured using BOD?

Biochemical Oxygen Demand (BOD) measures the amount of oxygen consumed by microorganisms to decompose organic matter in water. A lower BOD value indicates cleaner water, while a higher value signifies greater organic pollution.

Why is the ozone layer important?

The ozone layer, located in the stratosphere, absorbs most of the Sun's harmful ultraviolet (UV) radiation, protecting life on Earth from its damaging effects.

What causes the depletion of the ozone layer?

The depletion of the ozone layer is primarily caused by certain man-made chemicals, such as chlorofluorocarbons (CFCs), which release chlorine atoms that catalytically destroy ozone molecules.

Are all greenhouse gases harmful?

Greenhouse gases are essential for maintaining a habitable temperature on Earth by trapping some of the Sun's heat. However, an excessive increase in their concentration due to human activities leads to global warming and climate change.

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