Chapter 16: Environmental Chemistry – Chemistry 2nd Year Notes

This chapter covers Environmental Chemistry from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Environmental chemistry studies the chemical processes in our surroundings, the air, water and soil, and how human activities affect them. This chapter looks at the atmosphere, the main air and water pollutants, and the major environmental problems: acid rain, ozone depletion and the greenhouse effect.

It also introduces the control of pollution and the idea of green chemistry.

Learning Objectives

• Describe the main layers of the atmosphere and their importance.
• Identify the major air pollutants and their sources.
• Explain the causes and effects of acid rain.
• Explain ozone depletion and the role of CFCs.
• Explain the greenhouse effect and global warming.
• Describe water pollution and simple ways to control pollution.

Key Concepts

The Atmosphere

The atmosphere is divided into layers. The lowest is the troposphere, where we live and where weather occurs. Above it is the stratosphere, which contains the ozone layer that absorbs most of the harmful ultraviolet radiation from the Sun. Higher still are the mesosphere and the thermosphere. The atmosphere protects life, provides the gases we breathe and keeps the Earth warm enough to live on.

Air Pollution and Acid Rain

Air pollution is the presence of harmful substances in the air; most comes from burning fossil fuels. Major pollutants are carbon monoxide (poisonous, from incomplete burning), sulphur dioxide and oxides of nitrogen (from burning coal, oil and fuels), carbon dioxide (a greenhouse gas), CFCs (which destroy ozone) and smoke particles. When fuels containing sulphur and nitrogen are burned, they release sulphur dioxide and oxides of nitrogen, which react with oxygen and water vapour to form sulphuric and nitric acids that fall as acid rain. Acid rain damages plants and soil, corrodes buildings and statues, and kills fish.

Ozone Depletion and the Greenhouse Effect

The ozone layer in the stratosphere absorbs harmful ultraviolet radiation. Chlorofluorocarbons (CFCs), once used in refrigerators and aerosols, rise into the stratosphere where ultraviolet light breaks them to release chlorine atoms that destroy ozone, thinning the layer and letting more ultraviolet reach the Earth. In the lower atmosphere, greenhouse gases such as carbon dioxide, methane and water vapour let sunlight through but trap much of the heat the Earth radiates back; this natural greenhouse effect keeps the planet warm, but burning huge amounts of fossil fuels has raised carbon dioxide levels, trapping more heat and causing global warming.

Water Pollution and Its Control

Water pollution is caused by sewage, industrial waste, fertilizers and pesticides washing into rivers and lakes; it spreads disease and kills aquatic life. Pollution can be reduced by treating sewage and industrial waste before release, using cleaner fuels and catalytic converters in vehicles, controlling fertilizer and pesticide use, recycling waste and planting trees. Practising green chemistry, designing processes that make less waste and use safer chemicals, is an important long-term solution.

Important Definitions

Formulas & Rules

Item	Fact
Atmosphere layers	troposphere, stratosphere (ozone), mesosphere, thermosphere
Acid rain	sulphur dioxide and oxides of nitrogen give sulphuric and nitric acids
Ozone depletion	caused by CFCs releasing chlorine atoms
Greenhouse gases	carbon dioxide, methane, water vapour, CFCs
Control	cleaner fuels, catalytic converters, treat waste, plant trees

Diagrams & Illustrations

Layers of the atmosphere: a labelled diagram of the troposphere, stratosphere (with the ozone layer), mesosphere and thermosphere.

The greenhouse effect: a diagram showing sunlight reaching the Earth and heat being trapped by greenhouse gases, leading to warming.

Acid rain formation: a flow chart from burning fuels (sulphur dioxide and oxides of nitrogen) to acids forming in the air and falling as acid rain.

Solved Examples & Numericals

Example 1: Acid rain

Which gases are mainly responsible for acid rain, and what acids do they form? Sulphur dioxide and oxides of nitrogen, which react with oxygen and water to form sulphuric acid and nitric acid.

Example 2: Ozone layer

Why are CFCs harmful to the ozone layer? In the stratosphere ultraviolet light breaks CFCs to release chlorine atoms, which destroy ozone molecules and thin the ozone layer.

Example 3: Greenhouse gas

Name the main greenhouse gas and its chief source. Carbon dioxide; its chief source is the burning of fossil fuels in vehicles, factories and power stations.

Short Questions & Answers

Long Questions & Answers

Q1: Explain the causes and effects of acid rain.

Acid rain is rain that has become acidic because of pollutant gases in the air. Its main causes are the burning of fossil fuels containing sulphur and nitrogen in power stations, factories and vehicles, which release sulphur dioxide and oxides of nitrogen. High in the air these gases react with oxygen and water vapour to form sulphuric acid and nitric acid, which dissolve in the rain and fall to the ground. The effects are serious: acid rain makes soils and lakes acidic, damaging plants, killing fish and upsetting ecosystems; it strips nutrients from the soil and harms forests; and it attacks buildings, bridges and statues, especially those made of marble and limestone. Acid rain can be reduced by removing sulphur from fuels, trapping sulphur dioxide before it escapes, and using cleaner energy.

Q2: Explain ozone depletion and the greenhouse effect.

The ozone layer high in the stratosphere is vital because it absorbs most of the harmful ultraviolet radiation from the Sun. Ozone depletion is the thinning of this layer, caused mainly by chlorofluorocarbons that were once used in refrigerators, air-conditioners and aerosol sprays. When they drift into the stratosphere, ultraviolet light breaks them apart to release chlorine atoms, and each chlorine atom can destroy many ozone molecules, creating thin areas known as ozone holes; this lets more ultraviolet reach the surface, increasing skin cancer and eye damage, which is why CFCs have been banned. The greenhouse effect is a separate problem in the lower atmosphere. Gases such as carbon dioxide, methane and water vapour let sunlight through to warm the Earth but trap much of the heat it radiates back, keeping the planet warm; however, the huge amounts of carbon dioxide from burning fossil fuels have strengthened this effect, trapping more heat and causing global warming that melts glaciers, raises sea levels and changes the climate.

Q3: Discuss the sources of air pollution and how it can be controlled.

Air pollution is the presence of harmful substances in the atmosphere, and most of it comes from burning fossil fuels. Vehicles release carbon monoxide, oxides of nitrogen and unburnt hydrocarbons; power stations and factories burning coal and oil release sulphur dioxide and carbon dioxide; and various industries release smoke and dust. These pollutants cause breathing diseases, acid rain, smog, ozone depletion and global warming. Air pollution can be controlled by using cleaner fuels such as natural gas and removing sulphur from fuels; by fitting catalytic converters to vehicles to convert harmful gases into less harmful ones; by trapping smoke and gases from factory chimneys; by banning harmful chemicals such as CFCs; by recycling waste to reduce burning; and by planting trees, which absorb carbon dioxide and release oxygen. Practising green chemistry, designing processes that produce less waste and use safer materials, is an important long-term solution.

MCQs with Answers

Quick Revision Summary

• Atmosphere: troposphere, stratosphere (ozone), mesosphere, thermosphere.
• Main pollutants: carbon monoxide, sulphur dioxide, oxides of nitrogen, carbon dioxide, CFCs, particles.
• Acid rain: sulphur dioxide and oxides of nitrogen give sulphuric and nitric acids.
• Ozone depletion: CFCs release chlorine that destroys ozone.
• Greenhouse effect: carbon dioxide and methane trap heat, causing global warming.
• Control: cleaner fuels, catalytic converters, treat waste, ban CFCs, plant trees.

Exam Tips

• Match each pollutant to its source and harmful effect.
• For acid rain, name the gases (sulphur dioxide and oxides of nitrogen) and the acids.
• Distinguish ozone depletion (CFCs, ultraviolet) from the greenhouse effect (carbon dioxide, heat).
• Remember the ozone layer is in the stratosphere and blocks ultraviolet.
• List concrete control measures in pollution questions.