🌍 Environmental Chemistry

Secondary School Chemistry • Environment and Pollution

1. Introduction

Environmental chemistry studies the chemical substances and processes that occur in air, water and soil and how human activities affect the environment.

Learning Objectives

Explain different forms of environmental pollution.
Identify common air pollutants and their sources.
Explain acid rain and its effects.
Describe the greenhouse effect and climate change.
Explain water and soil pollution.
Describe eutrophication.
Explain methods of waste management.
Describe methods of water treatment.
Suggest ways of preventing environmental pollution.

2. Environmental Pollution

Pollution occurs when harmful substances or forms of energy are introduced into the environment at levels that cause harmful effects.

Type	Examples
Air pollution	Smoke, carbon monoxide, sulfur dioxide
Water pollution	Sewage, fertilisers, industrial chemicals
Soil pollution	Pesticides, plastics, heavy metals
Noise pollution	Excessive unwanted sound

3. Air Pollution

Air pollution occurs when harmful gases or particles enter the atmosphere.

Pollutant	Possible source	Effect
CO	Incomplete combustion	Reduces oxygen transport in blood
SO₂	Burning sulfur-containing fuels	Respiratory irritation and acid rain
NOₓ	High-temperature combustion	Acid rain and photochemical pollution
Particulates	Smoke, dust and combustion	Respiratory problems
CO₂	Combustion and respiration	Contributes to enhanced greenhouse effect

4. Carbon Monoxide

Carbon monoxide is produced mainly during incomplete combustion of carbon-containing fuels.

2C + O₂ → 2CO

Carbon monoxide is dangerous because it binds strongly to haemoglobin, reducing the blood's ability to transport oxygen.

Safety: Carbon monoxide is colourless and odourless, so dangerous concentrations may not be detected by human senses.

5. Sulfur Dioxide and Acid Rain

Sulfur dioxide can be released when sulfur-containing fuels are burned.

In the atmosphere, sulfur dioxide can undergo oxidation and react with water to form acidic substances.

SO₂ → sulfur oxides → sulfurous/sulfuric acids

Acid rain can:

Damage vegetation.
Acidify lakes and rivers.
Damage aquatic organisms.
Corrode metals.
Damage limestone and marble buildings.

6. Nitrogen Oxides

Nitrogen oxides are produced mainly during high-temperature combustion, such as in vehicle engines and some industrial processes.

They can contribute to acid deposition and photochemical pollution.

Exam idea: High combustion temperatures allow nitrogen and oxygen from air to react more readily, producing nitrogen oxides.

7. Greenhouse Effect

The greenhouse effect is the natural process by which certain gases in the atmosphere absorb and re-emit infrared radiation, helping to keep Earth warm.

Important greenhouse gases include:

Carbon dioxide
Methane
Water vapour
Nitrous oxide

The enhanced greenhouse effect occurs when human activities increase the concentration of greenhouse gases, contributing to global warming and climate change.

8. Global Warming and Climate Change

Global warming refers to the long-term increase in Earth's average temperature. Climate change is broader and includes long-term changes in temperature, rainfall, weather patterns and other climate characteristics.

Human activities that increase greenhouse gases

Burning coal, oil and other fossil fuels.
Deforestation.
Some agricultural activities.
Industrial processes.
Waste decomposition.

9. Ozone Layer

Ozone is a form of oxygen with the formula O₃. A region of the upper atmosphere contains relatively high concentrations of ozone and absorbs much of the Sun's harmful ultraviolet radiation.

Some substances, particularly chlorofluorocarbons (CFCs), can contribute to ozone depletion.

Reduced ozone protection can increase exposure to harmful ultraviolet radiation.

10. Water Pollution

Water pollution occurs when harmful substances enter water bodies and reduce water quality.

Pollutant	Possible source
Sewage	Domestic waste
Fertilisers	Agricultural runoff
Pesticides	Agricultural activities
Oil	Transport and industrial activities
Heavy metals	Mining and industry
Plastics	Poor waste disposal

11. Eutrophication

Eutrophication occurs when excessive nutrients, particularly nitrates and phosphates, enter a water body.

Process

Fertilisers or sewage add nutrients to water.
Algae grow rapidly.
An algal bloom develops.
Algae eventually die.
Microorganisms decompose the dead material.
Decomposition uses dissolved oxygen.
Oxygen concentration falls.
Aquatic organisms may die because of oxygen deficiency.

Key sequence:
Excess nutrients → algal bloom → decomposition → oxygen depletion → death of aquatic organisms.

12. Soil Pollution

Soil can become polluted by substances such as pesticides, plastics, industrial chemicals and heavy metals.

Possible effects include:

Reduced soil quality.
Harm to soil organisms.
Contamination of crops.
Contamination of groundwater.
Entry of toxic substances into food chains.

13. Waste Management

Waste management involves collecting, treating and disposing of waste in ways that reduce harm to people and the environment.

The waste hierarchy

Reduce
Reuse
Recycle
Recover
Dispose

Reducing waste at its source is generally preferable to disposing of it after it has been produced.

14. Recycling

Recycling involves processing waste materials so that they can be used again.

Material	Possible recycled product
Glass	New glass products
Paper	Recycled paper
Metal	New metal products
Some plastics	New plastic products

Recycling can reduce the amount of waste sent to disposal sites and conserve some raw materials.

15. Water Treatment

Water intended for drinking may require treatment before it is safe to consume.

Common stages

Screening to remove large objects.
Coagulation or flocculation where appropriate.
Sedimentation.
Filtration.
Disinfection.

Boiling can also make water microbiologically safer by killing many disease-causing microorganisms.

16. Sustainable Chemistry

Sustainable chemistry aims to reduce the environmental impact of chemical processes.

Reduce hazardous waste.
Use safer substances where possible.
Use energy efficiently.
Reduce unnecessary packaging.
Reuse and recycle materials.
Use renewable resources where appropriate.

17. Malawi Environmental Applications

Environmental chemistry can be applied to everyday problems such as waste disposal, water quality, agricultural runoff, deforestation, smoke from combustion and protection of water resources.

Example: Excess fertiliser washed from agricultural land into a lake or river can increase nutrient levels and contribute to eutrophication.

18. MANEB-style Questions

Question 1: Explain why carbon monoxide is dangerous to humans.

Question 2: Describe how excessive fertiliser use can contribute to fish deaths in a lake.

Question 3: State two effects of acid rain.

Question 4: Distinguish between the greenhouse effect and the enhanced greenhouse effect.

Answers:

Carbon monoxide binds strongly to haemoglobin and reduces the blood's ability to transport oxygen.
Nutrients enter the water, causing algal blooms. When the algae die, decomposition uses dissolved oxygen, reducing oxygen available to fish and other aquatic organisms.
Examples include damage to vegetation, acidification of water bodies and corrosion of materials.
The greenhouse effect is a natural warming process; the enhanced greenhouse effect results from increased concentrations of greenhouse gases, mainly due to human activities.

19. Practice Exercises

Define pollution.
Name two air pollutants.
What is produced during incomplete combustion of carbon?
Why is carbon monoxide dangerous?
What causes acid rain?
Name two greenhouse gases.
What is eutrophication?
State two effects of water pollution.
Give three methods of reducing waste.
State two stages involved in water treatment.

Answers

The introduction of harmful substances or energy into the environment at levels that cause harmful effects.
Examples: carbon monoxide, sulfur dioxide, nitrogen oxides and particulates.
Carbon monoxide and, depending on conditions, other products of incomplete combustion.
It reduces the blood's ability to transport oxygen.
Acidic substances formed from atmospheric pollutants such as sulfur and nitrogen oxides can be deposited with rain or other precipitation.
Carbon dioxide and methane.
Excessive nutrient enrichment of a water body that can lead to algal blooms and oxygen depletion.
Examples: harm to aquatic organisms and contamination of drinking water.
Reduce, reuse and recycle.
Examples: filtration and disinfection.

20. Common Mistakes

Do not confuse the natural greenhouse effect with the enhanced greenhouse effect.
Do not say that all greenhouse gases are pollutants.
Remember that eutrophication involves excess nutrients.
Do not confuse ozone depletion with global warming.
Give a clear chain of events when explaining eutrophication.

21. Exam Tips

For pollution questions, identify the pollutant, source, effect and control measure.
For eutrophication, remember the complete sequence from nutrients to oxygen depletion.
Use chemical equations when they strengthen an explanation.
When asked for environmental solutions, give practical and specific measures.