What you'll learn
This topic examines the chemical composition of air and water, how human activities affect these vital resources, and the environmental consequences of pollution. You'll study the composition of clean air, various pollutants and their sources, methods of water purification, and the greenhouse effect. This content is frequently tested through data interpretation questions and practical application scenarios.
Key terms and definitions
Fractional distillation — the separation of a mixture of liquids by boiling and condensing at different temperatures based on their boiling points
Greenhouse gases — gases in the atmosphere that trap heat energy from the sun, including carbon dioxide, methane and water vapour
Acid rain — precipitation with a pH below 5.6, formed when sulfur dioxide and nitrogen oxides dissolve in rainwater to form acids
Eutrophication — the enrichment of water bodies with excess nutrients (nitrates and phosphates), leading to excessive algal growth and oxygen depletion
Catalytic converter — a device fitted to vehicle exhausts that converts harmful gases (carbon monoxide, nitrogen oxides) into less harmful products using transition metal catalysts
Potable water — water that is safe to drink, containing low levels of dissolved salts and microorganisms
Carbon footprint — the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or activity
Thermal pollution — the addition of excess heat to water bodies, which reduces dissolved oxygen levels and harms aquatic life
Core concepts
Composition of the atmosphere
Clean, dry air has a remarkably consistent composition:
- Nitrogen: approximately 78%
- Oxygen: approximately 21%
- Argon: approximately 0.9%
- Carbon dioxide: approximately 0.04%
- Water vapour: variable (0-4%)
- Noble gases (neon, helium, krypton, xenon): trace amounts
The proportions of nitrogen and oxygen have remained relatively stable for approximately 200 million years. Carbon dioxide levels, however, have increased significantly since industrialisation began in the 1800s, rising from about 0.028% to current levels above 0.04%.
Fractional distillation of liquid air separates these components industrially:
- Air is filtered to remove dust and carbon dioxide
- Air is compressed and cooled to approximately -200°C to liquefy it
- The liquid air is warmed in a fractionating column
- Nitrogen (boiling point -196°C) boils off first and is collected at the top
- Argon (boiling point -186°C) is collected at middle levels
- Oxygen (boiling point -183°C) remains as liquid and is collected at the bottom
This process produces pure gases for industrial use: oxygen for steelmaking and medical applications, nitrogen for creating inert atmospheres and manufacturing ammonia, and argon for filling light bulbs.
Air pollution and environmental impact
Carbon dioxide sources and effects:
Carbon dioxide enters the atmosphere through:
- Combustion of fossil fuels (coal, oil, natural gas) in power stations, vehicles and industry
- Respiration by living organisms
- Volcanic activity
- Decomposition of organic matter
Carbon dioxide is removed by:
- Photosynthesis in plants and algae
- Dissolving in oceans and lakes
Rising atmospheric CO₂ concentrations contribute to the greenhouse effect. Greenhouse gases allow short-wavelength radiation from the sun to pass through the atmosphere but absorb and re-radiate long-wavelength infrared radiation from Earth's surface, trapping heat. This leads to global warming with consequences including:
- Rising sea levels from thermal expansion and melting ice
- Changes in weather patterns and increased extreme weather events
- Habitat loss and species extinction
- Shifts in agricultural zones
Carbon monoxide:
This colourless, odourless toxic gas forms during incomplete combustion of carbon-containing fuels when oxygen supply is limited:
2C + O₂ → 2CO
Sources include vehicle exhausts, faulty gas appliances and cigarette smoke. Carbon monoxide is poisonous because it binds irreversibly to haemoglobin in red blood cells, preventing oxygen transport. Even small concentrations can be fatal.
Oxides of nitrogen:
Nitrogen and oxygen react at the high temperatures in vehicle engines and power stations:
N₂ + O₂ → 2NO
This nitrogen monoxide oxidises further to nitrogen dioxide (NO₂):
2NO + O₂ → 2NO₂
Nitrogen dioxide is a brown toxic gas that causes respiratory problems. Both nitrogen oxides contribute to acid rain formation and photochemical smog in urban areas.
Sulfur dioxide:
Sulfur impurities in fossil fuels burn to produce sulfur dioxide:
S + O₂ → SO₂
Coal and crude oil naturally contain sulfur compounds. Sulfur dioxide causes respiratory problems and contributes significantly to acid rain formation.
Acid rain formation and effects:
Sulfur dioxide and nitrogen oxides dissolve in water droplets in clouds:
SO₂ + H₂O → H₂SO₃ (sulfurous acid) 2H₂SO₃ + O₂ → 2H₂SO₄ (sulfuric acid) 4NO₂ + 2H₂O + O₂ → 4HNO₃ (nitric acid)
Acid rain (pH below 5.6) causes:
- Damage to limestone buildings and metal structures through chemical weathering
- Acidification of lakes and rivers, harming aquatic life
- Damage to tree roots and leaves, reducing forest growth
- Release of toxic aluminium ions from soil, which damages plant roots
Reducing air pollution:
Catalytic converters in vehicle exhausts use platinum, palladium and rhodium catalysts to convert harmful gases:
- 2CO + 2NO → 2CO₂ + N₂
- Hydrocarbons + oxygen → carbon dioxide + water
Other methods include:
- Using low-sulfur fuels or removing sulfur before combustion (desulfurisation)
- Fitting scrubbers to power station chimneys to neutralise acidic gases with alkalis
- Improving public transport to reduce vehicle numbers
- Developing electric vehicles and renewable energy sources
- Implementing congestion charging in city centres
Water pollution
Agricultural pollution:
Artificial fertilisers containing nitrates and phosphates wash from farmland into rivers and lakes during rainfall. This causes eutrophication:
- Excess nutrients enter the water body
- Algae grow rapidly, forming an algal bloom
- The algal bloom blocks sunlight to plants below
- Underwater plants die and decompose
- Decomposing bacteria multiply and consume oxygen
- Fish and other aquatic animals die from oxygen depletion
Pesticides used in farming may also enter water systems, accumulating in food chains and harming wildlife.
Sewage pollution:
Untreated sewage introduces harmful bacteria, viruses and organic matter. Bacterial decomposition depletes dissolved oxygen, similar to eutrophication. Pathogens in sewage spread diseases including cholera, typhoid and dysentery.
Industrial pollution:
Factories may discharge:
- Heavy metals (lead, mercury, cadmium) which accumulate in organisms
- Toxic chemicals that poison aquatic life
- Hot water causing thermal pollution, which reduces oxygen solubility
Plastic pollution:
Non-biodegradable plastics persist in water bodies for hundreds of years. Microplastics enter food chains, potentially harming organisms and human health. Marine animals may ingest or become entangled in larger plastic items.
Water treatment and purification
Potable water must be free from harmful microorganisms and have acceptable levels of dissolved substances. Treatment processes vary depending on the source water quality.
Municipal water treatment:
- Screening: Large debris is filtered out
- Sedimentation: Water stands in settling tanks where suspended particles sink
- Filtration: Water passes through sand and gravel filters to remove smaller particles
- Chlorination: Chlorine gas or sodium hypochlorite is added to kill microorganisms
Some treatment works add fluoride (to reduce tooth decay) and adjust pH to prevent pipe corrosion.
Laboratory water purification:
Distillation produces pure water:
- Water is boiled to produce steam
- Steam passes through a condenser where it cools and condenses
- Pure water (distillate) is collected
- Dissolved salts and impurities remain in the boiling vessel
Distillation removes all dissolved substances but requires significant energy.
Desalination makes seawater drinkable using either distillation or reverse osmosis (forcing water through membranes that block dissolved ions). These methods are energy-intensive and expensive but essential in arid coastal regions.
Testing water purity
Chemical tests:
- pH testing: Pure water has pH 7; acidic or alkaline water indicates contamination
- Dissolved oxygen: Measured using sensors or chemical tests; lower levels indicate pollution
- Nitrate tests: Specific reagents produce colour changes proportional to nitrate concentration
- Heavy metal tests: Various chemical reagents detect specific metal ions
Biological indicators:
Certain organisms tolerate different pollution levels:
- Stonefly larvae and freshwater shrimp indicate clean water
- Bloodworms and rat-tailed maggots tolerate highly polluted water
Physical tests:
- Turbidity (cloudiness) indicates suspended solids
- Temperature affects dissolved oxygen levels
- Conductivity indicates dissolved ionic substances
Sustainable resource management
Reducing environmental impact:
Recycling conserves resources and reduces pollution:
- Metals: recycling aluminium saves 95% of the energy needed to extract from ore
- Glass: can be recycled indefinitely without quality loss
- Plastics: some types can be recycled, though sorting and contamination present challenges
- Paper: recycling reduces deforestation
Carbon footprint reduction:
Individuals and organisations can reduce greenhouse gas emissions by:
- Using renewable energy sources (solar, wind, hydroelectric)
- Improving energy efficiency in buildings and appliances
- Reducing meat consumption (livestock farming produces significant methane)
- Using public transport, cycling or walking instead of private vehicles
- Planting trees to absorb carbon dioxide
Water conservation:
Methods include:
- Fixing leaks in pipes and taps
- Installing water-efficient appliances
- Collecting rainwater for gardens
- Using grey water (from washing) for non-potable purposes
- Implementing drip irrigation in agriculture
Worked examples
Example 1: Calculating atmospheric composition
A student analyses 1000 cm³ of dry air. Calculate the volume of oxygen present.
Solution: Oxygen comprises approximately 21% of clean, dry air.
Volume of oxygen = 21% of 1000 cm³ = 21/100 × 1000 cm³ = 210 cm³
Marks awarded: Full marks for calculation showing working (2 marks total)
Example 2: Explaining acid rain formation
Describe how burning coal in power stations leads to acid rain formation and state one effect of acid rain. (4 marks)
Solution:
Coal contains sulfur (or sulfur compounds) as an impurity. (1 mark)
When coal burns, the sulfur reacts with oxygen to form sulfur dioxide: S + O₂ → SO₂ (1 mark)
Sulfur dioxide dissolves in water droplets in clouds and reacts to form sulfuric acid. (1 mark)
Effect: Acid rain damages limestone buildings/kills fish in lakes/damages tree roots (accept any reasonable effect). (1 mark)
Example 3: Evaluating pollution control methods
A city wants to reduce nitrogen oxide emissions from vehicles. Suggest two methods and explain how one works. (4 marks)
Solution:
Method 1: Fit catalytic converters to vehicle exhausts (1 mark)
Method 2: Improve public transport to reduce vehicle numbers/introduce congestion charging/promote electric vehicles (accept any sensible alternative) (1 mark)
Explanation: Catalytic converters contain platinum/palladium/rhodium catalyst (1 mark) which converts nitrogen oxides to nitrogen gas: 2NO → N₂ + O₂ or 2CO + 2NO → 2CO₂ + N₂ (1 mark)
Common mistakes and how to avoid them
• Confusing carbon monoxide and carbon dioxide: Carbon monoxide (CO) is the toxic gas from incomplete combustion; carbon dioxide (CO₂) is the greenhouse gas from complete combustion. Always check the formula and remember that CO binds to haemoglobin while CO₂ causes global warming.
• Incomplete acid rain explanations: Don't just say "sulfur dioxide causes acid rain." Explain that SO₂ dissolves in water and reacts to form sulfuric acid. Include the full mechanism for maximum marks.
• Misunderstanding eutrophication sequence: Learn the correct order: nutrients enter water → algae bloom → light blocked → plants die → bacteria decompose plants → oxygen depleted → fish die. Missing steps lose marks.
• Vague pollution reduction methods: Stating "use less fuel" is too general. Specify methods like "fit catalytic converters," "use public transport" or "switch to renewable energy." The more specific, the better.
• Forgetting the role of catalysts: In questions about catalytic converters, remember to name the metals (platinum, palladium, rhodium) and state that they speed up reactions without being used up.
• Not linking properties to effects: When discussing pollutants, connect their properties to consequences. For example: "nitrogen dioxide is toxic AND causes respiratory problems" or "plastic is non-biodegradable AND persists in the environment for hundreds of years."
Exam technique for "Air, water and the environment"
• Command word awareness: "Describe" requires you to state features and characteristics without explanation. "Explain" requires reasons or mechanisms using "because" or "so that." "Suggest" means the answer may not be in the syllabus—apply your knowledge to unfamiliar contexts.
• Use data effectively: Questions often include graphs showing CO₂ levels over time or tables of pollutant concentrations. Quote figures from the data in your answer: "Carbon dioxide increased from 0.028% in 1850 to 0.041% in 2020." This demonstrates analysis and earns marks.
• Equation accuracy: Chemical equations for combustion and acid formation appear frequently. Balance equations correctly and use correct formulae (SO₂ not S₂O, NO₂ not N₂O). Even one error loses marks.
• Longer answers require structure: For 4-6 mark questions, plan your answer with multiple distinct points. Use paragraph breaks or bullet points to show the examiner you're making separate points. Each valid scientific point typically earns one mark.
Quick revision summary
Clean air contains approximately 78% nitrogen and 21% oxygen. Burning fossil fuels releases carbon dioxide (greenhouse gas causing global warming), carbon monoxide (toxic from incomplete combustion), sulfur dioxide and nitrogen oxides (causing acid rain). Catalytic converters reduce vehicle pollution. Water pollution from fertilisers causes eutrophication through oxygen depletion. Potable water production involves filtration, sedimentation and chlorination. Distillation produces pure water by boiling and condensing. Reducing carbon footprints through recycling, renewable energy and efficiency improvements helps protect the environment.