C6: Global Challenges

What you'll learn

This topic examines how chemistry helps us understand and address major environmental and resource challenges facing our planet. You'll explore the composition of Earth's atmosphere, how human activities affect it, and the chemistry behind climate change, pollution, and sustainable resource use.

Key terms and definitions

Greenhouse gas — A gas that absorbs infrared radiation from Earth's surface and re-radiates it, contributing to the warming of the atmosphere (e.g. carbon dioxide, methane, water vapour)

Carbon footprint — The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event

Sustainable development — Meeting the needs of the present without compromising the ability of future generations to meet their own needs

Potable water — Water that is safe to drink, with sufficiently low levels of dissolved salts and microbes

Phytomining — The use of plants to absorb metal compounds from soil, which are then harvested and processed to extract the metal

Bioleaching — The use of bacteria to extract metals from low-grade ores by converting insoluble metal compounds into soluble ones

Life cycle assessment (LCA) — An evaluation of the environmental impact of a product through all stages: extraction and processing of raw materials, manufacturing, use, and disposal

Finite resource — A resource that cannot be replaced once used, or is replaced extremely slowly (e.g. fossil fuels, metal ores)

Core concepts

Earth's atmosphere composition and evolution

The current atmosphere is approximately:

• 78% nitrogen
• 21% oxygen
• 0.04% carbon dioxide
• Small amounts of noble gases (mainly argon) and water vapour

Evolution of the atmosphere:

During Earth's first billion years, the atmosphere consisted primarily of carbon dioxide with small amounts of water vapour, ammonia, and methane released by intense volcanic activity. There was little or no oxygen.

As Earth cooled, water vapour condensed to form oceans. Carbon dioxide dissolved in these oceans and reacted to form carbonate precipitates, which became sedimentary rocks (limestone). Marine organisms incorporated carbonates into shells and skeletons, further reducing atmospheric CO₂.

Approximately 2.7 billion years ago, photosynthetic organisms (initially cyanobacteria, later algae and plants) began releasing oxygen:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Oxygen levels gradually increased over billions of years. Some oxygen converted to ozone (O₃) in the upper atmosphere, forming a protective layer against harmful UV radiation. This allowed more complex life to develop on land.

The carbon cycle

Carbon continuously cycles between the atmosphere, living organisms, soil, and oceans through various processes:

Processes that add CO₂ to the atmosphere:

• Respiration by plants and animals
• Combustion of fossil fuels and biomass
• Decay and decomposition of organic matter
• Volcanic eruptions

Processes that remove CO₂ from the atmosphere:

• Photosynthesis by plants and phytoplankton
• Dissolution in oceans
• Formation of sedimentary carbonate rocks

The natural carbon cycle maintained relatively stable atmospheric CO₂ levels for thousands of years. However, human activities—particularly burning fossil fuels and deforestation—have disrupted this balance, increasing atmospheric CO₂ concentrations from approximately 280 ppm (pre-industrial) to over 415 ppm today.

Greenhouse effect and climate change

The greenhouse effect is a natural process essential for life on Earth. Greenhouse gases in the atmosphere absorb infrared radiation emitted from Earth's surface and re-radiate it in all directions, including back towards Earth. This keeps the planet warm enough to support life.

Key greenhouse gases:

• Water vapour (H₂O)
• Carbon dioxide (CO₂)
• Methane (CH₄)
• Nitrous oxide (N₂O)

Enhanced greenhouse effect:

Increasing concentrations of greenhouse gases due to human activities intensify the greenhouse effect, leading to global warming and climate change. Major human sources include:

• Carbon dioxide: Burning fossil fuels (coal, oil, natural gas) for electricity, transport, and heating; deforestation
• Methane: Agriculture (cattle digestion, rice paddies), landfill sites, extraction of fossil fuels
• Nitrous oxide: Agricultural fertilisers, industrial processes

Consequences of climate change:

• Rising global temperatures
• Melting polar ice caps and glaciers
• Sea level rise
• Changes in precipitation patterns and extreme weather events
• Ocean acidification (dissolved CO₂ forms carbonic acid)
• Ecosystem disruption and species extinction

Air pollution and atmospheric chemistry

Common air pollutants from combustion:

Carbon monoxide (CO): Produced by incomplete combustion of carbon-containing fuels when oxygen supply is limited. It is toxic because it binds irreversibly to haemoglobin, preventing oxygen transport in blood.

Particulates (PM): Small particles of unburnt carbon (soot) and other solids produced during incomplete combustion. They cause respiratory problems and contribute to global dimming by reflecting sunlight.

Sulfur dioxide (SO₂): Formed when sulfur impurities in fossil fuels burn: S + O₂ → SO₂

Sulfur dioxide dissolves in atmospheric water droplets to form sulfurous acid, contributing to acid rain.

Nitrogen oxides (NOₓ): Nitrogen and oxygen from air react at high temperatures in vehicle engines and power stations: N₂ + O₂ → 2NO 2NO + O₂ → 2NO₂

Nitrogen dioxide dissolves in water to form nitric acid, also contributing to acid rain.

Acid rain effects:

• Damages aquatic ecosystems (lakes become too acidic for fish)
• Damages plants and reduces crop yields
• Corrodes limestone buildings and metal structures
• Leaches nutrients from soil

Solutions:

• Catalytic converters in vehicles reduce NOₓ and CO emissions
• Flue gas desulfurisation removes SO₂ from power station emissions
• Low-sulfur fuels
• Alternative energy sources (renewable energy)

Water resources and treatment

Potable water is essential for human consumption but requires specific quality standards. It must have:

• Low levels of dissolved salts
• pH between 6.5 and 8.5
• No harmful microorganisms
• No toxic substances

UK water treatment process:

1. Screening: Removes large debris
2. Sedimentation: Particles settle out in settlement tanks
3. Filtration: Water passes through sand and gravel beds to remove remaining particles
4. Chlorination: Chlorine gas or chlorine compounds kill microorganisms

In areas with freshwater shortage, alternative sources include:

Desalination: Removing salt from seawater through:

• Distillation (heating to evaporate water, then condensing)
• Reverse osmosis (forcing water through selective membranes)

Both processes require significant energy, making them expensive.

Wastewater treatment:

1. Screening and grit removal: Large solids and grit removed
2. Sedimentation: Produces sewage sludge and effluent
3. Aerobic biological treatment: Bacteria break down organic matter in presence of oxygen
4. Anaerobic digestion of sludge: Bacteria break down sludge without oxygen, producing methane (useful fuel) and nutrient-rich digestate

Metal extraction and resource sustainability

Traditional extraction methods:

Metals more reactive than carbon are extracted by electrolysis of molten compounds (e.g. aluminium from aluminium oxide). Metals less reactive than carbon are extracted by reduction with carbon (e.g. iron from iron oxide in a blast furnace).

These methods require:

• Large amounts of energy
• Mining of finite ore resources
• Significant environmental disruption

Alternative methods for low-grade ores:

Phytomining:

1. Plants grown on soil containing metal compounds
2. Plants absorb and concentrate metals
3. Plants harvested and burned
4. Ash contains metal compounds in higher concentration
5. Metal extracted by displacement or electrolysis

Bioleaching:

1. Bacteria produce solutions that dissolve metals from ores
2. Metal compounds leached out in solution
3. Metal extracted by displacement with more reactive metal or electrolysis

Example: Copper extraction by displacement Cu²⁺(aq) + Fe(s) → Cu(s) + Fe²⁺(aq)

Advantages of phytomining and bioleaching:

• Economically viable for low-grade ores
• Less energy required
• Reduced landscape damage
• Can process mining waste

Disadvantages:

• Very slow processes
• Not suitable for large-scale immediate demand

Life cycle assessments and sustainability

A life cycle assessment (LCA) evaluates environmental impact across four stages:

1. Extraction and processing of raw materials:

   • Energy consumption
   • Habitat destruction
   • Pollution from mining and processing

2. Manufacturing and packaging:

   • Energy and water use
   • Waste production
   • Emissions from factories

3. Use and operation:

   • Energy consumption during use
   • Maintenance requirements
   • Lifespan of product

4. Disposal:

   • Recyclability
   • Biodegradability
   • Landfill space requirements
   • Pollution from incineration or decomposition

Limitations of LCAs:

• Difficult to quantify some impacts (e.g. visual pollution)
• Selective or biased data use
• Different weightings for different impacts
• Not always scientifically rigorous

Reducing environmental impact:

• Use renewable resources (replaced as quickly as used)
• Recycle materials to reduce extraction needs
• Reduce energy consumption
• Use renewable energy sources
• Design products for longevity and recyclability
• Choose biodegradable materials where appropriate

Worked examples

Example 1: Calculate the percentage composition of nitrogen and oxygen in the current atmosphere if it contains 78% nitrogen, 21% oxygen, and 1% other gases. Explain why this composition differs from the early atmosphere. (4 marks)

Answer:

• Nitrogen: 78% ✓
• Oxygen: 21% ✓
• Early atmosphere contained very little/no oxygen ✓
• Photosynthesis by plants/algae/cyanobacteria released oxygen over billions of years ✓

Example 2: A student investigates the effect of acid rain on limestone. They add dilute sulfuric acid to calcium carbonate chips and observe bubbles of gas. Write a balanced symbol equation for this reaction and explain how acid rain forms from sulfur dioxide. (4 marks)

Answer:

• CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ ✓
• (Accept balanced equation with correct formulae)
• Sulfur dioxide dissolves in water/rain ✓
• Forms sulfuric acid/H₂SO₄ ✓
• (Accept sulfurous acid/H₂SO₃ as intermediate)

Example 3: Compare the extraction of copper from a high-grade ore and a low-grade ore using bioleaching. Include one advantage and one disadvantage of bioleaching. (5 marks)

Answer:

• High-grade ore: extracted by heating with carbon/reduction OR smelting ✓
• Low-grade ore: bacteria convert insoluble copper compounds to soluble compounds ✓
• Copper ions in solution displaced by more reactive metal (e.g. iron/scrap iron) ✓
• Advantage: economically viable for low-grade ores OR less energy required OR less environmental damage ✓
• Disadvantage: very slow process OR not suitable for large-scale/immediate production ✓

Common mistakes and how to avoid them

• Confusing "potable" with "pure": Potable water is safe to drink but contains dissolved minerals; pure water contains only H₂O molecules. Don't use these terms interchangeably
• Incorrect greenhouse gas mechanism: Students often say greenhouse gases "trap heat" — be specific that they absorb infrared radiation and re-radiate it, including back to Earth
• Misunderstanding the early atmosphere: Don't state oxygen was always present. The early atmosphere had little/no oxygen; photosynthesis gradually increased oxygen levels over billions of years
• Incomplete combustion equations: When writing equations for incomplete combustion producing CO or C, ensure oxygen supply is limited and clearly state this in your answer
• Vague LCA descriptions: Give specific examples of impacts at each stage rather than general statements like "bad for environment"
• Mixing up phytomining and bioleaching: Phytomining uses plants to absorb metals; bioleaching uses bacteria to dissolve metals from ores

Exam technique for "C6: Global Challenges"

• "Suggest" questions: Use your knowledge to propose plausible solutions even if you haven't learned them explicitly. For example, suggesting ways to reduce carbon footprint requires applying principles of combustion and energy use
• Command word "evaluate": Present both advantages and disadvantages, then make a justified conclusion. In LCA questions, consider multiple stages and types of impact
• Extended response (6-mark) questions: Structure answers logically using clear scientific terminology. Link ideas using connectives. Include specific chemical examples (equations, named compounds) where relevant. Aim for 6-8 distinct points
• Calculation questions: Always show working for method marks. Include units in final answers. For percentage composition or carbon footprint calculations, ensure your percentages total 100% as a check

Quick revision summary

Earth's atmosphere evolved from volcanic CO₂ to the current 78% N₂, 21% O₂ through ocean formation (dissolving CO₂) and photosynthesis (producing O₂). The carbon cycle naturally balances atmospheric CO₂, but human activities increase greenhouse gases, enhancing the greenhouse effect and causing climate change. Combustion produces pollutants (CO, particulates, SO₂, NOₓ) that cause health problems and acid rain. Sustainable practices include treating water, extracting metals through phytomining/bioleaching, conducting life cycle assessments, and using renewable resources to reduce environmental impact.