Materials, Air and the Environment

The atmosphere

The Earth's atmosphere is approximately:

• Nitrogen (N₂): ~78%
• Oxygen (O₂): ~21%
• Argon (Ar): ~0.9%
• Carbon dioxide (CO₂): ~0.04%
• Trace amounts of other noble gases, water vapour.

Evolution of the atmosphere

The early Earth's atmosphere (4.5 billion years ago) had no oxygen. It was rich in CO₂, water vapour, nitrogen, and methane. Over billions of years:

1. Water vapour condensed into oceans.
2. Photosynthesis by early organisms (cyanobacteria) added O₂ and removed CO₂.
3. CO₂ dissolved in oceans and was locked in carbonate rock (CaCO₃).
4. O₂ built up to current levels.

Greenhouse effect

Certain gases — greenhouse gases — absorb and re-emit infrared radiation from Earth's surface, trapping heat in the atmosphere (the enhanced greenhouse effect when concentrations increase due to human activity).

Key greenhouse gases:

• CO₂: fossil fuel combustion, deforestation.
• CH₄ (methane): cattle, rice paddies, landfill, natural gas leaks.
• Water vapour (H₂O): natural but affected by climate feedback loops.
• N₂O: agriculture (fertilisers), combustion.

Consequences of enhanced greenhouse effect: global temperature rise; melting ice caps; rising sea levels; more extreme weather events; habitat loss; ocean acidification.

Combustion and air pollution

Fossil fuel combustion produces:

• CO₂: greenhouse gas → climate change.
• CO: toxic (binds to haemoglobin, preventing O₂ transport); formed in incomplete combustion.
• SO₂: from sulfur impurities in fuels; causes acid rain (SO₂ + H₂O + ½O₂ → H₂SO₄).
• NOₓ: from N₂ and O₂ in air at high combustion temperatures; causes acid rain and photochemical smog.
• Particulates (soot): respiratory problems, climate effects.

Acid rain (pH <5.6):

• Formed from SO₂ and NOₓ dissolving in water.
• Effects: damages limestone buildings (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂); kills aquatic life; damages trees.
• Solutions: use catalytic converters (cars), desulfurisation of flue gases, use cleaner fuels.

Catalytic converters

Fitted to car exhausts; use Pt, Pd, Rh catalysts.

• Convert CO → CO₂: 2CO + O₂ → 2CO₂
• Convert NOₓ → N₂: 2NO → N₂ + O₂
• Convert unburnt hydrocarbons → CO₂ + H₂O.

Sustainable materials

• Recycling: reduces need for raw materials; less energy than primary production.
• Biodegradable polymers: break down naturally (e.g. PLA from corn starch).
• Carbon footprint: total CO₂ (and equivalent GHG) emitted by an activity or product across its lifecycle.

The water cycle and water treatment

Freshwater is treated before supply:

1. Sedimentation: large particles settle.
2. Filtration through sand/gravel beds: removes smaller particles.
3. Chlorination: kills bacteria/pathogens.
4. pH adjustment with lime Ca(OH)₂: neutralises acidic water.

CCEA assessment context

CCEA Single Award Chemistry includes the "Materials" theme. Expect questions on:

• Identifying greenhouse gases and their sources.
• Equations for acid rain formation.
• Evaluating solutions to air pollution.
• Recycling and sustainable chemistry arguments.