Air (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

• The present composition of gases in the atmosphere has not changed much in 200 million years
  • About four-fifths of the air is nitrogen and one-fifth is oxygen
  • The remaining gases include carbon dioxide, water vapour and trace quantities of the noble gases

Pie chart of the current atmosphere

The two main gases in the air are nitrogen and oxygen

• In addition to the gases present naturally in our atmosphere, other gases are present due to human activities and are classed as air pollutants

Carbon dioxide

• Sources:
  • Complete combustion of carbon-containing fuels such as fossil fuels
  • For example, the complete combustion of methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

• Adverse effects: 
  • Increases global warming, which leads to climate change

Carbon monoxide

• Sources:
  • Incomplete combustion of carbon-containing fuels such as fossil fuels
  • For example, the incomplete combustion of gasoline / octane:

C₈H₁₈ + 9O₂ → 5CO + 2CO₂ + 9H₂O

• Adverse effects:
  • Toxic / poisonous
  • It combines with haemoglobin in the blood and prevents it from carrying oxygen

Particulates

• Sources:
  • Incomplete combustion of carbon-containing fuels such as fossil fuels can also produce particulates of carbon (soot)
  • For example, the incomplete combustion of methane can produce CO and C:

2CH₄ + 3O₂→ 2CO + 4H₂O

CH₄ + O₂→ C + 2H₂O

• Adverse effects:
  • Respiratory problems
  • Cancer

Methane

• Sources:
  • Waste gases from digestive processes of animals
  • Decomposition of vegetation
  • Bacterial action in swamps, rice paddy fields and landfill sites
• Adverse effects:
  • Increases global warming, which leads to climate change

Oxides of nitrogen

• Sources:
  • Reaction of nitrogen with oxygen in the presence of high temperatures, e.g. in car engines, high-temperature furnaces and when lightning occurs
  • It is also a product of bacterial action in the soil
• Adverse effects:
  • Produces photochemical smog 
  • Dissolves in rain to form acid rain which causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms
  • Pollutes crops and water supplies
  • Irritates lungs, throats and eyes and causes respiratory problems 

Sulfur dioxide

• Sources:
  • Combustion of fossil fuels containing sulfur compounds:

S + O₂ → SO₂  

• • Power stations are a major source of sulfur dioxide
• Adverse effects:
  • Dissolves in rain to form acid rain with similar effects as the acid rain caused by oxides of nitrogen

How acid rain is produced