Maintaining air quality

Air composition, pollutants and their control, acid rain, the carbon cycle, greenhouse gases and the ozone layer for G3 Pure and O-Level Chemistry.

SEC G3 Pure Chemistry 2027
6 lessons

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Before you begin

This topic is about the air: what clean air contains, which gases pollute it and how, how acid rain forms, how carbon moves between the air and living things, why extra greenhouse gases warm the Earth, and how CFCs thin the ozone layer.

Be comfortable with: fossil fuels (combustion and carbon dioxide emissions), oxides (acidic oxides and neutralisation) and rate of reaction (why a catalyst speeds up exhaust reactions).

Learning goals

Describe the composition of dry air by volume: about 78% nitrogen, 21% oxygen and the rest noble gases, mainly argon, and carbon dioxide
Name common air pollutants, such as carbon monoxide, methane, nitrogen oxides, ozone, sulfur dioxide and unburned hydrocarbons
State that carbon monoxide comes from incomplete combustion of carbon-containing substances
State that nitrogen oxides come from lightning and internal combustion engines
State that sulfur dioxide comes from volcanoes and burning fossil fuels
Describe the redox reactions in catalytic converters that remove combustion pollutants
Describe how calcium carbonate reduces the effects of acid rain and removes sulfur dioxide from flue gases
Discuss why carbon monoxide is toxic
Discuss how nitrogen dioxide and sulfur dioxide form acid rain and its effects on breathing and buildings
Discuss why the ozone layer matters and how chlorofluorocarbons (CFCs) deplete it
Describe how combustion, respiration and photosynthesis move carbon in the carbon cycle
Describe how the carbon cycle regulates the amount of carbon dioxide in the atmosphere
State that carbon dioxide and methane are greenhouse gases, give their sources, and describe the possible effects of rising levels, such as extreme weather and melting polar ice

Syllabus statements covered

describe the volume composition of gases present in dry air as being approximately 78% nitrogen, 21% oxygen and the remainder being noble gases (with argon as the main constituent) and carbon dioxide
name some common atmospheric pollutants, e.g. carbon monoxide; methane; nitrogen oxides (NO and NO2); ozone; sulfur dioxide; unburned hydrocarbons
state the sources of these pollutants as — carbon monoxide from incomplete combustion of carbon-containing substances
state the sources of these pollutants as — nitrogen oxides from lightning activity and internal combustion engines
state the sources of these pollutants as — sulfur dioxide from volcanoes and combustion of fossil fuels
describe the reactions used in possible solutions to the problems arising from some of the pollutants named in (b) — the redox reactions in catalytic converters to remove combustion pollutants (see also 10(d))
describe the reactions used in possible solutions to the problems arising from some of the pollutants named in (b) — the use of calcium carbonate to reduce the effect of ‘acid rain’ and in flue gas desulfurisation
discuss some of the effects of these pollutants on health and on the environment — the toxic nature of carbon monoxide
discuss some of the effects of these pollutants on health and on the environment — the role of nitrogen dioxide and sulfur dioxide in the formation of ‘acid rain’ and its effects on respiration and buildings
discuss the importance of the ozone layer and the problems involved with the depletion of ozone by reaction with chlorine-containing compounds, chlorofluorocarbons (CFCs)
describe the carbon cycle in simple terms, to include — the processes of combustion, respiration and photosynthesis
describe the carbon cycle in simple terms, to include — how the carbon cycle regulates the amount of carbon dioxide in the atmosphere (see also 11.1(d))
state that carbon dioxide and methane are greenhouse gases and may contribute to global warming; give the sources of these gases and describe the potential effects of increased levels of these greenhouse gases, including more extreme weather events and melting of polar ice.

Lessons

Work through them in order.

Maintaining air quality

Practise and check

Topic reference

Quick reference

Problem	Main gases	What happens	Ways to reduce it
Carbon monoxide poisoning	CO, from incomplete combustion	binds to haemoglobin; blood carries less oxygen	catalytic converters; enough air for burning
Acid rain	SO₂, nitrogen oxides	acids form in the air; lakes, trees, metals and limestone damaged	flue gas desulfurisation; catalytic converters
Enhanced greenhouse effect	CO₂, CH₄	more infrared kept in; global warming	burn less fossil fuel; stop methane leaks; protect forests
Ozone depletion	CFCs	chlorine destroys ozone; more UV reaches the ground	replace CFCs; collect old refrigerators

Core knowledge to remember

Clean, dry air is about 78% nitrogen and 21% oxygen; the rest is mainly argon, with other noble gases and carbon dioxide.
Carbon monoxide, methane, nitrogen oxides, ozone, sulfur dioxide and unburned hydrocarbons are air pollutants, each with its own sources and effects.
A catalytic converter changes carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen.
Photosynthesis removes carbon dioxide from the air; respiration and combustion return it.
Greenhouse gases absorb infrared radiation from the Earth; more of them enhances the greenhouse effect.
The ozone layer absorbs harmful ultraviolet radiation; CFCs release chlorine atoms that destroy ozone.

Common mistakes

Treating the normal gases of clean air as pollutants.
Mixing up acid rain, global warming and ozone depletion: each has its own gases, mechanism and effects.
Giving a pollutant without its source, or an effect without the gas that causes it.
Using infrared to explain the ozone layer, or ultraviolet to explain the greenhouse effect.