By the end of this lesson
- Identify the sources of sulfur dioxide and carbon monoxide.
- Explain the biological consequences of acid rain.
- Explain how carbon monoxide affects oxygen transport.
BIG QUESTION
How can a gas from a power station harm fish in a lake hundreds of kilometres away?
Sulfur dioxide dissolves in rain, is carried by wind and lowers the pH of distant lakes and soils.
Sulfur dioxide and acid rain
sulfur dioxideA gas released when fossil fuels containing sulfur are burned; it dissolves in rain water to form acid rain. (SO₂) is released when fossil fuels containing sulfur impurities, especially coal, are burned in power stations and some industries. Volcanoes are a natural source.
SO₂ dissolves in water droplets in clouds to form acidic solutions that fall as acid rainRain with a low pH, caused by sulfur dioxide and nitrogen oxides dissolving in water in the air; it damages trees and harms life in lakes., often far from the source.
Biological effects of acid rain
Acid rain lowers the pH of lakes and rivers. Many fish, amphibians and invertebrates cannot survive at low pH; for example, fish eggs may fail to hatch. Low pH can also release aluminium ions from soils, which damage fish gills.
On land, acid rain damages leaves and leaches mineral ions such as magnesium and calcium from soils, so trees grow poorly and may die. Sulfur dioxide also irritates the airways, worsening conditions such as asthma.
Carbon monoxide
carbon monoxideA poisonous gas in cigarette smoke that binds to haemoglobin more strongly than oxygen, reducing the oxygen the blood can carry. (CO) is produced by incomplete combustionBurning a fuel with too little oxygen, which produces carbon monoxide or soot as well as carbon dioxide. of carbon-containing fuels, for example in vehicle engines, faulty gas boilers and cigarette smoke. It is colourless and odourless.
CO binds to haemoglobinThe iron-containing red protein in red blood cells that binds oxygen reversibly, forming oxyhaemoglobin in the lungs and releasing oxygen in respiring tissues. much more strongly than oxygen does, forming carboxyhaemoglobin. Less oxygen is carried to tissues, causing tiredness, headaches and, at high concentrations, unconsciousness and death.
Reducing pollution
Removing sulfur from fuels, fitting scrubbers to power-station chimneys and using renewable energy reduce SO₂ emissions.
Catalytic converters, good ventilation and carbon monoxide detectors reduce the risks from CO.
Two air pollutants compared
Name the source and link it to a biological effect.
| Feature | Sulfur dioxide | Carbon monoxide |
|---|---|---|
| Main source | Burning fossil fuels containing sulfur. | Incomplete combustion of fuels. |
| How it causes harm | Forms acid rain; irritates airways. | Binds to haemoglobin. |
| Biological effect | Lower pH kills aquatic organisms and damages trees. | Less oxygen carried to tissues. |
| Reduction method | Desulfurisation and scrubbers. | Catalytic converters, ventilation, detectors. |
Language that earns precision
- acid rain
- Rain with a lower pH than normal, caused by dissolved pollutants such as sulfur dioxide.
- incomplete combustion
- Burning in insufficient oxygen, producing carbon monoxide or soot.
- carboxyhaemoglobin
- Haemoglobin combined with carbon monoxide.
Common misconceptions
Claim: Carbon monoxide causes acid rain.
Correction: Acid rain is caused mainly by sulfur dioxide and nitrogen oxides. CO harms by binding to haemoglobin.
Claim: Acid rain only affects the area near the factory.
Correction: Pollutants are carried by wind and may fall as acid rain far away.
Claim: Carbon monoxide damages lungs by burning them.
Correction: It is absorbed into blood and reduces oxygen transport.
Match the answer to the command word
For CO, write “binds to haemoglobin, so less oxygen is carried”, not just “stops breathing”.
For acid rain, give an effect on a named group: fish, trees or soil minerals.
Recall, apply, then justify
Explain how carbon monoxide reduces the oxygen supplied to body tissues.
Hint
Haemoglobin, binding and transport.
Suggested marking points
- Carbon monoxide binds (irreversibly/strongly) to haemoglobin.
- Less haemoglobin is available to combine with oxygen.
- So less oxygen is carried by red blood cells to tissues.
Describe how burning coal can lead to the death of fish in a lake.
Hint
Gas, rain, pH.
Suggested marking points
- Burning coal releases sulfur dioxide.
- Sulfur dioxide dissolves in rain to form acid rain.
- The lake pH falls, killing fish or their eggs.
Lichens are sensitive to sulfur dioxide. Explain how a survey of lichens could be used to compare air quality in two towns, and state one limitation.
Planning hint
Indicator species and a fair survey.
What a strong design includes
- Lichens act as indicator species for sulfur dioxide.
- More lichen species and greater cover suggest lower SO₂ levels.
- Survey the same type of surface, such as tree bark, in both towns.
- Use the same number of randomly chosen sample sites.
- Record lichen species and percentage cover.
- Limitation: other factors such as humidity or shade also affect lichen growth.