SO₂ makes acid rain; CO poisons haemoglobin
Sulfur dioxide (), from burning sulfur-rich fossil fuels, dissolves in rain as acid rain: it kills fish and invertebrates, damages tree leaves (less photosynthesis) and leaches soil ions toxic to plants. Rock and building damage are NOT biological. Carbon monoxide (), from incomplete combustion, binds haemoglobin as stable carboxyhaemoglobin, so blood carries less oxygen.
Greenhouse gases → enhanced effect → warming
Spec greenhouse gases: water vapour, , nitrous oxide (), methane (), CFCs. , and are NOT. They absorb outgoing infra-red and re-radiate some back. Extra gas from human activity (fossil fuels, deforestation, livestock, fertilisers) traps more heat — the enhanced greenhouse effect — causing global warming: melting ice, rising seas, shifting rainfall and species ranges.
Eutrophication: nitrate to oxygen crash
Six steps: (1) excess nitrate fertiliser leaches into water; (2) nitrate feeds an algal bloom; (3) the algal mat blocks sunlight from submerged plants; (4) those plants die (algae die too once nutrients run out); (5) decomposers break down the dead matter by aerobic respiration, using up dissolved oxygen; (6) fish and invertebrates cannot respire and die. Sewage (4.16) causes the same oxygen crash.
Drawn from real examiner reports.
Acid rain: give a BIOLOGICAL consequence
Statement 4.12 asks for the biological consequences of pollution. For acid rain, 'corrodes buildings' or 'weathers rock' are chemical/physical and score nothing. Creditworthy effects act on living things: acidified water kills fish and invertebrates; leaf damage cuts photosynthesis; leached ions make soil toxic. Always tie the answer to an organism.
June 2024 Paper 2B Q1(c)(i): rock erosion given as a consequence of sulfur dioxide/acid rain was not credited; a biological consequence was required.
CO and SO₂ are not greenhouse gases
When asked to name greenhouse gases, many candidates give carbon monoxide or sulfur dioxide. These are serious air pollutants but NOT greenhouse gases. The spec list: water vapour, carbon dioxide, nitrous oxide, methane and CFCs. is toxic and causes acid rain, but neither traps heat.
June 2023 Paper 1BR Q8(b)(i)–(ii): candidates named carbon monoxide and sulfur dioxide as greenhouse gases (wrong).
Nitrogen (N₂) vs nitrous oxide (N₂O)
Nitrogen gas () makes up about 78% of the air and is not a greenhouse gas. Nitrous oxide () is a different molecule and is a greenhouse gas. When a list asks "which is NOT a greenhouse gas", the answer is nitrogen — nitrous oxide is a common wrong pick. Watch the subscript: one atom of oxygen changes the meaning entirely.
November 2024 Paper 2B Q6(c): the gas that is NOT a greenhouse gas was nitrogen; nitrous oxide was a common wrong pick.
Decomposers remove the oxygen, not algae
The dissolved oxygen is used by decomposers, not by the algae. Candidates often write 'the algae use up all the oxygen' — wrong. The algae and plants die and become dead matter; bacteria and fungi then respire aerobically to break it down, consuming the oxygen. The algae cause the problem by growing and dying; the decomposers deoxygenate.
November 2024 Paper 2B Q6(a)(ii): weak answers attributed oxygen removal to the algae rather than to decomposers.
Don't skip the light-blocking step
A frequent gap is jumping straight from algal bloom to oxygen depletion. Keep the order: algal bloom → blocks sunlight → submerged plants cannot photosynthesise → they die → decomposers respire aerobically → oxygen removed → aerobic organisms die. Omitting the light-blocking step loses a mark; those dying plants would otherwise have added oxygen.
November 2024 Paper 2B Q6(a)(ii): weak answers missed the light-blocking step in the eutrophication sequence.
Evaluate GWP by amount, not per molecule
Do not quote global-warming-potential (GWP) figures in isolation. Although has a lower GWP per molecule than methane or nitrous oxide, it is released in far larger amounts and persists longest, making it the largest contributor overall. Weigh GWP against quantity emitted and persistence, then conclude.
June 2023 Paper 1BR Q8(b)(i)–(ii): evaluating GWP data, weak answers quoted figures without noting CO₂ contributes most because it is released in the largest amount and persists longest.
Chain: pollutant → change → biological effect
Most 4d questions say explain, not describe. Use a three-part chain: pollutant → environmental change → effect on a named organism or process. E.g. sulfur dioxide → acid rain → lower pH → fish die; carbon monoxide → carboxyhaemoglobin → tissues starved of oxygen.
Evaluate across every gas and criterion
For "evaluate which gas is most damaging", do not stop at one gas or one number. Consider ALL the gases given, compare GWP per molecule, amount released and persistence, then conclude with a reason. A one-sided answer, or figures with no judgement, caps the mark.
Answer the command word precisely
Check whether the question wants describe or explain, and whether it needs a biological consequence — an effect on a living organism or process. A chemical/physical answer (rock erosion) scores nothing. For 'explain', give the mechanism and a direction of change.
This sub-topic (statements 4.12–4.18B) covers how human activity pollutes the air and water, drives climate change, and alters ecosystems through eutrophication and deforestation.
| Pollutant | Source | Key biological consequence |
|---|---|---|
| Sulfur dioxide () | Burning sulfur-containing fossil fuels (power stations, industry) | Dissolves in rain to form acid rain → acidifies lakes, kills aquatic organisms; damages tree leaves, reducing photosynthesis |
| Carbon monoxide () | Incomplete combustion of fuels (car engines) | Binds much more strongly than oxygen to haemoglobin (forming a stable carboxyhaemoglobin), preventing the blood from carrying enough oxygen → tissues deprived of oxygen |
Note: rock erosion and building corrosion are NOT biological consequences and are not credited in biology mark schemes.
Spec-listed greenhouse gases: water vapour, carbon dioxide (), nitrous oxide (), methane (), CFCs. NOT greenhouse gases: carbon monoxide (), sulfur dioxide (), nitrogen ().
How it works: Solar radiation (short-wave) warms the Earth's surface. The surface re-emits infra-red (long-wave) radiation outwards. Greenhouse gases absorb this infra-red radiation and re-radiate some back towards Earth, keeping the planet warm. Human activities increase these gases, trapping more heat — the enhanced greenhouse effect — leading to global warming.
Human sources of each greenhouse gas:
| Gas | Main human sources |
|---|---|
| Burning fossil fuels; deforestation (fewer trees to absorb ); decomposition | |
| Methane () | Livestock digestion; paddy fields; landfill |
| Nitrous oxide () | Nitrogen fertilisers; vehicle exhausts |
| CFCs | Aerosols; refrigerants (now mostly banned) |
Consequences of global warming: melting ice caps raise sea levels; coastal and low-lying habitats are lost; changes in rainfall patterns alter species distributions; timing of seasonal events (flowering, migration) shifts.
The six-step sequence (learn in order):
Key mark-earning link: the oxygen is removed by decomposers respiring, not by the algae directly.
Water pollution by sewage (statement 4.16) triggers the same deoxygenation (steps 5–6): organic matter from sewage is broken down aerobically by decomposers, depleting dissolved oxygen.
Cutting down large areas of forest has multiple consequences:
Define eutrophication in the context of the 4BI1 specification.
State TWO greenhouse gases produced by human activities and, for each, give ONE human activity that produces it.
(2 marks)
