Environmental Chemistry: IGCSE Chemistry Exam Guide

Environmental chemistry appears on every 0620 paper but carries fewer marks per question than topics like stoichiometry or organic chemistry. The trade-off is that the questions are almost entirely recall-based: learn the pollutants, their sources, their effects and the solutions, and the marks are straightforward. The full content is under chemistry of the environment.

The pollutant table you must know

The exam draws from one table of information. Reproduce this and you can answer every pollutant question:

Pollutant	Source	Environmental/health effect	Reduction method
Carbon dioxide	Complete combustion of fossil fuels, respiration	Greenhouse gas, contributes to global warming	Use renewable energy, reduce fossil fuel use, carbon capture
Carbon monoxide	Incomplete combustion (limited oxygen supply)	Toxic: binds to haemoglobin, reduces oxygen transport in blood	Catalytic converters in cars, ensure complete combustion
Sulfur dioxide	Burning fossil fuels containing sulfur impurities	Acid rain (damages buildings, lakes, vegetation), respiratory problems	Flue gas desulfurisation, use low-sulfur fuels
Nitrogen oxides (NOx)	Formed in hot engines when N2 and O2 react at high temperatures	Acid rain, photochemical smog, respiratory problems	Catalytic converters
Particulates (soot)	Incomplete combustion	Respiratory diseases, reduces visibility, global dimming	Filters on exhausts, ensure complete combustion
Methane	Decomposition of organic matter, livestock, rice paddies	Greenhouse gas (more potent than CO2 per molecule)	Capture from landfill, reduce livestock emissions

The greenhouse effect: what the exam expects

The explanation that earns full marks follows three steps:

1. Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface.
2. The Earth re-emits longer-wavelength infrared radiation.
3. Greenhouse gases (CO2, methane, water vapour) absorb this infrared radiation and re-emit it in all directions, including back towards the Earth, keeping the surface warmer than it would otherwise be.

The enhanced greenhouse effect / global warming: Human activities (burning fossil fuels, deforestation) increase the concentration of CO2 and methane in the atmosphere. This causes more infrared radiation to be absorbed and re-emitted, raising the average global temperature.

Consequences the exam expects you to name: rising sea levels (thermal expansion of water and melting of ice caps), changing weather patterns, loss of habitats, and crop failure.

Acid rain: source, effect, reduction

The formation sequence the mark scheme wants:

1. Fossil fuels contain sulfur impurities. When burned, sulfur reacts with oxygen to form sulfur dioxide: S + O2 -> SO2.
2. Sulfur dioxide dissolves in rainwater to form sulfuric acid: SO2 + H2O + O -> H2SO4 (simplified). The exam sometimes accepts: SO2 dissolves in rainwater to form acid rain.
3. Nitrogen oxides form in car engines at high temperatures: N2 + O2 -> 2NO, then 2NO + O2 -> 2NO2. These dissolve in rainwater to form nitric acid.

Effects of acid rain:

• Corrodes limestone buildings and metal structures.
• Acidifies lakes and rivers, killing aquatic life.
• Damages leaves and roots of trees and crops.
• Releases toxic aluminium ions from soil into waterways.

Reduction: Catalytic converters (convert NOx to N2 and O2), flue gas desulfurisation (removes SO2 from power station exhaust using calcium oxide or calcium carbonate), using low-sulfur fuels.

Ozone depletion

The exam expects a clear causal chain:

1. CFCs were used as refrigerants, aerosol propellants and in foam blowing.
2. CFCs are very stable and drift up to the upper atmosphere without breaking down.
3. UV radiation breaks C-Cl bonds in CFCs, releasing chlorine radicals.
4. Chlorine radicals catalytically destroy ozone: Cl + O3 -> ClO + O2, then ClO + O -> Cl + O2. One chlorine atom destroys many ozone molecules.
5. Reduced ozone means more UV radiation reaches the surface, increasing skin cancer, cataracts and damage to crops.

Solution: Replace CFCs with HFCs (hydrofluorocarbons) that do not contain chlorine and do not deplete ozone.

Water treatment

The exam tests the steps in water purification for drinking:

1. Sedimentation: large particles settle out.
2. Filtration: water passes through sand beds to remove smaller suspended particles.
3. Chlorination: chlorine is added to kill bacteria and other microorganisms.

Some questions ask about distillation as a method to produce pure water from seawater (desalination), which is energy-intensive and expensive.

Worked exam question

Q (Paper 4): Explain how burning fossil fuels in a car engine can contribute to both acid rain and global warming. (4 marks)

Model answer: Burning fossil fuels produces carbon dioxide (1), which is a greenhouse gas that absorbs infrared radiation re-emitted from the Earth and contributes to global warming by increasing the greenhouse effect (1). At the high temperatures in the engine, nitrogen and oxygen from the air react to form nitrogen oxides (1), which dissolve in rainwater to form nitric acid, contributing to acid rain (1).

The four marks map to four distinct ideas: CO2 production, greenhouse mechanism, NOx formation, and acid rain formation. A student who writes only about CO2 earns at most 2 marks even with a perfect explanation.

Environmental chemistry is one of the most revision-efficient topics on the syllabus because the content is finite and the questions repeat closely. If this topic is still causing lost marks, a trial lesson can identify whether the issue is missing content or imprecise phrasing.