Understanding Chemistry of the environment: IGCSE Chemistry Explained

Why environmental chemistry matters for your IGCSE

Chemistry does not stop at the laboratory door. The reactions you study in class play out on a planetary scale every day, from the purification of drinking water in treatment plants across Europe and Asia to the formation of acid rain over industrialised regions. For IGCSE Chemistry (0620), the environment section connects core chemical principles to real-world consequences, and examiners expect you to make those connections confidently.

This topic spans three main areas: water, fertilisers, and air quality. Each one draws on ideas you have already met in earlier chapters (acids and bases, oxidation, bonding) and applies them to situations that affect communities worldwide. Whether you sit the Core or Extended tier, the environment questions reward precise chemical vocabulary paired with clear explanations of cause and effect.

Water: testing, purity, and treatment

Chemical tests for the presence of water

Two classic reagent tests confirm whether a liquid sample contains water:

These tests confirm the presence of water, but they do not prove the sample is pure water. A sample of seawater or cola would also turn cobalt(II) chloride paper pink. To confirm purity, you measure the boiling point (exactly 100 degrees Celsius at standard pressure) or the melting point (exactly 0 degrees Celsius). Any dissolved impurity shifts those values.

Why distilled water in the laboratory?

Tap water in Berlin, Lagos, or Singapore all contain dissolved ions and gases that vary by region. If you used tap water in a titration, those extra ions could interfere with your results. Distilled water has been boiled and condensed, removing dissolved solids and giving you a reliable, reproducible solvent for practical chemistry.

Substances found in natural water

Rivers, lakes, and groundwater contain a mix of dissolved and suspended substances. Some are beneficial, others harmful:

Water treatment plants remove many of these contaminants through a multi-stage process. During sedimentation, heavy particles settle to the bottom of large tanks. The water then passes through filtration beds (often layers of sand and gravel) to remove finer suspended matter. Finally, chlorination kills remaining bacteria and microbes, making the water safe to drink.

The approach to water treatment varies considerably between countries. In the Netherlands, advanced membrane filtration has reduced microplastic contamination to levels below detection limits. Japan employs ozone treatment alongside chlorination in many municipal plants. In many developing regions across sub-Saharan Africa and South Asia, basic sand filtration and solar disinfection remain the primary tools, and access to clean drinking water continues to be a pressing public health challenge.

Fertilisers and eutrophication

Plants need nitrogen, phosphorus, and potassium (NPK) to grow. Farmers apply fertilisers to supply these nutrients, and global food production depends on it. The Haber process, which you may have studied in the reversible reactions section, produces the ammonia that feeds most nitrogen-based fertilisers worldwide.

The problem begins when excess fertiliser washes off fields into rivers and lakes. This process, called eutrophication, follows a predictable chain of events:

1. Excess nitrates and phosphates enter the waterway through surface runoff.
2. Algae and aquatic plants grow rapidly on the surface (an algal bloom).
3. The dense surface layer blocks sunlight from reaching deeper plants, which die.
4. Bacteria decompose the dead plant matter, consuming large amounts of dissolved oxygen.
5. Fish and other aquatic organisms suffocate due to the depleted oxygen levels.

The Baltic Sea, bordered by nine European countries, provides one of the starkest examples of eutrophication on a continental scale. Agricultural runoff from Poland, Germany, and the Scandinavian nations has created seasonal "dead zones" where oxygen levels drop too low to support marine life. International agreements such as the Helsinki Convention have attempted to coordinate fertiliser regulation across borders, with mixed results.

In contrast, the restoration of Lake Erie on the US-Canada border shows that reducing phosphate inputs can reverse eutrophication over time, though the process takes decades of sustained effort.

Air quality and climate

Key air pollutants

Combustion of fossil fuels releases several pollutants into the atmosphere. The IGCSE syllabus focuses on the following:

Acid rain

When sulfur dioxide and nitrogen oxides dissolve in atmospheric moisture, they form sulfuric acid and nitric acid respectively. The resulting acidic precipitation damages limestone buildings (calcium carbonate reacts with the acid), kills trees by leaching nutrients from soil, and lowers the pH of lakes, harming aquatic ecosystems.

Scandinavia experienced severe acid rain damage in the 1970s and 1980s, largely caused by industrial emissions carried northward from the UK and Central Europe. International pressure through agreements like the Convention on Long-Range Transboundary Air Pollution (signed by 51 countries) led to desulfurisation technology in power stations and a measurable reduction in acid deposition across northern Europe.

The greenhouse effect

The greenhouse effect itself is not harmful. Without it, Earth's average temperature would be roughly -18 degrees Celsius rather than the current +15 degrees Celsius. The concern is the enhanced greenhouse effect: rising concentrations of CO₂, methane, and other greenhouse gases trap more infrared radiation in the atmosphere, pushing global temperatures upward.

The mechanism works as follows:

• Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface.
• The warm surface re-emits longer-wavelength infrared radiation.
• Greenhouse gases absorb and re-radiate this infrared radiation in all directions, including back toward the surface.
• The net effect is a warming of the lower atmosphere.

The 2015 Paris Agreement set a target of limiting global temperature rise to 1.5 degrees Celsius above pre-industrial levels. As of the mid-2020s, global emissions trajectories suggest this target will be extremely difficult to meet without rapid structural changes in energy production, transport, and agriculture across all major economies.

Ozone depletion

The ozone layer in the stratosphere absorbs harmful ultraviolet radiation from the Sun. Chlorofluorocarbons (CFCs), once widely used as refrigerants and aerosol propellants, catalyse the breakdown of ozone molecules. A single chlorine atom released from a CFC molecule can destroy thousands of ozone molecules before it is eventually removed from the stratosphere.

The Montreal Protocol of 1987, often cited as the most successful international environmental agreement, phased out CFC production across 197 signatory nations. Satellite monitoring confirms that the ozone layer has been slowly recovering since the early 2000s, a clear demonstration that coordinated global action can reverse chemical damage to the atmosphere.

Self-check: test your understanding

Common exam mistakes to avoid

• Confusing presence with purity: Cobalt(II) chloride paper turning pink proves water is present, not that the sample is pure water. Examiners penalise students who conflate these two ideas.
• Incomplete eutrophication sequences: Many students jump from "fertiliser enters water" straight to "fish die." The mark scheme requires every intermediate step: algal bloom, blocked sunlight, plant death, bacterial decomposition, oxygen depletion.
• Calling the greenhouse effect harmful: The natural greenhouse effect keeps Earth habitable. The problem is the enhanced greenhouse effect caused by increased concentrations of greenhouse gases. Precision matters here.
• Mixing up SO₂ and CO₂ effects: Sulfur dioxide causes acid rain. Carbon dioxide is a greenhouse gas. Both come from fossil fuel combustion, but their environmental impacts are distinct. State the specific effect for each pollutant.

Environmental chemistry questions on the IGCSE paper often carry 4-6 marks and require extended written responses. The strongest answers link chemical processes to their environmental consequences with specific vocabulary: name the pollutant, state its source, describe the chemical reaction, and explain the ecological impact. Practise writing these chains in full sentences, and you will find that the marks follow naturally.