Cambridge IGCSE Chemistry Notes 10: Chemistry of the Environment
Cambridge IGCSE Chemistry 0620 and 0971 notes on water, fertilisers, air pollutants, greenhouse gases, mitigation, catalytic converters and photosynthesis.
Topic 10 of Cambridge IGCSE Chemistry 0620 and 0971 connects chemical evidence and human activity to water quality, plant nutrition, air pollution and climate. Official sections 10.1 to 10.3 require exact tests, treatment stages, source-effect-strategy chains, the greenhouse mechanism, catalytic-converter chemistry and photosynthesis.
Test for the presence of water
Anhydrous cobalt(II) chloride changes from blue to pink when water is present.
Anhydrous copper(II) sulfate changes from white to blue when water is present.
These tests show that water is present. They do not by themselves show that a liquid is pure water, because an aqueous solution also produces the colour change.
Test water purity
Pure water has sharp melting and boiling points at a stated pressure. At standard atmospheric pressure, expected values are 0 °C and 100 °C.
Impurities can change the measured melting or boiling behaviour. A value close to the expected constant point supports purity, while a range or shifted result suggests impurities.
Pressure affects boiling point, so a careful conclusion uses the conditions provided rather than treating 100 °C as universal at every altitude and pressure.
Distilled water is used in practical chemistry because it contains fewer chemical impurities than tap water. Fewer dissolved ions reduce unwanted reactions and contamination.
Substances in natural water
Natural water may contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates from fertilisers, and phosphates from fertilisers and detergents.
Some contents are beneficial. Dissolved oxygen supports aquatic life, and some metal compounds provide essential minerals.
Others can be harmful. Some metal compounds are toxic; plastics can injure or be consumed by aquatic organisms; sewage can contain disease-causing microbes; and excess nitrates or phosphates can lead to deoxygenation and damage aquatic life.
The detailed stages of eutrophication are outside this Chemistry boundary. The assessed link is nutrient pollution to deoxygenation and aquatic harm.
Domestic water treatment
Domestic water treatment makes water safer and acceptable to use. It does not necessarily make it chemically pure.
- Sedimentation allows larger suspended solids to settle.
- Filtration removes remaining solid particles.
- Carbon removes tastes and odours by adsorption.
- Chlorination kills harmful microbes.
Each stage addresses a different problem. Filtration does not reliably kill microbes, while chlorination does not remove suspended stones or sand. Distillation and domestic treatment therefore have different purposes and products.
