Chemistry of the environment
Water and its treatment, fertilisers, the air and its pollutants, the greenhouse effect and how pollution is reduced.
📌 Key points
- Water turns anhydrous copper(II) sulfate from white to blue and anhydrous cobalt(II) chloride from blue to pink. Pure water melts at 0 °C and boils at 100 °C.
- Natural water can contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, and nitrates and phosphates from fertilisers, which cause deoxygenation of water.
- Water treatment: sedimentation and filtration remove solids; carbon removes tastes and odours; chlorination kills microbes.
- NPK fertilisers supply nitrogen, phosphorus and potassium for plant growth; ammonium salts and nitrates provide the nitrogen.
- Clean, dry air is about 78% nitrogen and 21% oxygen; the rest is mainly argon, with about 0.04% carbon dioxide.
- Pollutants: carbon monoxide (incomplete combustion, toxic); particulates (respiratory problems, cancer); oxides of nitrogen (car engines, acid rain, photochemical smog); sulfur dioxide (burning fuels containing sulfur, acid rain); methane (decomposing vegetation and animal digestion).
- Carbon dioxide and methane are greenhouse gases: they absorb and re-emit thermal energy, which causes global warming and leads to climate change.
- Catalytic converters remove carbon monoxide and nitrogen monoxide: 2CO + 2NO → 2CO2 + N2.
- Ways to reduce pollution: plant trees, use renewable energy and hydrogen-powered vehicles, use less fossil fuel, and remove sulfur dioxide from waste gases with calcium oxide (flue gas desulfurisation).
- Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy and chlorophyll: 6CO2 + 6H2O → C6H12O6 + 6O2.
📖 Key terms
- Greenhouse gas
- A gas such as CO2 or methane that absorbs and re-emits thermal energy in the atmosphere.
- Acid rain
- Rain made acidic by dissolved sulfur dioxide or oxides of nitrogen.
- Incomplete combustion
- Burning a fuel in too little oxygen, forming carbon monoxide or carbon.
- Chlorination
- Adding chlorine to water to kill microbes.
- Catalytic converter
- A device in a car exhaust that changes harmful gases into less harmful ones.
- Flue gas desulfurisation
- Removing sulfur dioxide from waste gases using calcium oxide.
✏️ Worked example
Explain how burning coal in a power station can cause acid rain, and give one way to reduce it.
- Coal contains sulfur compounds as impurities.
- When coal burns, the sulfur reacts with oxygen to form sulfur dioxide: S + O2 → SO2.
- Sulfur dioxide dissolves in rainwater and reacts to form acids, so the rain becomes acidic.
- Acid rain damages limestone buildings, kills fish and harms trees.
- Reduction: remove SO2 from the waste gases with calcium oxide (flue gas desulfurisation), or use low-sulfur fuels.
Match each pollutant to its source exactly: CO comes from incomplete combustion of carbon-containing fuels; oxides of nitrogen form when nitrogen and oxygen react at high temperature in engines.
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