Substances in Water from Natural Sources
Section: 10. Chemistry of the Environment | Syllabus: Cambridge IGCSE Chemistry (0620)
Substances Found in Water from Natural Sources
Water from natural sources (rivers, lakes, seas) is never completely pure - it may contain any of the following:
- Dissolved oxygen
- Metal compounds
- Plastics
- Sewage
- Harmful microbes
- Nitrates (from fertilisers)
- Phosphates (from fertilisers and detergents)
Beneficial and Harmful Substances
Some of these substances are beneficial, others are harmful:
| Substance | Beneficial or harmful? | Reason |
|---|---|---|
| Dissolved oxygen | Beneficial | needed by aquatic life for respiration |
| Some metal compounds | Beneficial | provide essential minerals for life |
| Some metal compounds | Harmful | toxic (e.g. lead or mercury compounds) |
| Some plastics | Harmful | harm aquatic life |
| Sewage | Harmful | contains harmful microbes which cause disease |
| Nitrates and phosphates | Harmful | lead to deoxygenation of water and damage to aquatic life |
Nitrates and phosphates act as fertilisers for algae, causing it to grow rapidly and use up the water's dissolved oxygen as it dies and decomposes - this is called eutrophication. Detailed knowledge of the eutrophication process is not required; you only need to know that the outcome is deoxygenation of the water, which damages aquatic life.
Worked Example
- Question: state why phosphates and some metal compounds found in river water are harmful. Give one reason for each. [2]
- Answer: phosphates - cause deoxygenation of the water; metal compounds - (some are) toxic
Common Mistakes
- ❌ Assuming all metal compounds in water are toxic
- ✓ only some metal compounds are toxic (e.g. lead, mercury) - others provide essential minerals and are beneficial
- ❌ Describing the full eutrophication process (algal bloom → light blocked → plants die → bacteria decompose them → oxygen used up) as if it were required exam content
- ✓ the syllabus only requires stating the outcome - nitrates and phosphates cause deoxygenation of water, damaging aquatic life
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