Natural water: useful substances and harmful contamination
Separate dissolved oxygen and useful minerals from potentially harmful compounds, plastics, sewage and nutrient runoff.
Section C10.1 · Water
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish dissolved substances from visible suspended solids.
- Explain why aquatic life can need dissolved oxygen.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which named substance can support oxygen-dependent aquatic life?
What you will learn
- Recall the required substances that natural water can contain and their stated sources.
- Explain beneficial and harmful effects without classifying every dissolved substance the same way.
Natural water is a mixture, with different possible effects
Water from natural sources may contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates from fertilisers and phosphates from fertilisers or detergents. Clear appearance does not exclude dissolved chemicals or microbes. The list describes possible contents, not proof that every river contains every item at the same concentration.
Dissolved oxygen supports aquatic life that needs it, and some metal compounds supply essential minerals for life. Other metal compounds can be toxic, and some plastics harm aquatic life. The compound identity and amount matter: a claim that all metal compounds are either helpful or poisonous is too broad. Sewage can contain harmful microbes that cause disease.
Excess nutrients can damage aquatic life
Nitrates and phosphates can enter water from the named fertiliser and detergent sources. In the required environmental comparison they can lead to deoxygenation of water and damage aquatic life. The syllabus does not require the detailed eutrophication sequence, so the essential link is nutrient input → oxygen depletion → harm to oxygen-dependent aquatic life.
Nutrients useful for plant growth in an appropriate crop setting can therefore have harmful effects when excessive amounts reach a water body. Likewise water containing oxygen is not automatically free of harmful microbes. Explain each relevant substance and effect instead of using a single label such as clean for an entire untested sample.
Sources for this lesson
Pause and explain
Which source pairing matches the required list?
Worked example
A supplied river description includes dissolved oxygen, sewage and fertiliser nitrates. Give one beneficial role and two potential harms.
Show the worked solution
- Dissolved oxygen supports suitable aquatic life.
- Sewage can introduce harmful microbes that cause disease.
- Nitrate input can lead to deoxygenation, harming aquatic life that relies on oxygen.
Answer Dissolved oxygen can support life; sewage microbes and nutrient-linked deoxygenation can cause harm.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
List the seven required possible contents of natural water, including the nitrate and phosphate sources.
Give me a hint
Separate oxygen, compounds, solid pollution and biological contamination.
Compare my reasoning
- Dissolved oxygen and metal compounds can be present.
- Plastics, sewage and harmful microbes are other possible contents.
- Nitrates can come from fertilisers; phosphates from fertilisers and detergents.
Dissolved oxygen; metal compounds; plastics; sewage; harmful microbes; fertiliser nitrates; fertiliser/detergent phosphates.
Look for these in your work
- All seven entries are included.
- The two nutrient sources are correctly attached.
Give two beneficial roles and four required potential harms associated with the named water contents.
Give me a hint
Do not put all metal compounds in one universal category.
Compare my reasoning
- Dissolved oxygen supports aquatic life; some metal compounds supply essential minerals.
- Some metal compounds can be toxic and some plastics harm aquatic life.
- Sewage microbes can cause disease; nitrates/phosphates can lead to deoxygenation and aquatic damage.
Benefits: oxygen and essential minerals. Harms: toxic compounds, plastics, disease-causing microbes and nutrient-linked oxygen depletion.
Look for these in your work
- Benefits and harms refer to specific contents.
- The nutrient harm is explained rather than labelled pollution only.
A fertiliser spill enters a pond. A report calls the spill harmless because fertiliser helps crops, and calls the pond safe because it still looks clear. Evaluate both inferences.
Give me a hint
Useful in one setting and visually clear do not settle the pond's composition or effects.
Compare my reasoning
- Fertiliser nutrients can lead to deoxygenation in a water body and damage aquatic life.
- Crop benefit does not establish unlimited runoff is harmless.
- Clear appearance does not exclude dissolved substances or harmful microbes, so it cannot support the proposed safety claim.
Crop benefit and clarity do not establish harmless pond water; nutrient-linked oxygen depletion and unseen contents remain relevant.
Look for these in your work
- Both flawed inferences are addressed.
- The explanation uses the named environmental effects.
Common mistakes
- Treating clear water as evidence that no dissolved contaminant or microbe is present.
- Calling all metal compounds toxic or all beneficial.
- Calling oxygen depletion an increase in dissolved oxygen.
- Assuming plant-growth benefits prove unlimited river runoff is harmless.
What can you explain now?
What is the required harmful effect of nitrates and phosphates reaching water?
Compare with the explanation
They can lead to deoxygenation and damage aquatic life.
Less dissolved oxygen can harm organisms that require it; the full eutrophication mechanism is not required here.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall the water-content list, connect each relevant substance with a benefit or harm, and explain why clarity alone is insufficient evidence.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.