Water
CIE IGCSE Chemistry· 10.1· 25 min read
1. Sources of Water and Common Contaminants★☆☆☆☆⏱ 10 min
The main global sources of potable (drinking) water are freshwater sources: rivers, lakes, reservoirs, and underground aquifers. Saltwater from oceans is only used for drinking water in arid regions with limited freshwater, via energy-intensive desalination processes.
Potable water
Water that is safe for human consumption, free from harmful levels of microbes, toxic chemicals, and sewage contaminants.
Water from natural sources is not pure: it may contain a range of dissolved and suspended substances. The syllabus requires you to recall these substances: dissolved oxygen; metal compounds; plastics; sewage; harmful microbes; nitrates (from fertilisers); and phosphates (from fertilisers and detergents). Some of these substances are beneficial, while others are potentially harmful.
Substance in natural water | Beneficial or harmful? | Effect |
|---|---|---|
Dissolved oxygen | Beneficial | Needed by aquatic life (fish, invertebrates) for respiration |
Some metal compounds | Beneficial | Provide essential minerals for living organisms |
Some metal compounds | Harmful | Certain metal compounds are toxic |
Plastics | Harmful | Harm aquatic life |
Sewage | Harmful | Contains harmful microbes that cause disease |
Nitrates (from fertilisers) and phosphates (from fertilisers and detergents) | Harmful | Lead to deoxygenation of water and damage to aquatic life |
Name three substances that may be present in water from a natural source, and state whether each is beneficial or potentially harmful.
- 1
- Dissolved oxygen: beneficial, because it is needed by aquatic life for respiration.
- 2
- Sewage: harmful, because it contains harmful microbes that cause disease.
- 3
- Nitrates and phosphates (from fertilisers/detergents): harmful, because they lead to deoxygenation of the water and damage aquatic life.
2. Domestic Water Treatment Process★★☆☆☆⏱ 12 min
Raw (untreated) water is made safe for domestic use through the treatment stages required by the 0620 syllabus. You must be able to describe each stage and its purpose.
Sedimentation: The water is left to stand so that insoluble solids settle out under gravity as sludge, which is removed.
Filtration: The water is passed through filter beds (e.g. sand and gravel) to remove the remaining fine suspended solids.
Carbon: The water is passed through activated carbon, which removes unpleasant tastes and odours.
Chlorination: Chlorine is added to kill microbes (bacteria), sterilising the water.
Explain the purpose of passing water through carbon and then adding chlorine during domestic water treatment.
- 1
- Carbon (activated carbon) removes unpleasant tastes and odours from the water, improving its quality for drinking.
- 2
- Chlorine kills microbes (harmful bacteria) in the water, sterilising it to prevent waterborne diseases.
3. Testing Water Purity★★☆☆☆⏱ 10 min
Pure water
Chemically pure H₂O with no dissolved contaminants, with a fixed boiling point of 100°C and melting point of 0°C at standard atmospheric pressure.
You must recall tests to confirm the presence of water, and a test to confirm that water is chemically pure. There are two named reagents that detect the presence of water: anhydrous copper(II) sulfate and anhydrous cobalt(II) chloride.
Presence of water (anhydrous copper(II) sulfate): Add the sample to white anhydrous copper(II) sulfate. If water is present, the solid turns blue as hydrated copper(II) sulfate forms: .
Presence of water (anhydrous cobalt(II) chloride): Add the sample to blue anhydrous cobalt(II) chloride. If water is present, the solid turns pink as hydrated cobalt(II) chloride forms: .
Purity of water: Measure the melting point and boiling point. Pure water melts at exactly 0°C and boils at exactly 100°C at 1 atm pressure; dissolved impurities lower the melting point below 0°C and raise the boiling point above 100°C.
A student tests a tap water sample and finds its boiling point is 101.5°C. Explain what this result shows about the sample, and state the result of an anhydrous copper(II) sulfate test on the same sample.
- 1
- The boiling point is higher than 100°C, so the tap water is not chemically pure: it contains dissolved solid impurities (e.g. mineral salts).
- 2
- The anhydrous copper(II) sulfate will still turn from white to blue (and anhydrous cobalt(II) chloride from blue to pink), because the sample contains water even though it is not pure.
4. Uses of Water★☆☆☆☆⏱ 8 min
Water has two broad categories of use that you must recall for Core exams: domestic (household) use and industrial use.
Domestic uses: Drinking, cooking, washing clothes/utensils, personal hygiene, watering garden plants.
Industrial uses: As a solvent for chemical manufacturing processes, as a coolant for power station turbines and factory machinery, irrigation of agricultural crops, food and beverage production.
Explain why water is a suitable coolant for industrial power station turbines.
- 1
- Water has a very high specific heat capacity, meaning it can absorb very large amounts of heat energy without a large increase in its own temperature.
- 2
- This makes it a low-cost, effective material to cool hot turbine parts and prevent overheating damage.
5. Common Pitfalls
Wrong move:
Referring to potable water as 'pure water'
Why:
Potable water is safe to drink but may contain low levels of harmless dissolved minerals, so it is not chemically pure.
Correct move:
Always distinguish between potable (safe to drink) and pure (no dissolved contaminants) water in responses.
Wrong move:
Claiming the anhydrous copper(II) sulfate test confirms pure water
Why:
The test only confirms water is present, even if other impurities are also in the sample.
Correct move:
Use boiling point measurement to test for pure water, and copper sulfate only to test for the presence of water.
Wrong move:
Naming ozone or UV light as water sterilisation methods for Core exams
Why:
The Core syllabus only requires you to recall chlorination as the standard sterilisation step for domestic water treatment.
Correct move:
Only name chlorine as the sterilisation chemical in Core exam responses.
Wrong move:
Listing screening or adding aluminium sulfate (flocculation) as 0620 water-treatment steps
Why:
These are not part of the 0620 syllabus; they belong to other exam boards' potable-water schemes.
Correct move:
Recall only the three syllabus ideas: sedimentation and filtration (remove solids), carbon (remove tastes and odours), and chlorination (kill microbes).
Wrong move:
Stating desalination is a common global water source
Why:
Desalination is very energy-intensive and expensive, so it is only used in arid regions with no accessible freshwater sources.
Correct move:
Name rivers, lakes, reservoirs and aquifers as the most common global sources of potable water.
6. Quick Reference Cheatsheet
Concept | Core Syllabus Key Details |
|---|---|
Potable water definition | Safe to drink, free from harmful microbes/contaminants |
Water treatment steps | Sedimentation & filtration (remove solids) → carbon (remove tastes/odours) → chlorination (kill microbes) |
Test for water presence | White anhydrous CuSO₄ → blue; or blue anhydrous CoCl₂ → pink |
Test for pure water | Melts at 0°C and boils at 100°C at 1 atm pressure |
Key industrial uses | Coolant, solvent for reactions, crop irrigation |
7. Frequently Asked
Is potable water the same as pure water?
No. Potable water is safe to drink but may contain low levels of harmless dissolved minerals, while pure water has no dissolved contaminants at all.
What is the difference between testing for water and testing for pure water?
The anhydrous copper(II) sulfate test (white to blue), or the anhydrous cobalt(II) chloride test (blue to pink), only confirms water is present, regardless of impurities. To test for pure water, measure its melting and boiling points: pure water melts at exactly 0°C and boils at exactly 100°C at 1 atm pressure.
Going deeper
What's Next
Now that you have mastered core water chemistry content, you are ready to move on to other topics in the Chemistry of the Environment unit for CIE IGCSE Chemistry 0620. Next, you will learn about air quality and common air pollutants, followed by the carbon cycle and greenhouse gas emissions. These topics are frequently tested together with water content in Section B structured questions, so revise them in sequence to build a complete understanding of environmental chemistry for your exam. You can also practice structured response questions to test your recall of treatment steps and purity tests under exam conditions.