WaterCambridge IGCSE Chemistry: Revision notes
Section 1
How do we test for the presence of water?
Two chemical tests are used to detect water in substances:
Test 1: Anhydrous Cobalt(II) Chloride
- Anhydrous cobalt(II) chloride is blue in colour
- When water is present, it absorbs the water and changes to pink (hydrated cobalt(II) chloride forms)
- This is a reversible colour change and is used to confirm water presence
Test 2: Anhydrous Copper(II) Sulfate
- Anhydrous copper(II) sulfate is white in colour
- When water is present, it absorbs the water and changes to blue (hydrated copper(II) sulfate forms)
- This test is also used to confirm and detect water in samples
Both tests rely on the substance changing colour when it becomes hydrated (absorbs water). The colour change is immediate and reversible.
Examiners expect you to name which colour the test substance changes TO, not just that it changes colour. Specify: cobalt(II) chloride turns from blue to pink, and copper(II) sulfate turns from white to blue.
Section 2
How do we test for the purity of water?
Pure water has specific, fixed physical properties that can be used to test its purity:
Melting Point Test
- Pure water has a melting point of 0 °C
- If the melting point is below 0 °C, the water contains dissolved impurities
- The presence of solutes lowers the melting point (freezing point depression)
Boiling Point Test
- Pure water has a boiling point of 100 °C (at atmospheric pressure)
- If the boiling point is above 100 °C, the water contains dissolved impurities
- The presence of solutes raises the boiling point (boiling point elevation)
Why Use These Tests? Melting and boiling points are intensive properties that depend only on the purity of the sample, making them reliable indicators of water purity. Any deviation from these standard values indicates the presence of dissolved substances.
A sample of water boils at 102 °C. This is above 100 °C, so the water is NOT pure – it contains dissolved impurities that have raised its boiling point. Similarly, water that melts below 0 °C is also impure because dissolved substances lower the melting point.
Section 3
Why is distilled water used in practical chemistry?
Distilled water is the standard liquid used in laboratory practical work because:
- It is pure – it contains virtually no dissolved or suspended impurities
- It contains no chemical impurities that could interfere with experimental reactions
- It ensures accurate and reliable results because variables are controlled
- It prevents unwanted side reactions caused by impurities in the water
- It is consistent from sample to sample, allowing experiments to be repeated reproducibly
How is Distilled Water Made? Distilled water is produced by boiling water and condensing the steam back into liquid form. This process leaves impurities behind because they do not evaporate with the water.
Comparison with Tap Water Tap water contains dissolved minerals, chlorine, and other substances added for safety and taste. These impurities can affect reaction rates, products, and the accuracy of experimental measurements.
When answering questions about why distilled water is used, always explain WHAT impurities it lacks and HOW this affects the experiment. Simply saying 'it's pure' is incomplete – examiners want to see the link to experimental reliability.
Using tap water in chemistry is like trying to measure your height while standing on uneven ground – the background noise (impurities) interferes with getting an accurate measurement. Distilled water is the level ground.
Section 4
What contaminants are found in natural water sources?
Natural water sources (rivers, lakes, groundwater) contain various substances dissolved or suspended in them:
| Substance | Source | Effect |
|---|---|---|
| Dissolved oxygen | Atmosphere, aquatic plants | Beneficial – essential for fish and aquatic life to breathe |
| Metal compounds | Rock weathering, soil minerals | Mixed – some are essential minerals (calcium, magnesium); some are toxic (lead, mercury) |
| Plastics | Human pollution, breakdown of larger items | Harmful – damage aquatic organisms, enter food chains, break into microplastics |
| Sewage | Human waste, urban runoff | Harmful – contains disease-causing microbes and pathogens |
| Nitrates and phosphates | Fertiliser runoff from agriculture | Harmful – cause excess algal growth, leading to deoxygenation and dead zones |
Key Distinction:
- Beneficial substances support aquatic ecosystems and are necessary for life
- Harmful substances damage water quality and pose risks to health and ecosystems
Students often incorrectly classify ALL metal compounds as harmful. Remember: some are essential nutrients (like calcium and magnesium in mineral water), while others (like lead or cadmium) are toxic. The exam will test your ability to distinguish between them.
Section 5
How does the water treatment process work?
Domestic water supply treatment involves three main stages to make water safe for drinking:
Stage 1: Sedimentation and Filtration
- Water from the source (river/reservoir) is collected in large settling tanks
- Heavy solids (sand, gravel, dead organisms) settle to the bottom due to gravity (sedimentation)
- Lighter particles and dissolved impurities pass through to the next stage
- The water is then passed through sand and gravel filters to remove remaining suspended solids and particles
- This stage removes visible impurities and turbidity
Stage 2: Activated Carbon Filtration
- Activated carbon has a large surface area with many pores
- It adsorbs (absorbs onto the surface) tastes, odours, and some chemical impurities
- This improves the palatability (taste and smell) of the water
- It can also remove some pesticides and organic compounds
Stage 3: Chlorination
- Chlorine gas or chlorine compounds are added to the water
- Chlorine kills harmful microbes including bacteria, viruses, and parasites that cause disease
- This provides protection against waterborne diseases (cholera, typhoid, dysentery)
- Chlorine remains in the water at low levels to provide residual protection as it travels through pipes to homes
Overall Purpose: These three stages work together to remove physical impurities, improve taste and odour, and most importantly, eliminate disease-causing microbes.
Exam questions often ask 'why' each stage is needed. Your answer should explain WHAT is removed (solids, tastes/odours, or microbes) and WHY this matters (health, safety, or palatability). Avoid just naming the stages without explaining their purpose.
If a question asks why chlorination is necessary: Chlorine kills disease-causing microbes (bacteria, viruses) in water, preventing waterborne diseases like cholera and typhoid. This protects public health.
Must Know
- Water tests: Anhydrous cobalt(II) chloride turns blue to pink when water is present; anhydrous copper(II) sulfate turns white to blue when water is present
- Purity tests: Pure water has a melting point of 0 °C and a boiling point of 100 °C; deviations indicate dissolved impurities
- Distilled water is used in practical chemistry because it contains no chemical impurities that could interfere with experiments and affect results
- Natural water contaminants: Dissolved oxygen (beneficial), metal compounds (mixed), plastics (harmful), sewage (harmful microbes), nitrates and phosphates (cause eutrophication)
- Water treatment: Sedimentation and filtration remove solids, activated carbon removes tastes and odours, and chlorination kills harmful microbes
- Beneficial vs harmful: Dissolved oxygen and some mineral compounds are essential; toxic metals, plastics, sewage, and excess nutrients are harmful to aquatic life and human health
That's the notes covered.
Carry on to the next subtopic.