Unit 9 Water
Key unit competence: Analyse the sources, properties, composition, purification and pollution of water to evaluate its importance in daily life and maintain environmental sustainability.
Key ideas
- Sources of water: rain, rivers, lakes (Kivu, Muhazi, Burera), springs, wells, boreholes and the sea. Water covers about 71% of the Earth's surface but very little is fresh water that we can use.
- Pure water is colourless, odourless and tasteless; it boils at 100 °C and freezes at 0 °C at standard pressure; its density is 1 g/cm3 at 4 °C; ice is less dense than water and floats.
- Tests for water: it turns anhydrous (white) copper(II) sulfate blue and anhydrous (blue) cobalt(II) chloride paper pink. Purity is shown by boiling at exactly 100 °C.
- Treatment of drinking water: screening, sedimentation, coagulation (alum), filtration (sand), chlorination (kills germs), storage. At home: boiling, filtering, using chlorine tablets or solar disinfection.
- The water cycle: evaporation from oceans and lakes, transpiration from plants, condensation into clouds, precipitation (rain), run-off and infiltration into the ground.
- Water pollutants: sewage, fertilisers and pesticides, industrial waste, oil, plastics, detergents and mining waste. Polluted water spreads diseases (cholera, typhoid, dysentery) and kills aquatic life (eutrophication). Prevent it by treating sewage and waste, using toilets, and limiting fertiliser use near water.
Common misconceptions
- "Clear water is always safe to drink." No: Clear water can contain germs and dissolved poisons.
- "Anhydrous copper(II) sulfate tests for pure water." No: It shows that water is present, not that the water is pure.
- "Boiling water removes all impurities." No: Boiling kills germs but dissolved salts and chemicals remain.
Worked examples
Example 1. Describe two tests to show that a liquid contains water and one test to show it is pure.
Solution
Test 1: add anhydrous copper(II) sulfate; it turns from white to blue.
Test 2: dip blue cobalt(II) chloride paper; it turns pink.
Purity: pure water boils at exactly 100 °C (or freezes at 0 °C) at standard pressure.
Example 2. Explain how fertilisers used on farms can kill fish in a lake.
Solution
Rain washes nitrates and phosphates into the lake.
These nutrients cause rapid growth of algae (algal bloom).
Algae die and bacteria decompose them, using up dissolved oxygen.
Fish die from lack of oxygen (eutrophication).
Practice questions
List three sources of water.
Show answer
Rain, rivers, lakes, springs, wells, boreholes (any three).
Common mistake: Listing tap water, which comes from these sources.
Give two physical properties of pure water.
Show answer
Colourless, odourless, tasteless, b.p. 100 °C, f.p. 0 °C, density 1 g/cm3 (any two).
Common mistake: Saying it is blue.
Name the chemical used to test for the presence of water and give the colour change.
Show answer
Anhydrous copper(II) sulfate: white to blue.
Common mistake: Giving blue to white.
Why is chlorine added to drinking water?
Show answer
To kill disease-causing germs.
Common mistake: Saying it removes mud.
Name the stages of the water cycle.
Show answer
Evaporation, transpiration, condensation, precipitation, run-off, infiltration.
Common mistake: Leaving out condensation.
Give two diseases caused by drinking polluted water.
Show answer
Cholera, typhoid, dysentery, diarrhoea (any two).
Common mistake: Naming malaria, which is spread by mosquitoes.
State three uses of water at home.
Show answer
Drinking, cooking, washing, cleaning, watering gardens (any three).
Common mistake: Giving industrial uses only.
Explain why lakes freeze from the top in cold countries.
Show answer
Ice is less dense than water, so it floats and forms a layer on top.
Common mistake: Saying cold air is heavier.
Give two ways to prevent water pollution.
Show answer
Treat sewage; do not dump waste in rivers; use fertilisers carefully; build latrines away from water (any two).
Common mistake: Saying 'stop using water'.
Explain why boiled water may still not be pure.
Show answer
Boiling kills germs but dissolved salts and chemicals remain.
Common mistake: Saying boiling removes everything.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which disease is spread by contaminated water? a. pink 2. Cobalt(II) chloride paper turns from blue to ______ with water. b. Cholera 3. In water treatment, alum is added to: c. ice is less dense than liquid water 4. Ice floats on water because: d. make small particles clump together and settle Show answer
1 → b (Cholera) · 2 → a (pink) · 3 → d (make small particles clump together and settle) · 4 → c (ice is less dense than liquid water)
- Cholera bacteria are spread through dirty water.
- Blue cobalt chloride paper becomes pink when wet.
- Coagulation makes tiny particles stick together so they settle or are filtered.
- Water expands on freezing, so ice is less dense.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
eutrophication · condensation · transpiration · run-off- Fertilisers washed into a lake cause too much algae to grow. This is called .
- The process by which water vapour forms clouds is .
- Rain falling on land and flowing into rivers is called .
Show answer
1: eutrophication · 2: condensation · 3: run-off
- Extra nutrients cause algal blooms that use up oxygen.
- Cooling vapour condenses into tiny droplets.
- Run-off is surface flow of water to rivers and lakes.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which method gives the purest water? a. 100 °C 2. At standard pressure, pure water boils at: b. Lake Muhazi 3. Which process in water treatment kills germs? c. Chlorination 4. Which is a source of fresh water in Rwanda? d. Distillation Show answer
1 → d (Distillation) · 2 → a (100 °C) · 3 → c (Chlorination) · 4 → b (Lake Muhazi)
- Distillation removes dissolved substances and germs.
- This fixed boiling point shows water is pure.
- Chlorine kills micro-organisms.
- Lake Muhazi is a freshwater lake in Rwanda.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
transpiration · water is added · 100 °C · 4 °C- Loss of water vapour from plant leaves is called .
- Anhydrous copper(II) sulfate turns from white to blue when .
- Water's density is greatest at .
Show answer
1: transpiration · 2: water is added · 3: 4 °C
- Plants release water vapour through their leaves.
- Water combines with it to form blue hydrated copper(II) sulfate.
- Water is densest at 4 °C, so lakes freeze from the top.