SCIJHS 1 • Term 3Topic 12Free Trial Lesson
Water: Properties, Water Cycle & Purification
Explore physical and chemical properties of water, the hydrological cycle, water pollution, hard vs soft water, and purification methods.
Curated Video Lesson
Visual explanation and practical step-by-step walk-through
Comprehensive Study Notes
Aligned with Ghana NaCCA & WAEC BECE syllabus standards
Topic Introduction & Real-World Context:
Water is the lifeblood of Ghana. Major rivers like the Volta, Pra, and Ankobra provide drinking water, irrigation, and hydroelectric power. However, challenges like illegal gold mining (galamsey) causing heavy water turbidity emphasize the critical need for water conservation and scientific purification.
What You Will Master in This Lesson (NaCCA Objectives):
State the physical and chemical properties of pure water and describe tests for water.
Explain the stages of the hydrological (water) cycle in nature.
Distinguish between hard water and soft water, their causes, advantages, and disadvantages.
Describe domestic and municipal water purification methods used in Ghana.
1. Physical & Chemical Properties of Water
Water (H₂O) is a chemical compound consisting of two hydrogen atoms bonded to one oxygen atom in a 2:1 ratio.
• Physical Properties of Pure Water:
- Colorless, odorless, tasteless, and transparent liquid at room temperature.
- Fixed Boiling Point: Exactly 100°C at normal atmospheric pressure (760 mmHg / 1 atm).
- Fixed Freezing Point: Exactly 0°C at normal atmospheric pressure.
- Maximum Density: Has its maximum density of 1.0 g/cm³ (1,000 kg/m³) at 4°C. (Expands upon freezing, which is why ice floats!).
- Neutral to Litmus: Does not change the color of blue or red litmus paper (pH = 7.0).
- "Universal Solvent": Dissolves an extraordinarily wide range of ionic and polar substances.
• Chemical Tests for the Presence of Water:
1. Anhydrous Copper(II) Sulphate Test:
- Turns from WHITE powder to bright BLUE crystals when water is added.
- CuSO₄ (white) + 5H₂O → CuSO₄·5H₂O (blue).
2. Cobalt(II) Chloride Paper Test:
- Turns from BLUE to PINK in the presence of moisture/water.
• Purity Test for Water:
- Note: Chemical tests only show that water is PRESENT; they do NOT prove water is PURE!
- To prove purity: Determine its boiling point (must boil sharply at 100°C) or freezing point (must freeze sharply at 0°C). Dissolved impurities raise the boiling point and lower the freezing point.
Key Takeaway: Test for presence of water: White CuSO₄ turns blue, or blue cobalt chloride turns pink. Test for PURITY: Boils sharply at 100°C.
Real-World Application: Adding salt to water when boiling yam raises the boiling point above 100°C, cooking the yam faster.
2. The Hydrological (Water) Cycle
The water cycle is the continuous, natural circulation of water between the Earth's surface and the atmosphere, driven by solar energy:
• Key Stages in the Cycle:
1. Evaporation: Solar heat warms oceans, rivers, and lakes, converting liquid water into water vapor that rises into the atmosphere.
2. Transpiration: Plant roots absorb water from the soil, which travels to leaves and evaporates into the air through open stomata.
3. Condensation: As warm water vapor rises into the cooler upper atmosphere, it cools and condenses into tiny liquid water droplets around dust particles, aggregating to form CLOUDS.
4. Precipitation: When water droplets in clouds become too large and heavy to remain suspended, gravity pulls them down as RAIN (or drizzle, hail, snow).
5. Infiltration & Percolation: Rainwater soaks into the soil and porous rock layers to replenish underground water aquifers (forming well and borehole water).
6. Surface Run-off: Water flows over land into streams, rivers, and ultimately returns to the sea to restart the cycle.
Key Takeaway: Water cycle stages: Evaporation + Transpiration → Condensation → Precipitation → Run-off & Infiltration.
Real-World Application: Deforestation in the Ashanti region reduces transpiration, contributing to localized reductions in rainfall and microclimate drying.
3. Hard Water vs Soft Water and Water Purification
• Hard Water vs Soft Water:
- Soft Water: Lathers readily and easily with soap (e.g. distilled water, pure rainwater).
- Hard Water: Does NOT lather easily with soap; forms a sticky, insoluble grey curd or scum. Caused by dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺) picked up when water flows through limestone or gypsum rocks.
• Types of Hardness:
1. Temporary Hardness: Caused by dissolved Calcium hydrogencarbonate [Ca(HCO₃)₂]. Can be removed simply by BOILING (converts Ca(HCO₃)₂ into insoluble calcium carbonate scale/fur: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂).
2. Permanent Hardness: Caused by dissolved Calcium sulphate (CaSO₄) or Magnesium sulphate (MgSO₄). Cannot be removed by boiling; requires chemical treatment (adding washing soda / sodium carbonate Na₂CO₃) or ion-exchange resins.
• Advantages & Disadvantages of Hard Water:
- Advantages: Tastes better due to minerals; calcium builds strong bones and teeth; prevents lead poisoning by coating pipes.
- Disadvantages: Wastes soap; forms "fur" (kettle scale) that damages boilers and pipes; stains laundry.
• Water Purification Stages (e.g. Ghana Water Company Limited):
1. Screening: Metal bars filter out large debris (sticks, plastics, dead vegetation).
2. Aeration: Water is sprayed into air to introduce oxygen and expel foul gases (H₂S).
3. Coagulation & Flocculation: ALUM (potassium aluminium sulphate) is added to clump fine colloidal clay particles into heavy flakes ("flocs").
4. Sedimentation: Water sits in large basins where heavy flocs settle to the bottom as sludge.
5. Sand Filtration: Water passes through layers of coarse gravel, fine sand, and charcoal to remove remaining suspended impurities.
6. Chlorination (Disinfection): A controlled amount of CHLORINE gas or bleach is added to kill pathogenic disease-causing bacteria.
Key Takeaway: Temporary hardness is removed by boiling; permanent hardness requires chemicals. GWCL steps: Screen → Aerate → Coagulate (Alum) → Settle → Filter → Chlorinate.
Real-World Application: The white chalky crust ("fur") on the inside of Ghanaian electric kettles is calcium carbonate deposited from boiling hard borehole water.
Common Mistakes Students Make in BECE Examinations:
⚠️Saying that turning white copper sulphate blue proves water is PURE—it only proves water is PRESENT. Only a fixed boiling point (100°C) proves purity.
⚠️Thinking boiling removes permanent hardness—boiling only removes TEMPORARY hardness.
⚠️Confusing the roles of Alum (coagulates suspended dirt) and Chlorine (kills germs and bacteria).
Teacher's BECE Exam Pro-Tips:
⭐In describing large-scale water treatment, always name the chemical agents: Alum for coagulation; Chlorine for killing bacteria/sterilization.
⭐Remember the soap reaction with hard water: Forms scum and wastes soap because Ca²⁺/Mg²⁺ ions precipitate soap molecules.
Quick Revision Summary Checklist:
Know physical properties of pure water (boils at 100°C, freezes at 0°C).
Know chemical tests: Anhydrous CuSO₄ (white → blue) and Cobalt chloride (blue → pink).
Explain the water cycle stages: Evaporation, Transpiration, Condensation, Precipitation.
Explain hard vs soft water and municipal water purification steps.
Step-by-Step Worked Examples (2)
Real BECE exam-standard problems with complete solution steps
Example 1: Purity Test for Liquid Water
Problem StatementA clear colorless liquid is provided in a beaker. Describe a physical test to determine whether the liquid is pure water.
Step-by-Step Solution:
Step 1: Set up apparatus to determine the boiling point of the liquid using a thermometer and heat source.
Step 2: Heat the liquid until it boils vigorously and record the temperature.
Step 3: If the liquid boils sharply at exactly 100°C at normal atmospheric pressure, it is pure water.
Alternative: Determine its freezing point; pure water freezes sharply at 0°C.
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Key Takeaway / Exam Rule: Impurities raise the boiling point above 100°C and lower the freezing point below 0°C.
Example 2: Explaining How Alum Purifies Water
Problem StatementIn community and municipal water treatment in Ghana, why is alum (potassium aluminum sulphate) added to turbid muddy water?
Step-by-Step Solution:
Step 1: Alum acts as a coagulant (chemical flocculating agent).
Step 2: It neutralizes negative electrical charges on tiny suspended colloidal clay and dirt particles.
Step 3: The particles clump together into larger, heavier flakes called "flocs" that settle rapidly to the bottom by sedimentation.
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Key Takeaway / Exam Rule: Coagulation with alum aggregates microscopic suspended dirt particles so they settle quickly.
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