Water
Introduction
Water is essential for life and is often called the 'universal solvent' because of its ability to dissolve a wide range of substances. ICSE Class 9 covers the chemical properties of water, hardness, purification, and pollution.
Water as a Universal Solvent
Water can dissolve more substances than any other liquid. This is due to its polar nature.
Polar Nature: Water molecules have a bent shape with a partial positive charge on hydrogen atoms and a partial negative charge on oxygen atoms. This polarity allows water to dissolve ionic compounds and polar molecules.
Substances that dissolve in water: Common salt, sugar, oxygen (small amounts), carbon dioxide Substances that do not dissolve in water: Oil, wax, sand, fats
Importance of waters solvent property:
- Transport of nutrients in living organisms (blood is mostly water)
- Chemical reactions in aqueous solutions
- Washing and cleaning
Hard and Soft Water
Hard Water
Water that does not produce a good lather with soap.
Causes: Dissolved calcium and magnesium salts (bicarbonates, chlorides, sulphates)
Types of Hardness
| Type | Cause | Removal |
|---|---|---|
| Temporary hardness | Ca(HCO₃)₂ and Mg(HCO₃)₂ | Boiling, Clarks method |
| Permanent hardness | CaCl₂, CaSO₄, MgCl₂, MgSO₄ | Washing soda, ion exchange, distillation |
Temporary Hardness
Removal by boiling: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂
The CaCO₃ precipitate (scale) settles down, making water soft.
Permanent Hardness
Removal by washing soda: CaSO₄ + Na₂CO₃ → CaCO₃ + Na₂SO₄
Ion exchange method: Uses ion exchange resins to replace Ca²⁺ and Mg²⁺ with Na⁺ or H⁺.
Advantages and Disadvantages of Hard Water
| Advantages | Disadvantages |
|---|---|
| Tastes better | Wastes soap/detergent |
| Beneficial for bones (Ca²⁺) | Forms scale in boilers/kettles |
| Reduces risk of heart disease | Reduces efficiency of heaters |
Water Pollution
Sources of Water Pollution
- Industrial waste: Chemicals, heavy metals, hot water
- Agricultural runoff: Fertilisers, pesticides, animal waste
- Domestic sewage: Human waste, detergents, food waste
- Oil spills: From ships and offshore drilling
Effects of Water Pollution
- Eutrophication: Excess nutrients cause algal blooms, depleting oxygen
- Bioaccumulation: Toxic substances accumulate in the food chain
- Diseases: Cholera, typhoid, dysentery (from contaminated water)
- Death of aquatic life: Due to toxins and oxygen depletion
Control Measures
- Treatment of industrial waste before release
- Proper sewage treatment
- Reduced use of chemical fertilisers
- Ban on dumping plastics and chemicals in water bodies
Water Treatment (Purification for Town Supply)
Steps in a water treatment plant:
- Sedimentation: Heavy particles settle at the bottom
- Coagulation: Alum (potash alum) added to clump fine particles together
- Filtration: Water passes through layers of sand, gravel, and charcoal
- Chlorination: Chlorine added to kill bacteria and microorganisms
- Aeration: Air is mixed to improve taste and remove dissolved gases
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| All dissolved solids cause hardness | Only Ca²⁺ and Mg²⁺ salts cause hardness |
| Boiling removes all hardness | Boiling removes only temporary hardness |
| Hard water is completely useless | Hard water has some health benefits |
| Soft water is chemically pure | Soft water still contains dissolved substances |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Hard water and softening methods | 4-5 marks | Very common |
| Water pollution and control | 3-4 marks | Common |
| Water treatment process | 3-4 marks | Common |
| Temporary vs permanent hardness | 3 marks | Frequently asked |
Self-Test
Q1: What causes temporary and permanent hardness in water?
Q2: Explain how boiling removes temporary hardness. Write the chemical equation.
Q3: List the steps involved in water purification for town supply.
Q4: What is eutrophication? How does it affect aquatic life?
Q5: Differentiate between hard water and soft water.
