Water
1. Composition of Water
Water is a COMPOUND composed of HYDROGEN and OXYGEN in the ratio 2:1 by volume (1:8 by mass).
Electrolysis of water: 2H₂O → 2H₂ + O₂
At CATHODE: Hydrogen gas (2 volumes) At ANODE: Oxygen gas (1 volume)
Synthesis of water: 2H₂ + O₂ → 2H₂O
'Water covers about 71% of the Earth's surface. Only 2.5% of it is FRESH water, and less than 1% is ACCESSIBLE for human use.'
2. Physical Properties of Water
| Property | Value/Description |
|---|---|
| Boiling point | 100°C (at sea level) |
| Freezing point | 0°C |
| Density (at 4°C) | 1 g/cm³ (MAXIMUM) |
| Colour | Colourless |
| Odour | Odourless |
| Taste | Tasteless |
| Specific heat capacity | HIGH (4200 J/kg°C) |
Anomalous expansion of water: Water CONTRACTS when cooled from room temperature until 4°C. BELOW 4°C, it EXPANDS. This means ICE is LESS dense than water → ice FLOATS. 'This is UNIQUE to water. Most substances are denser as solids.'
3. Universal Solvent
Water is called the UNIVERSAL SOLVENT because it can DISSOLVE a VERY LARGE number of substances.
Factors affecting solubility:
- Temperature: MOST solids dissolve MORE at higher temperature
- Pressure: Affects solubility of GASES (higher pressure → more gas dissolves)
- Nature of solute: POLAR and IONIC substances dissolve well in water
4. Hard and Soft Water
| Property | Soft Water | Hard Water |
|---|---|---|
| Lather with soap | FORMS easily | Forms SCUM (precipitate) |
| Contains | Only H₂O | Ca²⁺, Mg²⁺ ions (dissolved) |
Types of Hardness
| Type | Cause | Removal |
|---|---|---|
| Temporary | Dissolved Ca(HCO₃)₂ and Mg(HCO₃)₂ (calcium/magnesium hydrogen carbonates) | BOILING — decomposes bicarbonates to INSOLUBLE carbonates |
| Permanent | Dissolved CaSO₄, MgSO₄, CaCl₂, MgCl₂ (sulphates/chlorides) | DISTILLATION, ion exchange, or chemical treatment (washing soda) |
Removing Temporary Hardness
Boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑ Mg(HCO₃)₂ → MgCO₃↓ + H₂O + CO₂↑
The CaCO₃ and MgCO₃ form the 'FUR' (scale) in kettles.
Removing Permanent Hardness
Washing soda method: CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄ MgSO₄ + Na₂CO₃ → MgCO₃↓ + Na₂SO₄
Ion exchange method: Ca²⁺ and Mg²⁺ are EXCHANGED for Na⁺ or H⁺.
5. Disadvantages of Hard Water
- WASTES soap (forms scum instead of lather)
- Causes SCALE in boilers and pipes (reduces efficiency, can cause EXPLOSIONS)
- Makes clothes look DULL
- Spots on glassware and utensils
- Alters the taste of water (in some cases)
6. Water Treatment and Purification
Large-Scale Treatment (Municipal)
| Stage | Process | Purpose |
|---|---|---|
| 1. SEDIMENTATION | Gravity settling | Removes LARGE suspended particles |
| 2. FILTRATION | Sand/gravel layers | Removes FINER particles |
| 3. CHLORINATION | Adding chlorine | KILLS bacteria and germs |
| 4. AERATION | Spraying air | Removes odour, adds oxygen |
Household Purification
| Method | How it Works |
|---|---|
| BOILING | Kills ALL germs (most RELIABLE) |
| FILTER (candle) | Porous candle traps impurities |
| UV purification | Ultraviolet light KILLS microorganisms |
| RO (Reverse Osmosis) | Removes DISSOLVED salts |
| Distillation | Evaporates and condenses → PURE water |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Ice sinks in water' | Ice FLOATS because it is LESS dense (0.92 g/cm³) than water (1 g/cm³) |
| 'Hard water means it is difficult to drink' | Hard water IS drinkable. 'Hard' refers to its behaviour with soap, not its potability |
| 'Distilled water is safe to drink' | Distilled water is PURE but LACKS essential minerals. Prolonged use is NOT recommended |
| 'Boiling removes permanent hardness' | Boiling removes ONLY temporary hardness. Permanent hardness needs chemical treatment |
ICSE Exam Focus (5–7 marks)
- 2-mark questions: Composition of water, physical properties
- 3-mark questions: Difference between hard and soft water
- 4-mark questions: Types of hardness — causes and removal
- 6-mark questions: Water purification methods — diagram and explanation
Self-Test
Q1. Why is ice less dense than water? A1. Water EXPANDS when it freezes below 4°C (anomalous expansion). Ice crystals have OPEN hexagonal structure with more space between molecules, making it less dense.
Q2. What is the difference between temporary and permanent hardness of water? A2. Temporary hardness is caused by bicarbonates and can be removed by BOILING. Permanent hardness is caused by sulphates/chlorides and requires CHEMICAL treatment or distillation.
Q3. How can temporary hardness be removed? A3. By BOILING: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑. The CaCO₃ precipitates out.
Q4. Why does hard water not form lather with soap? A4. The Ca²⁺ and Mg²⁺ ions in hard water react with soap (sodium stearate) to form INSOLUBLE calcium/magnesium stearate (SCUM) instead of lather.
Q5. List three stages of municipal water treatment. A5. (1) Sedimentation — settling of suspended particles. (2) Filtration — removing fine particles through sand/gravel. (3) Chlorination — adding chlorine to kill bacteria.
Q6. Why is water called the 'universal solvent'? A6. Water can dissolve MORE substances than any other liquid due to its POLAR nature. It dissolves ionic compounds, polar covalent compounds, acids, bases, and many gases.
