Water
1. Importance of Water
Water is ESSENTIAL for life. It covers about 71% of the Earth's surface.
Uses of Water
| Use | Examples |
|---|---|
| Domestic | Drinking, cooking, washing, bathing |
| Agricultural | Irrigation of crops, livestock |
| Industrial | Cooling, cleaning, chemical processes |
| Recreational | Swimming, boating |
| Power generation | Hydroelectricity, thermal power plant cooling |
2. Composition of Water
Water is a COMPOUND made of HYDROGEN and OXYGEN.
Chemical formula: H₂O Ratio by volume: 2 parts hydrogen : 1 part oxygen. Ratio by mass: 1 part hydrogen : 8 parts oxygen.
Electrolysis of Water
When an electric current is passed through water (with a little acid added for conductivity):
- At the NEGATIVE electrode (CATHODE): HYDROGEN gas is produced (2 volumes).
- At the POSITIVE electrode (ANODE): OXYGEN gas is produced (1 volume).
Equation: 2H₂O → 2H₂ ↑ + O₂ ↑
Synthesis of Water
2H₂ + O₂ → 2H₂O (hydrogen burns in oxygen to form water).
3. Sources of Water
| Source | Type | Examples |
|---|---|---|
| Surface water | Fresh water | Rivers, lakes, ponds, streams |
| Groundwater | Fresh water | Wells, tube wells, springs |
| Rainwater | Fresh water | Direct precipitation |
| Oceans and seas | Saline water | 97% of Earth's water — salty, undrinkable |
| Glaciers and ice caps | Fresh water (frozen) | ~2% of Earth's water — locked in ice |
Water Availability
- Only ~3% of Earth's water is FRESH.
- Of this, ~2% is locked in glaciers and ice caps.
- Only ~1% is available for human use (rivers, lakes, groundwater).
4. Water Cycle (Hydrological Cycle)
The continuous movement of water between the Earth's surface and the atmosphere.
Stages of the Water Cycle
| Process | Description |
|---|---|
| Evaporation | Sun heats water → turns into water vapour |
| Transpiration | Plants release water vapour through leaves |
| Condensation | Water vapour cools → forms clouds |
| Precipitation | Water falls as rain, snow, hail, sleet |
| Runoff | Water flows over land into rivers, lakes, oceans |
| Infiltration | Water seeps into the ground (groundwater) |
The water cycle is a NATURAL purification process — evaporation leaves impurities behind.
5. Water Purification Methods
Simple Methods
| Method | What It Removes | Limitations |
|---|---|---|
| Boiling | Kills germs, bacteria, viruses | Does NOT remove dissolved impurities |
| Filtration | Insoluble impurities | Does NOT remove dissolved impurities or germs |
| Sedimentation + Decantation | Heavy suspended particles | Slow, does NOT remove fine particles |
| Chlorination | Kills germs | May leave taste |
Large-Scale Purification (Water Treatment Plant)
- Sedimentation: Large particles settle at bottom.
- Filtration: Water passes through layers of sand, gravel, and charcoal.
- Chlorination: Chlorine is added to kill germs.
- Aeration: Air is mixed in to improve taste.
Distillation
Water is boiled → steam is condensed → PURE WATER (distilled water).
- Removes ALL impurities (dissolved and undissolved).
- Used in laboratories, batteries, and medical applications.
- EXPENSIVE for large-scale use.
6. Hard and Soft Water
Soft Water
Water that lathers EASILY with soap.
Examples: Rainwater, distilled water.
Hard Water
Water that does NOT lather easily with soap (forms a white SCUM/precipitate).
Causes: Dissolved calcium and magnesium salts (bicarbonates, chlorides, sulphates).
Types of Hardness
| Type | Cause | How to Remove |
|---|---|---|
| Temporary hardness | Ca(HCO₃)₂, Mg(HCO₃)₂ | Boiling (decomposes bicarbonates) |
| Permanent hardness | CaSO₄, MgCl₂, CaCl₂ | Distillation, adding washing soda |
7. Water Conservation
Why Conserve Water?
- Only 1% of Earth's water is available freshwater.
- Demand is increasing (population, agriculture, industry).
- Many regions face water SCARCITY.
Methods of Water Conservation
- Rainwater harvesting: Collecting rainwater for later use.
- Drip irrigation: Watering plants at roots — reduces wastage.
- Fixing leaks: Saving thousands of litres per year.
- Recycling wastewater: Treating and reusing water.
- Turning off taps when not in use.
8. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Composition of water and electrolysis | 2-3 marks | Very High |
| Water cycle stages | 2-3 marks | High |
| Water purification methods | 3 marks | Very High |
| Hard vs soft water | 2 marks | Medium |
| Water conservation | 2 marks | Medium |
Common Mistakes
- Saying water is an ELEMENT — it is a COMPOUND of hydrogen and oxygen.
- In electrolysis: thinking oxygen collects at the cathode (it collects at the ANODE).
- Confusing evaporation and boiling (evaporation occurs at all temperatures, boiling at a fixed temperature).
- Boiling does NOT remove dissolved salts — only kills germs.
Self-Test (5 Questions)
Q1. The chemical formula of water is: (1 mark)
- A) HO
- B) H₂O₂
- C) H₂O
- D) HO₂
Q2. 'During electrolysis of water, which gas collects at the cathode?' (1 mark)
Q3. 'Name the process by which water changes from vapour to liquid in the water cycle.' (1 mark)
Q4. 'How does boiling remove temporary hardness?' (2 marks)
Q5. 'What is rainwater harvesting?' (2 marks)
Answers
A1. C) H₂O. A2. Hydrogen gas (2 volumes at cathode). A3. Condensation. A4. Boiling decomposes calcium bicarbonate into calcium carbonate (insoluble — forms scale/precipitate), carbon dioxide, and water. This removes temporary hardness. A5. Rainwater harvesting is the COLLECTION and STORAGE of rainwater for future use (e.g., recharging groundwater, watering plants, household use).
