🔬 1. Water Chemistry
Water is a covalently bonded molecule (H₂O) and is:
- A universal solvent – dissolves many ionic and polar substances.
- Involved in many chemical reactions, including hydrolysis.
- Neutral in its pure state (pH = 7), but may become acidic or basic due to dissolved substances.
Water is essential in daily life and industry due to:
- Its high specific heat capacity – stabilizes temperature.
- Its use as a coolant and reactant (e.g., in photosynthesis).
- Its ability to dissolve salts, gases, and organic substances.
🧪 2. Essential Substances in Water
Natural water sources (e.g., rivers, lakes) contain:
- Dissolved oxygen (O₂) – essential for aquatic life.
- Mineral salts – like calcium (Ca²⁺), magnesium (Mg²⁺), nitrates (NO₃⁻), etc.
- Carbon dioxide (CO₂) – balances aquatic pH.
- Sometimes fluoride (F⁻) – added for dental health (in treated water).
- Chlorine (Cl₂) – added during purification to kill microbes.
⚠️ 3. Major Water Pollutants, Their Sources and Effects
| Pollutant | Source | Effect |
|---|---|---|
| Nitrates/Phosphates (fertilizers) | Agricultural runoff | Causes eutrophication: algae bloom, oxygen depletion, death of aquatic life |
| Sewage | Domestic and industrial waste | Contains pathogens, causes oxygen depletion |
| Heavy metals (Pb²⁺, Hg²⁺, Cd²⁺) | Industrial waste, old pipes | Toxic to humans and animals; accumulates in food chains |
| Oil | Oil spills, shipping | Blocks oxygen, harms aquatic birds and fish |
| Detergents | Household wastewater | Can form foam, be toxic to aquatic life |
| Thermal pollution | Power plants | Reduces dissolved oxygen, affects aquatic ecosystems |
💧 4. Purification of Water (River to Tap)
Main Steps:
- Screening
- Removes large debris like twigs and plastics using metal screens.
- Sedimentation
- Water is held in tanks so heavy particles settle down.
- Filtration
- Water passes through layers of sand and gravel to remove fine particles.
- Chlorination
- Chlorine is added to kill bacteria and pathogens.
- pH Adjustment
- Acids or alkalis may be added to neutralize pH.
- Fluoridation (Optional)
- Fluoride may be added to help prevent tooth decay.
Result: Safe drinking water (tap water).
🌊 5. Getting Pure Water from Sea Water (Desalination)
Two main processes:
- Simple Distillation:
- Sea water is boiled.
- Water vaporizes, leaving salts behind.
- Vapor is then condensed into pure water.
- Reverse Osmosis (advanced, not always required in exams):
- Water is forced through a semipermeable membrane that blocks salt.
Used in desert countries or islands where fresh water is scarce.
✅ Exam Tips:
- Define water pollution: the contamination of water by harmful substances.
- Know key pollutants, their sources, and effects.
- Understand each step of water purification clearly.
- Be able to compare distillation with natural purification.
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