This lesson explains the types of water hardness, methods of softening water, water purification processes, and the chemistry behind each method.

1. Core Concepts (Short Notes)

25.1 Hard Water

Water that does not easily form lather with soap. Caused by dissolved calcium and magnesium salts.

Two types:

• Temporary hardness → due to bicarbonates (Ca(HCO₃)₂, Mg(HCO₃)₂)

• Permanent hardness → due to chlorides & sulfates (CaCl₂, MgSO₄)

25.2 Problems Caused by Hard Water

• Wastes soap (forms scum)

• Causes scale (CaCO₃) in boilers & kettles

• Reduces efficiency of heating systems

25.3 Temporary Hardness Removal

(a) Boiling

Ca(HCO₃)₂ → CaCO₃ ↓ + CO₂ + H₂O

(b) Adding Slaked Lime (Ca(OH)₂)

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ ↓ + 2H₂O

25.4 Permanent Hardness Removal

(a) Ion Exchange (Zeolite Method)

Na₂Zeolite + Ca²⁺ → CaZeolite + 2Na⁺

Regeneration uses brine (NaCl solution).

(b) Synthetic Resins

• Cation exchange resin removes Ca²⁺, Mg²⁺

• Anion exchange resin removes Cl⁻, SO₄²⁻

(c) Washing Soda (Na₂CO₃)

Ca²⁺ + CO₃²⁻ → CaCO₃ ↓

25.5 Water Purification for Drinking

Common steps:

• Sedimentation: heavy particles settle

• Filtration: sand filters remove fine particles

• Chlorination: kills bacteria (Cl₂)

• Aeration: removes odors, oxidizes Fe²⁺ → Fe³⁺

• Flocculation: chemicals like alum help settle colloids

25.6 Desalination

Removing salt from seawater:

• Reverse Osmosis (RO): high pressure forces water through semipermeable membrane.

• Distillation: evaporation + condensation.

2. Key Reactions to Remember

Temporary Hardness Removal

Ca(HCO₃)₂ → CaCO₃ + CO₂ + H₂O

Washing Soda Softening

Ca²⁺ + CO₃²⁻ → CaCO₃ ↓

Chlorination

Cl₂ + H₂O → HOCl + HCl HOCl is the disinfecting agent.

3. Tips & Tricks for Exams

• Boiling works ONLY for temporary hardness.

• Permanent hardness needs chemical treatment.

• Ion exchange resins are often tested in diagram questions.

• RO is the most modern & effective desalination method.

• Temporary hardness forms scale when boiled; permanent hardness does not.

• Know the advantages of chlorination but also that excess forms harmful byproducts.

4. Important Points to Remember

• Hardness is caused by Ca²⁺ and Mg²⁺ ions.

• Temporary hardness can be removed by heating; permanent hardness cannot.

• Ion exchange produces very soft (deionized) water.

• Water purification ensures safe drinking water.

RO is widely used in domestic water filters.