Introduction to Impurities in Water | Engineering Chemistry Notes

Introduction to Impurities in Water

Pure water is a theoretical concept (H₂O) and is rarely found in nature. Natural water always contains a variety of impurities that affect its quality, usability, and treatment requirements. In Engineering Chemistry, understanding these impurities is fundamental to designing water purification and treatment processes.

1. Classification of Impurities in Water

Suspended Impurities: Sand, clay, silt, organic matter, microorganisms.
Colloidal Impurities: Finely divided particles of silica, clay, or organic substances that do not settle easily.
Dissolved Impurities
- Gases: CO₂, O₂, N₂, H₂S, SO₂, etc.
- Mineral salts: Ca²⁺, Mg²⁺, Na⁺, Cl^-, SO₄^2-, HCO₃^-, etc.
- Organic compounds: Natural organics, industrial effluents, agricultural chemicals.
Microbiological Impurities: Bacteria, viruses, algae, protozoa.

Figure: Classification of impurities in natural water

2. Effects of Impurities

Suspended matter: Causes turbidity, sedimentation, and microbial growth.
Colloidal matter: Stable, resists settling, requires coagulation.
Dissolved gases:
- CO₂ → Corrosion of pipes, acidity.
- O₂ → Supports corrosion and microbial growth.
- H₂S → Foul odor, toxicity.
Dissolved salts:
- Ca²⁺, Mg²⁺ → Hardness of water.
- Cl^-, SO₄^2- → Salinity, scaling, taste issues.
Microbiological: Pathogenic organisms cause waterborne diseases (cholera, typhoid, dysentery).

3. Engineering Relevance

Knowledge of impurities is essential for:

Design of water treatment plants
Prevention of boiler scaling and corrosion
Ensuring safe drinking water
Industrial applications (textile, paper, food, pharmaceuticals)

Typical Water Impurities (Examples)

Type	Examples	Effects
Suspended	Sand, clay, silt	Turbidity, sedimentation
Colloidal	Silica, organic matter	Stable, causes color
Dissolved gases	CO₂, O₂, H₂S	Corrosion, foul odor
Dissolved salts	Ca²⁺, Mg²⁺, Cl^-, SO₄^2-	Hardness, scaling, taste
Microbiological	Bacteria, viruses, protozoa	Diseases

Quick Revision Points

Impurities in water exist in suspended, colloidal, dissolved, and microbiological forms.
Hardness is caused by Ca²⁺ and Mg²⁺ salts.
Corrosion is enhanced by dissolved O₂ and CO₂.
Pathogens in water cause waterborne diseases.
Understanding impurities is essential for designing water treatment methods.