MEV 013: Unit 06 - Water Chemistry

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UNIT 6: WATER CHEMISTRY

6.0 Introduction

Water is not only vital for life, but also central to Earth's physical, chemical, and biological systems. The chemistry of water governs its behavior in natural and engineered environments. This unit explores water’s molecular structure, distribution, properties, and the various chemical and biological constituents that influence water quality and its environmental significance.

6.1 Objectives

After completing this unit, you will be able to:

  • Understand the structure and distribution of water on Earth.
  • Describe the chemical and physical properties of water.
  • Explain the hydrological cycle and significance of water sources.
  • Differentiate between freshwater and seawater chemistry.
  • Identify major chemical species in water and assess water quality.
  • Understand the role of gases, organic matter, and humic substances in aquatic systems.

6.2 Distribution of Water

Water exists in multiple reservoirs:

  • Oceans and Seas: ~97.5% (saline)
  • Glaciers and Ice Caps: ~1.7%
  • Groundwater: ~0.75%
  • Lakes, Rivers, and Soil Moisture: ~0.01%
  • Atmosphere and Biosphere: <0.01%

Freshwater availability is limited and unevenly distributed, creating the need for sustainable management.

6.3 Chemistry of Water – Structure and Polarity

  • Water Molecule (H₂O): Two hydrogen atoms covalently bonded to one oxygen atom.
  • Bond Angle: ~104.5°
  • Polarity: Unequal electron distribution leads to a partial positive charge on hydrogen and a partial negative charge on oxygen, making water a polar solvent.
  • Hydrogen Bonding: Responsible for many unique properties like high boiling point, surface tension, and solubility.

6.4 Properties of Water

Key properties:

  • Universal Solvent: Dissolves a wide variety of substances.
  • High Specific Heat: Moderates Earth's climate.
  • Cohesion and Adhesion: Surface tension and capillary action.
  • Density Anomaly: Ice is less dense than liquid water.
  • Transparency: Supports photosynthesis in aquatic ecosystems.

6.5 Hydrology

Study of movement, distribution, and quality of water on Earth.

6.5.1 Groundwater and Water Table

  • Groundwater: Water stored in soil and rock formations.
  • Water Table: Upper surface of the saturated zone.
  • Aquifers and recharge rates are vital in assessing groundwater availability and sustainability.

6.6 Sources and Uses of Water: The Hydrological Cycle

The Hydrological Cycle includes:

  1. Evaporation
  2. Condensation
  3. Precipitation
  4. Infiltration
  5. Runoff
  6. Transpiration

Uses of Water: Domestic, agricultural, industrial, recreational, ecological.

6.7 Physical and Chemical Properties of Freshwater and Seawater

6.7.1 Physical Properties

Property

Freshwater

Seawater

Density

~1.00 g/cm³

~1.025 g/cm³

Freezing Point

0°C

-1.9°C

Conductivity

Low

High (due to salt ions)

Light Penetration

Greater

Variable

6.7.2 Chemical Properties

  • Freshwater: Lower concentration of dissolved ions.
  • Seawater: High concentrations of Na⁺, Cl⁻, Mg²⁺, SO₄²⁻, K⁺, Ca²⁺.
  • pH: Seawater is slightly alkaline (~8.1); freshwater varies with geology and pollution.

6.7.3 Environmental Significance

  • Determines aquatic biodiversity, nutrient cycling, and water usability.
  • Chemical imbalances can lead to eutrophication, acidification, and pollution.

6.8 Coagulation and Sedimentation

Coagulation: Addition of chemicals (e.g., alum, ferric chloride) to destabilize colloidal particles.

Sedimentation: Settling of coagulated particles under gravity.

Both processes are essential in water treatment plants to remove suspended solids and turbidity.

6.9 Water Quality

Water quality assessment includes physical, chemical, and biological parameters.

6.9.1 Physical Characteristics

  • Color
  • Odor
  • Taste
  • Turbidity
  • Temperature

6.9.2 Chemical Characteristics

  • pH
  • Dissolved Oxygen (DO)
  • Biochemical Oxygen Demand (BOD)
  • Chemical Oxygen Demand (COD)
  • Hardness
  • Nutrients (N, P)
  • Heavy Metals (Pb, Hg, Cd)

6.9.3 Biological Characteristics

  • Microorganisms: Bacteria (e.g., E. coli), viruses, algae.
  • Biological Indicators: Macroinvertebrates indicating pollution levels.

6.10 Chemical Species in Water

6.10.1 Organic Components in Water

  • Natural: Carbohydrates, amino acids, fatty acids, lignins.
  • Anthropogenic: Pesticides, hydrocarbons, detergents.
  • Impact: Oxygen depletion, toxicity, bioaccumulation.

6.10.2 Inorganic Components in Water

  • Cations: Ca²⁺, Mg²⁺, Na⁺, K⁺
  • Anions: Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻
  • Nutrients: Nitrate, phosphate → cause eutrophication in excess.

6.11 Distribution of Gases in Water

6.11.1 Oxygen in Water

  • Sources: Diffusion from air, photosynthesis.
  • DO levels indicate water health.
  • Low DO → hypoxia, harmful to aquatic life.

6.11.2 Carbon Dioxide in Water

  • Dissolves as H₂CO₃ (carbonic acid).
  • Maintains pH balance and supports aquatic photosynthesis.
  • CO₂ ↔ HCO₃⁻ ↔ CO₃²⁻ equilibrium is important in buffering capacity.

6.12 Organic Matter and Dissolved Humic Substances in Water

  • Organic Matter: Derived from plant and animal decay.
  • Humic Substances: High molecular weight, colored, acidic compounds.
    • Categories: Humic acid, fulvic acid, humin.
  • Influence:
    • Metal ion complexation
    • Light absorption (brown water)
    • Organic carbon content

6.13 Let Us Sum Up

Water chemistry is a foundational field in environmental science and hydrology. Water's structure and polarity make it a powerful solvent and key player in global biogeochemical cycles. This unit covered the distribution, properties, and chemistry of water, alongside its quality parameters, chemical species, and the dynamics of gases and organic matter. Understanding these aspects is crucial for managing water resources sustainably.

6.14 Keywords

  • Hydrological Cycle-The continuous movement of water on, above, and below the Earth's surface
  • Polarity-Unequal sharing of electrons in a molecule causing partial charges
  • Coagulation-Process of destabilizing suspended particles in water
  • Sedimentation-Settling of particles under gravity
  • Dissolved Oxygen (DO)-Measure of oxygen available for aquatic life
  • Biochemical Oxygen Demand-Amount of oxygen consumed by microorganisms while decomposing organic matter
  • Hardness-Presence of Ca²⁺ and Mg²⁺ in water
  • Humic Substances-Complex organic molecules from decomposition, affecting water chemistry
  • Eutrophication-Excessive nutrient enrichment leading to algal blooms and oxygen depletion

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