water

19.1 Introduction

Water is a vital chemical, covering approximately 80% of the Earth's surface and comprising about two-thirds of human body weight.
Survival without water is limited to just a few days, while one can survive several weeks without food.
An average adult requires about 2 liters of water daily to maintain health.
Importance of Water:
- Water is an excellent solvent, necessary for various daily activities including drinking, washing, and irrigation.
- Industrial applications also rely on large quantities of water for processes such as cooling and electricity generation.
- In many developing countries, the lack of access to clean water results in millions of deaths each year.
Topics Covered in This Chapter:
- The properties of water as an excellent solvent.
- Water treatment processes and water pollution.
- Analysis of water's composition through experiments.
- Sewage treatment processes.

The water cycle illustrates how water circulates on Earth.
Pure water is rare; rainwater, while relatively pure, still contains dissolved gases like oxygen, nitrogen, and carbon dioxide.
As water moves over soil, it dissolves minerals, leading to varying levels of hardness.

Hard Water: Water that doesn’t easily form lather with soap, characterized by the presence of dissolved Ca²⁺ or Mg²⁺ ions.
Temporary Hardness: Can be removed by boiling, mainly caused by the presence of calcium hydrogencarbonate (Ca(HCO₃)₂).
- Formation mechanism:
  ext{CaCO}3 + ext{H}2 ext{CO}3 ightarrow ext{Ca(HCO}3)_2
Upon boiling, calcium hydrogencarbonate decomposes to form insoluble calcium carbonate:
ext{Ca(HCO}3)₂ ightarrow ext{CaCO}3 + ext{CO}2 + ext{H}2O

Hard water leads to soap wastage, as soap reacts with calcium and magnesium ions to form an insoluble grey precipitate (scum).
With modern detergents less affected by hardness, soap’s use in laundering has diminished.

Permanent Hardness: Cannot be removed by boiling, caused by dissolved calcium sulfate (CaSO₄) or magnesium sulfate (MgSO₄).

Distillation: Boiling water to remove all dissolved solids and liquids, but impractical on a large scale.
Washing Soda (Sodium Carbonate):
- Formula: ext{Na}2 ext{CO}3 ullet 10 ext{H}_2 ext{O}
- Reacts with calcium ions to form an insoluble precipitate of calcium carbonate:
  ext{Ca}^{2+} + ext{CO}3^{2-} ightarrow ext{CaCO}3 ext{ (solid)}
Ion-Exchange Resins: Exchange calcium and magnesium ions for sodium ions, often used in household softeners.
- Reaction example:
  ext{Ca}^{2+} + 2 ext{R}^{Na}
  ightarrow ext{R}^{Ca} + 2 ext{Na}^{+}
- The resin is regenerated by passing a concentrated sodium chloride solution through it.

Each household in Ireland uses on average about 300 liters of water per day.
Water treatment ensures that drinking water is safe, clean, and acceptable for consumption, meeting specific safety and aesthetic standards.

Screening: Removal of large debris via a mesh (e.g., twigs, plastic bags).
Flocculation: Coagulation of small particles into larger clumps ('flocs') assisted by chemicals like aluminium sulfate.
Sedimentation: Water allowed to stand in large tanks to let solids settle out, with over 90% of particles removed.
Filtration: Water flows through sands which remove remaining suspended solids.
Chlorination: Small quantities of chlorine added to kill harmful microorganisms, closely monitored to avoid unpleasant tastes.
Fluoridation: Addition of fluoride compounds to promote dental health, maintained at about 1 ppm.
pH Adjustment: Adjustments are made to ensure water is neutral, using lime or sulfuric acid as needed.

Key Chemicals and Their Functions:
- Aluminium Sulfate: Used for flocculation.
- Chlorine: For sterilizing.
- Fluorine Compounds: For reducing tooth decay.
- Lime/Calcium Hydroxide: For raising pH.
- Sodium Carbonate: For softening.
- Sulfuric Acid: For lowering pH.

Definition: Measure of oxygen consumed by microbial activity in a water sample over 5 days at 20°C in darkness.
Calculation involves comparing initial and final dissolved oxygen levels:
- A high B.O.D. indicates heavy pollution.

Over-enrichment of water with nutrients leads to excessive plant and algal growth, reducing oxygen availability for aquatic life.

Toxic metals (e.g., lead, mercury, cadmium) accumulate in the environment, typically from industrial waste.
Heavy metals are removed through precipitation methods.

Primary Treatment: Physical separation of solids through screening and sedimentation.
Secondary Treatment: Biological treatment using bacteria to decompose organic matter via processes like activated sludge.
Tertiary Treatment: Removal of remaining nutrients (nitrates and phosphates). Can involve chemical precipitates or denitrifying bacteria.

pH Measurement: Using pH sensors to determine water acidity or alkalinity.
Atomic Absorption Spectrometry: Detects heavy metals based on light absorption characteristics of elements.
Colorimetry: Measures concentrations of colored substances based on light absorption, applicable in assessing chlorine levels in water.