Water: Composition, Properties, and Treatment

Water is an essential chemical compound consisting of two hydrogen atoms and one oxygen atom. In the JAMB syllabus, we focus on its chemical behavior, hardness, and industrial treatment processes.

1. Composition and Synthesis

Water is formed when hydrogen burns in oxygen. The reaction is expressed as:
2H₂ + O₂ → 2H₂O
By volume, two volumes of hydrogen combine with one volume of oxygen to form two volumes of steam (at temperatures above 100°C). This confirms that the composition of water by volume is 2:1 (Hydrogen:Oxygen).

2. Water as a Solvent and Dissolved Gases

Water is known as a 'universal solvent' because it dissolves more substances than any other liquid. Atmospheric gases like Oxygen, Nitrogen, and Carbon (IV) Oxide dissolve in water. Their biological significance includes:

• Oxygen: Essential for the respiration of aquatic organisms.
• Carbon (IV) Oxide: Needed by aquatic plants for photosynthesis and helps in the formation of shells (calcium carbonate) by marine animals.

3. Hard and Soft Water

Water is described as hard if it does not lather easily with soap. This is caused by the presence of dissolved calcium and magnesium ions.

Types of Hardness

• Temporary Hardness: Caused by dissolved Calcium hydrogen trioxocarbonate (IV) [Ca(HCO₃)₂] or Magnesium hydrogen trioxocarbonate (IV). It can be removed by boiling.
• Permanent Hardness: Caused by the presence of Calcium or Magnesium sulphates and chlorides (e.g., CaSO₄, MgCl₂). It cannot be removed by boiling.

Softening Methods

• Boiling: Removes temporary hardness by decomposing Ca(HCO₃)₂ into insoluble CaCO₃ (kettle fur).
• Addition of Slaked Lime: Ca(OH)₂ reacts with hydrogen trioxocarbonates.
• Addition of Washing Soda: Na₂CO₃ removes both temporary and permanent hardness.
• Ion-Exchange (Permutit): Replaces Ca²⁺ and Mg²⁺ ions with Na⁺ ions.

4. Treatment of Water for Town Supply

The process of making water safe for domestic use involves several steps:

• Coagulation/Flocculation: Alum (Potassium aluminium sulphate) is added to make small dirt particles clump together.
• Sedimentation: The heavy clumps settle at the bottom of the tank.
• Filtration: Water passes through sand and gravel beds to remove remaining impurities.
• Disinfection (Chlorination): Chlorine is added to kill germs and bacteria. Sometimes fluorine is added for dental health.

5. Water of Crystallization and Related Phenomena

• Water of Crystallization: The fixed amount of water chemically combined in a crystal (e.g., CuSO₄·5H₂O).
• Efflorescence: The loss of water of crystallization to the atmosphere (e.g., Na₂CO₃·10H₂O).
• Deliquescence: The absorption of enough water from the atmosphere to form a solution (e.g., CaCl₂, NaOH, FeCl₃).
• Hygroscopy: The absorption of moisture from the atmosphere without forming a solution (e.g., H₂SO₄, Silica gel, CuO).