Water

Water is known as the universal solvent because it can dissolve many substances effectively.

Sources of Water

• Natural sources: Rainwater, well water, spring water, and sea water
• Treated sources: Distilled water, pipe-borne water, and chlorinated water

Types of Water

Water exists in two main types:

• Soft water: Easily forms lather with soap
• Hard water: Does not lather easily due to dissolved salts

Structure of Water

A water molecule (H₂O) has oxygen as its central atom. Oxygen’s electronic configuration is 1s² 2s² 2p⁴. It contains two lone pairs and two unpaired electrons that form covalent bonds with two hydrogen atoms. This results in a V-shaped or bent molecular geometry with a bond angle of approximately 105° due to repulsion between the lone pairs.

Laboratory Preparation of Water

Dry hydrogen gas is ignited in air, producing steam. When the steam contacts a cold surface, it condenses to form water.

Physical Properties of Water

• Boiling point: 100°C
• Freezing point: 0°C
• Maximum density: 1 g/cm³ at 4°C
• Neutral to litmus

Chemical Properties of Water

1. Reacts with electropositive metals to form alkalis and release hydrogen gas:
   2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

   Metals like Mg and Zn react with steam. Cu, Au, Ag, and Hg do not react with water.

2. Reacts with non-metals like chlorine to form acidic solutions:
   H₂O(l) + Cl₂(g) → HCl(aq) + HOCl(aq)

Test for Water

1. Turns white anhydrous copper(II) tetraoxosulphate(VI) blue
2. Turns blue cobalt(II) chloride paper pink

Note: These tests indicate the presence of water but are not exclusive to pure water.

Hardness of Water

Hard water does not easily form lather with soap. It gains hardness from soluble salts such as CaSO₄, MgSO₄, and Ca(HCO₃)₂ present in the soil.

Types of Hardness

• Temporary hardness: Caused by Ca(HCO₃)₂
• Permanent hardness: Caused by CaSO₄, MgSO₄, CaCl₂, MgCl₂

Removal of Temporary Hardness

• By boiling:
  
  Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
• By adding slaked lime:
  
  Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s) + 2H₂O(l)

Effects of Temporary Hardness

• Build-up of fur in kettles and boilers
• Formation of stalagmites and stalactites in caves

Removal of Permanent Hardness

• By adding washing soda:
  
  Na₂CO₃(aq) + CaSO₄(aq) → CaCO₃(s) + Na₂SO₄(aq)
• By adding caustic soda:
  
  2NaOH(aq) + CaSO₄(aq) → Ca(OH)₂(s) + Na₂SO₄(aq)
• Ion exchange using sodium zeolite:
  
  CaSO₄(aq) + Na-zeolite → Ca-zeolite (insoluble) + Na₂SO₄(aq)

Advantages of Hard Water

• Tastes better than soft water
• Contains calcium salts that strengthen teeth and bones
• Helps crustaceans like snails and crabs build shells
• Does not dissolve lead, making it safe in lead pipes

Disadvantages of Hard Water

• Causes fur in kettles and boilers
• Wastes soap
• Unsuitable for dyeing and tanning processes

Treatment of Water for Municipal Supply

1. Coagulation: Chemicals like potash alum (KAl(SO₄)₂) or sodium aluminate (NaAlO₂) are added to help particles clump together.
2. Sedimentation: The flocs settle at the bottom of the tank.
3. Filtration: Water passes through filter beds to remove any remaining dirt.
4. Chlorination (Disinfection): Chlorine is added to kill germs. Iodine and fluorine may also be added to prevent goiter and tooth decay.