💧 Water and Its Properties

Why Water is Essential for Life

Water (H₂O) is the most abundant molecule in living organisms and possesses unique properties critical for life.

Molecular Structure

Polarity:

• Oxygen is more electronegative than hydrogen
• Creates partial negative charge (δ-) on oxygen
• Creates partial positive charge (δ+) on hydrogens
• Bent molecular geometry (104.5° bond angle)

Hydrogen Bonding:

• Weak attractions between water molecules
• δ+ hydrogen attracted to δ- oxygen of another molecule
• Each water molecule can form up to 4 hydrogen bonds

Key Properties of Water

1. Cohesion and Adhesion

Cohesion: Water molecules stick to each other (enables surface tension, water transport) Adhesion: Water molecules stick to other polar surfaces (capillary action)

2. High Specific Heat Capacity

• Requires significant energy to change temperature
• Stabilizes body temperature and moderates climate

3. High Heat of Vaporization

• Evaporative cooling (sweating, transpiration)

4. Ice Floats

• Lower density as solid due to crystalline structure
• Insulates aquatic life in winter

5. Excellent Solvent

• Dissolves polar and ionic substances
• Forms hydration shells around ions

Hydrophobic vs. Hydrophilic

Hydrophilic: Polar molecules that dissolve in water Hydrophobic: Nonpolar molecules that don't dissolve in water Amphipathic: Molecules with both regions (e.g., phospholipids)

pH and Buffers

Water ionization: H₂O ⇌ H⁺ + OH⁻

• pH = -log[H⁺]
• pH 7 = neutral; <7 = acidic; >7 = basic
• Buffers resist pH changes