1: Water properties

Section 1: Foundations - 1: Water Properties

Water ($H_2O$) is the fundamental solvent of life, and its unique properties are dictated by its molecular structure and polarity. The oxygen atom is highly electronegative, creating a significant dipole moment: partial negative charge ($\delta^-$) on oxygen and partial positive charges ($\delta^+$) on the hydrogens. This polarity enables hydrogen bonding, where the $\delta^+$ hydrogen of one water molecule is attracted to the $\delta^-$ oxygen of another. Each water molecule can form up to four hydrogen bonds, resulting in a dynamic, tetrahedral network.

This hydrogen bonding network underpins water's exceptional physical properties:

• High Specific Heat Capacity: Absorbing large amounts of heat with minimal temperature change buffers organisms and environments against rapid temperature fluctuations.
• High Heat of Vaporization: Significant energy is required to convert liquid water to vapor, providing an efficient cooling mechanism (evaporative cooling).
• High Surface Tension & Cohesion: Strong hydrogen bonding at the air-water interface creates a "skin-like" surface, crucial for capillary action in plants and cell structures. Cohesion (water-water attraction) also contributes to water transport.
• Lower Density of Ice: Hydrogen bonds in ice form an open, crystalline lattice, making ice less dense than liquid water. This allows ice to float, insulating aquatic habitats below.

Water's polarity makes it an excellent solvent, particularly for polar and ionic compounds (salts, sugars, amino acids). Charged or polar solutes become surrounded by water molecules in hydration shells. Oxygen atoms orient towards cations ($\delta^-$ to positive ion), while hydrogens orient towards anions ($\delta^+$ to negative ion). Hydrophobic molecules disrupt the hydrogen bonding network, leading to the hydrophobic effect, a major driving force in protein folding and membrane formation.

Water undergoes autoionization: $2H_2O \rightleftharpoons H_3O^+ + OH^-$. The equilibrium constant for this reaction defines the ion product of water ($K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$ at 25°C). This establishes the neutral point of the pH scale (pH 7) and is the foundation for understanding acid-base chemistry (pH and buffers) in biological systems. Water's ability to act as both a weak acid and a weak base is critical for biochemical reactions.