Hydrophilic vs Hydrophobic Molecules and Water Properties

Discussion introduces a contrast: hydrophobic vs hydrophilic molecules; this is a trial run, with some molecules not yet known or seen.
The goal is to understand why hydrophilic (water-loving) molecules behave differently from hydrophobic (water-fearing) ones in biological contexts.

Water is a polar covalent molecule.
The symbol \delta^- denotes a partial negative charge on the oxygen atom, reflecting its higher electronegativity relative to hydrogen.
The hydrogens carry partial positive charges (often denoted \delta^+).
These polarity features drive water’s hydrogen bonding and interactions with other molecules.

The associations between water molecules (hydrogen bonds) are transient in liquid water; each bond is relatively weak.
However, collectively these interactions are numerous enough to give water unique properties (high cohesion, surface tension, etc.).
The combination of many weak bonds leads to strong overall cohesion in liquid water.

Water has a high heat capacity, meaning it can absorb a lot of heat with only a modest increase in temperature.
This is described by the relation Q = m c \Delta T, where:
- Q is the heat added,
- m is the mass,
- c is the specific heat capacity, and
- \Delta T is the change in temperature.
For water, the specific heat capacity is approximately c \approx 4.18\ \mathrm{J\,g^{-1}\,K^{-1}}, explaining why heating water requires a substantial amount of energy to raise its temperature by 1 Kelvin (or 1 °C).

Water expands when it freezes, which is unusual among common liquids.
This expansion has important consequences for life and environmental processes (e.g., ice floating on liquid water, insulation of aquatic ecosystems).

Cohesion: water molecules stick to each other through hydrogen bonds.
Adhesion: water molecules also stick to other substances, such as the surfaces inside plant xylem.
Together, adhesion and cohesion help pull water up from the roots through the plant (aided by capillary action and the cohesion-tension mechanism).
In plants, this pull helps water reach wherever it is needed for growth and metabolic processes.

When thinking about cohesion and adhesion, it can be helpful to imagine climbing with a straw or climbing on rocks:
- You need adhesion to the side to prevent slipping;
- The cohesive forces between water molecules help maintain a continuous column.

Polar bears and penguins rely on sea ice as a critical habitat.
With global temperatures rising, sea ice is shrinking, which has ecological and climatic implications.

When salts (e.g., sodium chloride, NaCl) dissolve in water, hydration shells form around ions.
The water molecules orient themselves according to charge:
- Positive hydrogens (the \delta^+ regions) are attracted to negative oxygens around anions or negatively charged species.
- The negative oxygens (the \delta^- regions) are attracted to the positive cations (e.g., Na+).
In the case of NaCl, Na+ typically becomes surrounded by the lone pairs and partial negative charges on water oxygens, while Cl− becomes surrounded by the partial positive charges on water hydrogens.

A water molecule can transfer a hydrogen ion (a proton, H^+) to another water molecule.
This transfer leads to the formation of the hydronium ion:
- Reaction: \mathrm{H2O} + \mathrm{H2O} \rightarrow \mathrm{H_3O^+} + \mathrm{OH^-}
The result is a pair of ions in solution: hydronium (H3O+) and hydroxide (OH−).

Water’s polarity (polar covalent bonds with \delta^- on O and \delta^+ on H) drives hydrogen bonding and many of water’s life-supporting properties.
Although individual hydrogen bonds are weak, their abundance governs water’s high heat capacity, cohesion, adhesion, and solvent capabilities.
Water’s solvent properties enable the dissolution of salts via hydration shells and enable proton transfer, influencing pH-related chemistry in aqueous systems.
The physical properties of water (high heat capacity, expansion on freezing) have profound biological and ecological implications, including plant transport, climate stability, and habitat integrity.