Table of Contents
Water has several unusual physical and chemical properties that make it uniquely suited as the medium of life. In this chapter, we will focus on these specific properties and what they mean for living organisms, without yet going into the special topic of water’s autoprotolysis (which has its own chapter).
Molecular Structure and Polarity
A water molecule consists of one oxygen atom and two hydrogen atoms: its formula is $\mathrm{H_2O}.$
The molecule has a bent (V-shaped) geometry, not a straight line. Because oxygen is more electronegative than hydrogen, the shared electrons are drawn more strongly toward the oxygen atom.
As a result:
- The region around the oxygen atom carries a partial negative charge $\delta^-,$
- The regions around the hydrogen atoms carry partial positive charges $\delta^+.$
This separation of partial charges makes water a polar molecule. Polarity is the basis for most of water’s important properties.
Hydrogen Bonding
The polarity of water molecules allows them to attract each other. The slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a neighboring molecule. This weak but important interaction is called a hydrogen bond.
Key features:
- Hydrogen bonds are weaker than covalent bonds within the molecule, but stronger than many random intermolecular forces.
- Each water molecule can form several hydrogen bonds with neighboring molecules.
- The constant breaking and reforming of hydrogen bonds creates a dynamic, cohesive network.
This network of hydrogen bonds underlies many of water’s unusual behaviors.
Cohesion, Adhesion, and Surface Tension
Cohesion: Water–Water Attraction
Cohesion is the tendency of water molecules to stick to each other due to hydrogen bonding.
Consequences:
- Water forms droplets rather than scattering into a fine mist.
- Water columns (for example, in plant stems) can remain continuous over long distances.
Adhesion: Water–Other Surfaces
Adhesion is the attraction between water molecules and other polar or charged surfaces.
Consequences:
- Water can “climb” along surfaces, such as the walls of narrow tubes, glass slides, or plant cell walls.
- Water spreads on many surfaces rather than forming perfect spheres.
Surface Tension
At the boundary between water and air, water molecules are pulled more strongly toward other water molecules than toward the air. This creates a kind of “skin” on the surface known as surface tension.
Effects observable in everyday life and biology:
- Small objects (e.g., a carefully placed needle) can rest on the water surface without sinking, despite being denser than water.
- Some insects can walk on water surfaces.
- Droplet formation and the shape of water drops on leaves are influenced by surface tension.
Capillary Action
Capillary action arises from the combination of cohesion and adhesion.
When water is in a very narrow tube or in tiny pores:
- Adhesion pulls water upward along the walls (water sticks to the tube),
- Cohesion pulls additional water molecules along (water sticks to water).
As a result, water rises spontaneously in thin tubes without external pressure. The narrower the tube, the higher the rise. This phenomenon is fundamental in biological water transport through fine structures.
Water as an Excellent Solvent
Because of its polarity, water is often called the “universal solvent” (although it does not dissolve everything).
Dissolving Ionic Compounds
In ionic compounds such as sodium chloride (kitchen salt), ions are held together in a crystal lattice. In water:
- The negative pole of water ($\delta^-$ at the oxygen) orients toward positive ions (cations),
- The positive pole ($\delta^+$ at the hydrogens) orients toward negative ions (anions).
Water molecules form hydration shells around each ion, separating them from each other and keeping them in solution.
Dissolving Polar Molecules
Many polar molecules (e.g. sugars, some amino acids) have regions with partial positive and negative charges.
- These regions interact with the partial charges of water molecules.
- Hydrogen bonds or other polar interactions form between water and the solute.
- The molecules disperse evenly: they become dissolved.
Hydrophilic vs. Hydrophobic
- Hydrophilic (“water-loving”) substances: dissolve well in water; they are ionic or polar.
- Hydrophobic (“water-fearing”) substances: do not dissolve well; they are nonpolar (for example, many lipids and oils).
The differing behavior of hydrophilic and hydrophobic substances in water is crucial for many biological structures, especially membranes.
High Specific Heat Capacity
Water can absorb or release large amounts of heat with relatively small changes in its own temperature. This is expressed as high specific heat capacity: the energy needed to raise the temperature of 1 gram of a substance by $1^\circ\mathrm{C}.$
Because hydrogen bonds must be partly broken for water molecules to move faster (i.e., for temperature to rise), much of the added energy goes into disrupting bonds rather than simply increasing motion.
Biological and environmental relevance:
- Water-rich organisms maintain more stable internal temperatures.
- Large bodies of water moderate climate by buffering temperature fluctuations between day and night and between seasons.
High Heat of Vaporization
To convert liquid water into water vapor, many hydrogen bonds must be broken. The energy required for this phase change is water’s heat of vaporization, which is unusually high.
Consequences:
- Evaporation of water removes a lot of heat from a surface.
- This is the basis for evaporative cooling, such as sweating or transpiration (details of these processes are covered in other chapters).
Density Anomaly of Water and Ice
Most substances become denser as they cool and thus occupy less volume. Water behaves differently near its freezing point:
- Water has its maximum density at about $4^\circ\mathrm{C}.$
- As water cools below this temperature and approaches $0^\circ\mathrm{C},$ a more ordered hydrogen-bond network develops.
- In ice, water molecules are arranged in an open, lattice-like structure with more empty space between them.
As a result, ice is less dense than liquid water and floats on top.
Biological consequences:
- In a cooling lake, denser water at $4^\circ\mathrm{C}$ sinks, while colder water and ice remain above.
- Ice forms an insulating layer on the surface, while water beneath can remain liquid and above freezing.
- Aquatic organisms can survive winter in unfrozen water beneath the ice cover.
High Cohesive Strength and Viscosity
Due to the extensive hydrogen bonding:
- Water has relatively high cohesive strength (its molecules resist being pulled apart),
- Its viscosity (internal resistance to flow) is moderate yet sufficient for stable flow patterns in biological systems.
This balance of cohesion and viscosity:
- Helps maintain continuous water columns in narrow tubes.
- Allows efficient circulation and transport without being either too “sticky” or too “runny.”
Transparency
Liquid water is largely transparent to visible light. This is essential because:
- Light can penetrate through water, enabling photosynthesis in aquatic environments down to certain depths.
- Visual sensing is possible for organisms living in water.
Different wavelengths of light are absorbed to different degrees, leading to changes in light quality with depth, but overall transparency remains a key property.
High Surface Heat Capacity and Thermal Conductivity
In addition to specific heat capacity, water has:
- A high surface heat capacity: surfaces of bodies of water can store or release large amounts of heat.
- Good thermal conductivity: heat is transmitted relatively efficiently through water.
Together, these properties promote:
- Even temperature distribution within aquatic habitats,
- Efficient heat transfer within water-rich tissues.
Compressibility and Incompressibility
Liquid water is only slightly compressible under normal pressures. For biological systems this means:
- Water-filled structures (cells, vessels) can support shape and stability.
- Water can transmit pressure effectively, which is used, for example, to create internal pressure in cells and tissues.
Summary of Key Properties
The crucial properties of water for life include:
- Polarity and hydrogen bonding,
- Strong cohesion and adhesion, leading to surface tension and capillary action,
- Excellent solvent ability for ionic and polar substances,
- High specific heat capacity and high heat of vaporization,
- Density anomaly (ice less dense than liquid water),
- Moderate viscosity with high cohesive strength,
- Transparency to visible light,
- Effective heat storage and transfer,
- Low compressibility.
Together, these features make water not just a common substance, but a uniquely suitable medium in which life can arise, persist, and function.