Water is fundamental to life on Earth, covering roughly 70% of the planet’s surface and composing a large majority of living organisms. Its ability to support biological systems is linked to a set of unique physical and chemical behaviors. These five properties—cohesion, adhesion, high specific heat, solvent action, and the density anomaly—stem from the structure of the individual water molecule. Understanding this molecular foundation reveals why water behaves so differently from nearly every other liquid.
The Molecular Foundation
A single water molecule, H₂O, is formed when one oxygen atom covalently bonds with two hydrogen atoms. Oxygen is significantly more electronegative than hydrogen, pulling the shared electrons closer to itself. This unequal sharing creates a molecule with an uneven distribution of electric charge.
The oxygen side acquires a partial negative charge, while the two hydrogen sides each acquire a partial positive charge. This charge separation makes water a “polar” molecule, acting like a tiny magnet with distinct positive and negative poles. This polarity allows adjacent water molecules to attract one another, forming weak forces known as hydrogen bonds.
These hydrogen bonds occur when the partially positive hydrogen atom of one molecule is drawn toward the partially negative oxygen atom of a neighboring molecule. Although a single hydrogen bond is weak, water molecules form extensive, interconnected networks of these bonds in the liquid state, which is the underlying mechanism responsible for water’s distinguishing characteristics.
Thermal Stability and Movement
A consequence of the extensive hydrogen bond network is cohesion, the attraction between water molecules themselves. This strong mutual attraction allows water to develop surface tension, creating a thin, elastic “skin” on its surface that can support small insects. The attraction of water molecules to other substances is called adhesion, which is strong with materials that possess electrical charges.
Working together, cohesion and adhesion produce capillary action, allowing water to climb upward against gravity. This property enables the transport of water and dissolved nutrients from the roots to the leaves of tall plants. The ability to stick to both itself and other surfaces is fundamental to fluid movement in biological systems.
High Specific Heat
Water also exhibits a high specific heat, which is the amount of energy required to raise the temperature of a substance. When heat energy is applied to water, much of that energy is used to break the numerous hydrogen bonds rather than increasing the molecules’ kinetic energy. This means water can absorb or release large amounts of heat with only a minor change in its own temperature. This thermal stability helps moderate global climate fluctuations and allows living organisms to maintain a relatively constant internal body temperature.
Interaction with Other Substances
Solvent Action
Water’s capacity as a solvent is frequently described as the “universal solvent” because it dissolves more substances than any other liquid. Water’s polarity is the reason for this ability, as the charged ends of the molecule can surround and separate other charged or polar compounds. For instance, when table salt (an ionic compound) is added to water, the oxygen end attracts the positive sodium ions, while the hydrogen ends attract the negative chloride ions, pulling the salt crystal apart into a solution. This allows water to transport vital nutrients and dissolved gases throughout the environment and within living cells.
Density Anomaly
The final unique property is the density anomaly, which describes how solid water (ice) is less dense than its liquid form. As liquid water cools below 4 degrees Celsius, the hydrogen bonds begin to lock the molecules into a stable, open crystalline lattice structure. This arrangement spaces the molecules farther apart than they are in the denser liquid state, causing ice to float when it forms at 0 degrees Celsius. This floating layer of ice insulates the water beneath it, preventing entire bodies of water from freezing solid and allowing aquatic life to survive the winter.