Water is abundant across Earth, existing in various forms from oceans to atmospheric vapor. Its presence is fundamental for all known life, supporting diverse ecosystems and facilitating essential functions within living organisms.
The Polarity of Water
A water molecule (H₂O) has one oxygen atom bonded to two hydrogen atoms. Oxygen is more electronegative than hydrogen, pulling shared electrons in the covalent bonds closer to itself. This creates partial negative charges near the oxygen and partial positive charges near each hydrogen atom.
The water molecule has a bent shape. This bent geometry ensures the partial positive charges of the hydrogens are on one side and the partial negative charge of the oxygen is on the other, making the molecule polar. This polarity allows water molecules to attract each other, forming hydrogen bonds. These bonds are weaker than covalent bonds but collectively strong, influencing many of water’s behaviors.
Water’s Stickiness: Cohesion, Adhesion, and Surface Tension
Hydrogen bonds contribute to water’s “stickiness,” manifesting as cohesion, adhesion, and surface tension.
Cohesion is the attraction between water molecules, causing them to stick together. This is evident when water forms dome-like shapes on surfaces, such as a drop on a leaf.
Adhesion describes water molecules’ attraction to other substances. This allows water to “climb” narrow tubes, like those in plants, a process known as capillary action. Water molecules adhere to the tube’s walls while cohesively pulling other water molecules along, enabling water and dissolved nutrients to move upward against gravity.
Surface tension arises from the strong cohesive forces at the surface of water. Water molecules at the surface are more strongly attracted to their neighboring water molecules than to the air above, creating a taut, film-like layer. This phenomenon allows some insects to walk on the water’s surface without sinking, as their weight is distributed over this strong surface layer.
How Water Handles Heat and Cold
Water has a high specific heat capacity, meaning it can absorb and release substantial heat with minor temperature changes. This characteristic is directly linked to the extensive hydrogen bonding. A considerable amount of energy is required to break these bonds before water molecules increase their temperature.
This property helps regulate Earth’s climate, allowing large bodies of water like oceans to absorb vast quantities of solar energy, moderating temperature fluctuations. The high specific heat also contributes to the stability of internal temperatures in living organisms.
Water displays a unique density anomaly: it is most dense at approximately 4°C. Unlike most substances that become denser as they cool, water expands as it cools from 4°C to 0°C and freezes. This expansion upon freezing means that ice is less dense than liquid water, which is why ice floats. Ice forms an insulating layer on the surface, protecting aquatic life beneath from extreme cold.
Water as a Universal Solvent
Water is known as the “universal solvent” due to its ability to dissolve a wide array of substances. This solvent capacity is a direct consequence of water’s polarity.
The partial positive and negative charges on water molecules allow them to attract and surround the charged components of other molecules, such as the ions in salts or other polar molecules. When substances like salt dissolve, water’s positive hydrogen ends are attracted to negative ions, and its negative oxygen end to positive ions. This attraction disrupts the bonds holding the solute together, dispersing its components. This property is fundamental for biological processes, enabling the transport of nutrients, minerals, and waste products within organisms.