Water is the most familiar substance on Earth, yet its properties are highly unusual compared to almost any other liquid. This simple molecule, composed of two hydrogen atoms and one oxygen atom, is the medium in which all known life processes take place. Water exhibits physical and chemical characteristics that are foundational to the planet’s climate and the existence of biological organisms.
The Molecular Basis: Polarity and Hydrogen Bonds
The remarkable properties of water begin with its bent molecular structure. Oxygen is a highly electronegative atom, meaning it attracts shared electrons more strongly than the hydrogen atoms it is bonded to. This uneven sharing creates a polar covalent bond, giving the oxygen end a partial negative charge and the hydrogen ends partial positive charges.
This molecular polarity allows water molecules to form weak electrostatic attractions with their neighbors. The partially positive hydrogen atom of one molecule is drawn to the partially negative oxygen atom of an adjacent molecule, forming a hydrogen bond. These numerous hydrogen bonds link water molecules into a constantly shifting network, causing water’s distinctive behaviors, such as temperature stability and its ability to dissolve substances.
Thermal Stability and Climate Regulation
Water possesses an unusually high specific heat capacity. Energy added to water must first be used to break the extensive network of hydrogen bonds before the molecules can increase their motion and raise the temperature. This high heat capacity allows large bodies of water, such as oceans, to absorb vast amounts of solar energy without experiencing drastic temperature fluctuations.
By absorbing and slowly releasing heat, these reservoirs regulate the global climate and moderate air temperatures in coastal regions. This principle also applies to living organisms, where the high water content prevents rapid temperature changes, maintaining stable internal conditions. Water also has a high heat of vaporization, requiring a large amount of energy to transform liquid water into a gas, which enables effective cooling mechanisms like evaporative sweating.
Water as the Universal Solvent
Water’s polarity makes it an exceptional solvent, earning it the designation of the “universal solvent” because it can dissolve more substances than any other liquid. Ionic compounds, such as sodium chloride, are readily dissolved because the partially negative oxygen atoms are attracted to positive ions, and the partially positive hydrogen atoms are attracted to negative ions. This attraction surrounds the ions, pulling them away from the crystal structure and dispersing them in a sphere of hydration.
Water can also dissolve other polar molecules, like sugars, by forming new hydrogen bonds with them. This dissolving power is essential for life, allowing water to transport dissolved nutrients and chemical reactants throughout biological systems and flush out metabolic waste products. Non-polar molecules like oils and fats are repelled and do not dissolve.
Cohesion, Adhesion, and Surface Tension
Water molecules exhibit strong cohesive forces due to their extensive hydrogen bonding. This internal stickiness is responsible for surface tension, where molecules at the surface are pulled inward and laterally by their neighbors, creating a taut “skin” that allows some insects, like water striders, to walk across the surface.
Water also displays adhesion, which is its attraction to molecules of a different substance. When cohesion and adhesion work together, they produce capillary action, allowing water to move against the force of gravity in narrow spaces. This phenomenon is vital for plants, enabling water to travel from the roots up through the xylem vessels to the leaves.
The Density Anomaly: Why Ice Floats
Unlike most substances, water in its solid form (ice) is less dense than its liquid form, causing it to float. Liquid water reaches its maximum density at approximately \(4^\circ\text{C}\). As the temperature drops below this point, hydrogen bonds force the molecules to spread out and lock into a rigid, open, hexagonal crystalline lattice structure.
This structured arrangement in ice takes up about 9% more space than the randomly packed molecules in the liquid state. The resulting lower density of ice is a fundamental ecological safeguard, as floating ice forms an insulating layer on the surface of lakes and oceans. This layer prevents the water below from freezing solid, allowing aquatic life to survive the winter in the insulated liquid environment.