The water molecule, a combination of two hydrogen atoms and one oxygen atom, is the foundation for life on Earth due to its polarity. Polarity arises because oxygen is more electronegative than hydrogen, pulling the shared electrons closer to itself. This uneven sharing creates a partial negative charge on the oxygen side and partial positive charges on the hydrogen sides. This unique structure, where electrical charge is distributed unevenly, grants water properties unlike almost any other substance.
Water’s Role as a Universal Solvent
The partial positive and negative charges on the water molecule allow it to interact strongly with other charged substances, making it an exceptional solvent. When an ionic compound like table salt enters water, the positive hydrogen ends surround the negative chloride ions, while the negative oxygen end surrounds the positive sodium ions. This process, called hydration, pulls the compound apart, dissolving the substance and allowing the ions to disperse.
Water is frequently termed the “universal solvent,” even though it cannot dissolve everything. Substances that readily dissolve, such as salts and sugars, are called hydrophilic, or “water-loving,” because their polar nature allows them to form hydrogen bonds with water. Conversely, nonpolar molecules like oils and fats are hydrophobic, or “water-fearing,” and clump together because water molecules are more attracted to each other. Water’s capacity to dissolve and transport chemicals, including nutrients and waste products, forms the basis of biological fluids like blood plasma and cell cytoplasm.
Cohesion, Adhesion, and Surface Tension
The polarity of water allows the partial positive charge of one molecule to be attracted to the partial negative charge of a neighbor, forming a hydrogen bond. This continuous attraction between water molecules is known as cohesion. Cohesion is powerful enough to create surface tension, which is the resistance of water’s surface to being broken or stretched.
At the boundary between water and air, molecules are pulled inward and sideways, creating a dense, film-like layer that allows small insects to walk across the surface. Water also exhibits adhesion, the attraction of water molecules to other charged or polar surfaces, such as the cellulose walls inside a plant’s vascular tissue. The combined forces of cohesion and adhesion enable capillary action, the mechanism that draws water upward through the narrow tubes of a plant’s xylem, delivering water from the roots to the leaves.
Temperature Stability
The network of hydrogen bonds imparts exceptional resistance to temperature change, giving water a high specific heat. This means water can absorb or release a large amount of thermal energy before its own temperature changes significantly. When heat is applied, much of the energy is used to break the existing hydrogen bonds rather than increasing the speed of the molecules, which would raise the temperature.
This property helps stabilize global climates, as large bodies of water like oceans absorb significant heat during the day or summer with only minor temperature fluctuations. Water also possesses a high heat of vaporization, requiring substantial energy to convert liquid water into a gas. This feature is responsible for the effectiveness of evaporative cooling, where water molecules absorb heat from a surface—such as human skin during sweating—as they transition into water vapor, carrying away excess heat.
Density Changes When Freezing
One of water’s unusual properties is that its solid form, ice, is less dense than its liquid form, causing it to float. As liquid water cools, hydrogen bonds force the molecules into a more structured, crystalline lattice arrangement below 4°C. This fixed lattice structure is more open, spacing the water molecules about nine percent farther apart than they are in the liquid state.
The resulting increase in volume translates to a lower density, which is why ice floats on the surface of lakes and rivers. This floating layer acts as an insulating barrier, preventing the water below from freezing solid and allowing aquatic life to survive the winter. If ice were denser than liquid water, bodies of water would freeze from the bottom up, making survival nearly impossible for most aquatic organisms.