The ability of water to stick to itself, known as cohesion, is exceptionally high compared to other common liquids. This powerful self-attraction is responsible for many of water’s unique characteristics, from forming droplets to sustaining life in plants. The explanation for this strong force lies entirely within the geometry and electrical properties of the single water molecule, \(\text{H}_2\text{O}\). The arrangement of atoms generates powerful electrical forces that act as the molecular “glue,” causing water molecules to cling tightly to one another.
The Water Molecule’s Unique Structure
The water molecule is composed of one oxygen atom bonded to two hydrogen atoms (\(\text{H}_2\text{O}\)). The actual geometry is bent or angular, not linear. This shape results from the electron distribution around the central oxygen atom.
The oxygen atom forms covalent bonds by sharing electrons with the hydrogen atoms. It also has two pairs of non-bonding electrons (lone pairs). These four electron clouds repel each other, pushing the two hydrogen atoms into a bent arrangement.
The resulting \(\text{H-O-H}\) bond angle is approximately \(104.5^\circ\). This non-linear, angular geometry prevents the electrical forces within the molecule from canceling each other out, allowing the molecule to develop distinct positive and negative sides.
How Polarity Creates Uneven Charge Distribution
Polarity describes a separation of electrical charge within a molecule, arising from a difference in electronegativity between the atoms. Oxygen is significantly more electronegative than hydrogen, meaning it strongly attracts the shared electrons in the \(\text{O-H}\) bond.
The shared electrons spend more time near the oxygen nucleus. This unequal sharing causes the oxygen atom to acquire a partial negative electrical charge (\(\delta^-\)), while the hydrogen atoms acquire partial positive charges (\(\delta^+\)).
Due to the molecule’s bent shape, these partial charges do not cancel out. The two partial positive hydrogen ends are concentrated on one side, and the partial negative oxygen end is concentrated on the other. This creates a powerful molecular dipole, making each water molecule a tiny magnet with distinct positive and negative poles, which is necessary for strong cohesive behavior.
The Formation of Hydrogen Bonds
The slight positive charge on the hydrogen end of one water molecule is strongly attracted to the negative charge on the oxygen end of a neighboring molecule. This electrostatic attraction between adjacent molecules is defined as a hydrogen bond. These bonds are much weaker than the covalent bonds holding the atoms together within a single molecule.
Each water molecule can potentially form up to four hydrogen bonds with its nearest neighbors. Although hydrogen bonds are relatively weak individually, liquid water is densely packed with these attractions. This ability to form an extensive, three-dimensional network gives water its high degree of self-attraction.
The combined force of countless hydrogen bonds acting simultaneously creates water’s high cohesion. In liquid water, these bonds are constantly and rapidly breaking and reforming. This dynamic and extensive network ensures that water molecules remain closely linked, generating a strong collective internal force.
Observable Effects of High Cohesion
The strong cohesive force manifests in several easily observable phenomena, primarily surface tension. Surface tension is the resistance of the liquid’s surface to rupture when placed under stress. Water molecules at the surface are pulled inward and sideways by their neighbors, creating a tight, film-like layer.
This surface tension causes water to form spherical droplets rather than immediately flattening out. It also allows small, light objects, such as certain insects, to walk across the water’s surface without breaking the molecular film. Water exhibits one of the highest surface tension values among common non-metallic liquids.
Cohesion also works in tandem with adhesion—water’s attraction to other substances—to produce capillary action. This mechanism allows water to move upward against gravity within a narrow space, such as a thin glass tube or the tiny vessels inside a plant stem. The cohesive forces maintain the column of water, while the adhesive forces between the water and the vessel walls pull the entire column higher. This combined action is fundamental for transporting water from the roots to the highest leaves of trees.