Cohesion (chemistry)

Causes and common forces

The underlying mechanisms that produce cohesion include several types of intermolecular interaction. Common contributors are:

• Hydrogen bonding: directional, relatively strong attractions that occur between molecules containing H bonded to electronegative atoms (for example, water).
• Dipole–dipole interactions: attractions between polar molecules aligned so that opposite ends attract.
• Dispersion (London) forces: weak, transient attractions present in all molecules, significant in nonpolar substances.

These interactions can be explored more fully in surveys of intermolecular forces: Intermolecular forces overview.

Manifestations and examples

Cohesion gives rise to observable effects in everyday materials. Surface tension, a direct consequence of cohesive forces at a liquid's interface, allows small objects to float or insects to walk on water. Liquids tend to minimize surface area, forming spherical droplets when cohesion dominates adhesion to a surface. For instance, mercury beads into near-spherical droplets on many surfaces because its cohesive forces are strong relative to adhesive forces with those surfaces.

Measurement and influencing factors

Quantities that reflect cohesion include surface tension and cohesive energy density. Both depend on temperature, pressure, and chemical composition: heating generally reduces cohesive attraction, while dissolved substances or contaminants can increase or decrease measurable values. Experimental techniques for characterizing cohesion include tensiometry and calorimetric approaches that probe intermolecular binding energies.

Importance, examples, and distinctions

Cohesion is central to many natural and engineered processes. In plant physiology, cohesive forces among water molecules help transmit tension through the xylem during transpiration. In coatings, adhesives and wetting behavior are controlled by the balance between cohesion and adhesion — the latter being attraction between unlike materials. Understanding the distinction between cohesion (like-to-like attraction) and adhesion (like-to-unlike attraction) is key when predicting whether a liquid will spread or bead on a surface. Further reading and applied discussions are available at: Surface and interfacial effects.

Notable facts

While cohesion is often discussed for liquids, similar concepts apply to solids and polymers where intermolecular or interatomic bonding determines mechanical cohesion. The term is therefore used across chemistry, materials science, and biology to describe the internal attractions that keep a substance intact.