Cohesion in chemistry describes the tendency of molecules within a substance to stick together. This concept governs how liquids and solids maintain their form and interact with their surroundings. These attractive forces determine many of the physical properties we observe in materials. Understanding this internal attraction is foundational to grasping the behavior of matter, especially in the liquid state.
Defining Cohesion: The Attraction Between Like Molecules
Cohesion is the attractive force that exists between molecules of the same type. This internal attraction allows a mass of liquid to resist being pulled apart, holding the substance intact. When cohesive forces are significant, the substance minimizes its surface area as molecules pull inward toward their neighbors.
This inward pull results in the macroscopic phenomenon known as surface tension, where the liquid surface behaves like an elastic film. Water molecules at the surface are attracted only by molecules below and to the side, creating an unbalanced net force pulling inward. This force allows small, light objects, like certain insects, to rest on the water’s surface. Strong cohesion also explains why liquids form spherical droplets, such as dew on a leaf, as molecules cluster together to achieve the smallest possible surface area.
The Intermolecular Forces Driving Cohesion
Cohesion relies on the specific types and strengths of intermolecular forces (IMFs) present between the molecules. These forces are weaker than the chemical bonds holding a molecule together but determine the substance’s physical state and bulk properties. The strongest cohesive force is the hydrogen bond, exhibited prominently in water.
Hydrogen bonds form when a hydrogen atom bonded to a highly electronegative atom, like oxygen or nitrogen, is attracted to another electronegative atom nearby. Water’s high cohesive strength is due to each molecule’s ability to form up to four of these strong attractions with neighboring water molecules. Substances without hydrogen bonding rely on weaker forces, such as dipole-dipole interactions, which occur between polar molecules with permanent positive and negative ends.
The weakest cohesive forces are London Dispersion Forces (LDFs), which exist in all molecules, including non-polar ones. LDFs are temporary, induced attractions created by the fleeting movement of electrons around a molecule. The overall cohesive strength of a material is the sum of these IMFs. For example, mercury has an extremely high cohesive force due to its strong metallic bonding characteristics.
Cohesion vs. Adhesion: A Crucial Distinction
While cohesion is the attraction between like molecules, adhesion is the attractive force between molecules of different substances. These two forces often work together to determine how a liquid interacts with a solid surface. If the cohesive forces within the liquid are stronger than the adhesive forces between the liquid and the surface, the liquid will bead up.
A drop of water on a waxed car hood demonstrates this, as water molecules prefer to cling to each other rather than the non-polar wax. Conversely, if the adhesive forces are stronger, the liquid will spread out and wet the surface, such as water on clean glass.
Capillary action, the movement of a liquid through a narrow tube, provides a clear example of both forces in balance. The adhesive forces between the liquid molecules and the tube walls pull the liquid upward against gravity. Simultaneously, the cohesive forces hold the rising column together, allowing water to travel from the roots to the leaves in plants.