Cohesive Forces
Definition and meaning of Cohesive Forces in chemistry.
Cohesive forces are the specific intermolecular attractions that hold molecules of the same substance together. These molecular attractions include hydrogen bonds, dipole-dipole interactions, and London dispersion forces. They dictate exactly how tightly the individual molecules cling to one another in a liquid.
In more detail
These internal attractive forces control many physical properties of any chemical substance. They directly determine the exact physical phase of a material at a given temperature. The overall strength of these connections governs important properties like surface tension and viscosity.
Viscosity is a property that measures how much a liquid resists flowing when poured. A liquid with strong cohesive forces strongly resists being pulled apart by outside pressure. Water molecules naturally form extensive hydrogen bonds with all their neighboring liquid molecules.
This interaction creates an inward pull that minimizes the overall surface area of the liquid. The resulting tension creates a thin, invisible skin on the surface of the water. Scientists distinguish cohesive forces from a different concept called adhesive forces.
Adhesive forces describe the chemical attraction between molecules of completely different substances. The balance between cohesion and adhesion determines exactly how a liquid behaves on a solid surface. Strong cohesion makes a liquid bead up into a small, tight sphere. Strong adhesion makes that same liquid spread out and wet the surface completely.
Key facts
| Field | Physical Chemistry |
|---|---|
| Underlying forces | Hydrogen bonding, dipole-dipole interactions, and London dispersion forces |
| Key measurable effect | Directly controls the surface tension and viscosity of a liquid |
| Classic example | Water molecules bonding together tightly to form a spherical drop |
| Contrasted with | Adhesive forces that act between completely different chemical materials |
Water molecules create very strong cohesive forces through constant network hydrogen bonding. This intense molecular attraction gives pure water an unusually high surface tension. The strong surface skin allows a light water strider insect to walk right across a pond. The cohesive forces pull the water molecules inward and away from the surrounding air. This inward pull is why small water droplets form round beads on a waxy green leaf. The water simply prefers to stick to itself rather than spreading onto the waxy plant surface.
Frequently asked questions
How do cohesive forces fundamentally differ from adhesive forces?
Cohesive forces act between molecules of the exact same chemical substance. Adhesive forces act between molecules of different substances, like water sticking to a glass window.
Why does liquid water have such unusually strong cohesive forces?
Water molecules easily form strong hydrogen bonds with each of their close neighbors. These specific bonds pull the water tightly together and create a high surface tension.
What happens if the adhesive forces are stronger than the cohesive forces?
The liquid will spread out in a thin film to maximize contact with the surface. This exact effect is why pure water wets a clean glass window completely.