Cohesive Forces
Definition and meaning of Cohesive Forces in chemistry.
Cohesive forces are the intermolecular attractive forces, such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces, that hold like molecules of a substance together.
In more detail
Cohesion arises from the same intermolecular forces that determine a substance's phase and physical properties, and its strength governs surface tension, viscosity, boiling point, and capillary rise. Liquids with strong cohesion, like water, resist being pulled apart and tend to minimize their surface area, producing a "skin"-like surface. Cohesive forces are distinguished from adhesive forces, which describe attraction between unlike molecules, such as a liquid and a container wall; the balance between the two determines whether a liquid wets a surface or beads up on it.
Key facts
| Field | Physical Chemistry |
|---|---|
| Underlying forces | Hydrogen bonding, dipole-dipole interactions, London dispersion forces |
| Key measurable effect | Surface tension |
| Classic example | Water (H2O) |
Water molecules form extensive hydrogen bonds with one another, producing strong cohesive forces that give water an unusually high surface tension. This is why a water strider can walk across a pond's surface and why small water droplets bead into spheres on a waxy leaf rather than spreading out.
Frequently asked questions
How do cohesive forces differ from adhesive forces?
Cohesive forces act between molecules of the same substance, holding it together; adhesive forces act between molecules of different substances, causing one to stick to another, such as water clinging to glass.
Why does water have unusually strong cohesive forces?
Water molecules form hydrogen bonds with each other, which are stronger than typical dipole-dipole or dispersion forces, giving water high surface tension, high boiling point, and pronounced capillary action.