Cohesion
Definition and meaning of Cohesion in chemistry.
Cohesion is the mutual physical attraction between molecules of the exact same chemical substance. Intermolecular forces like hydrogen bonding and van der Waals forces cause this important pulling effect. This mutual attraction constantly holds the individual chemical molecules together in a tight group.
In more detail
Cohesion determines many bulk properties of a liquid that we can easily see and measure. It directly controls the surface tension and the thickness or viscosity of a fluid. It also causes a liquid to strongly resist separating into tiny isolated droplets.
Water shows unusually strong cohesion compared to most other common liquids found on Earth. The water molecules form an extensive, temporary network of hydrogen bonds with one another. These strong chemical bonds give water one of the highest surface tensions of any standard liquid.
It is very important to keep cohesion distinct from a related concept called adhesion. Adhesion instead describes the physical attraction between molecules of two completely different substances. A simple example of adhesion is water clinging to a clean glass window surface.
The active interplay of cohesion and adhesion produces fascinating physical effects in nature. This delicate balance creates the capillary action that pulls water up inside a tall plant stem. It also causes the curved meniscus shape you see when measuring liquid in a narrow glass tube.
Key facts
| Field | Physical Chemistry |
|---|---|
| Main causes | Hydrogen bonding, dipole-dipole forces, and standard van der Waals forces |
| Key related property | Directly controls the observable surface tension of a chemical liquid |
| Contrasted with | Adhesion, which is the physical attraction between completely different substances |
| Classic example | Water molecules tightly sticking together to form a spherical liquid drop |
Imagine a single drop of water sitting on a freshly waxed car hood. The drop forms into a near-perfect sphere instead of spreading out flat across the metal. The wax coating strongly repels the water, making the adhesive forces extremely weak. The strong, hydrogen-bonded cohesive forces between the water molecules pull the liquid surface inward. The water naturally tries to minimize its overall surface area by forming a tight round bead. The molecules pull together tightly because they prefer each other over the waxy surface.
Frequently asked questions
What is the main difference between the concepts of cohesion and adhesion?
Cohesion is the specific mutual attraction between molecules of the exact same substance. Adhesion is the chemical attraction between molecules of two completely different physical substances.
Why does plain liquid water have such incredibly strong cohesion?
Water molecules form extensive hydrogen bonds with all of their close neighbors. This specific intermolecular force is comparatively strong and pulls the water molecules tightly together.
How does cohesion relate to the surface tension of a liquid?
The cohesive molecules at the surface do not have identical neighbors resting above them. They pull much more strongly on the molecules beside and below them.