Adhesion
Definition and meaning of Adhesion in chemistry.
Adhesion is the attractive physical force between molecules of two completely different substances. It causes dissimilar materials to stick together when their surfaces touch each other. This attraction relies on weak intermolecular forces acting across the physical boundary between the materials.
In more detail
Students often confuse adhesion with cohesion in chemistry classes. Cohesion describes the attraction between molecules of the exact same substance. Adhesion describes the specific attraction between two different materials.
The strength of an adhesive bond depends on the chemical properties of both surfaces. Polar molecules generally create much stronger adhesive forces than nonpolar molecules. This happens because polar molecules have partial positive and negative charges that act like tiny magnets.
These charges form hydrogen bonds or strong electrostatic attractions with the opposing surface. Van der Waals forces also contribute to adhesion, especially when surfaces are perfectly flat and close together. Chemists use the principles of adhesion to invent stronger industrial glues and durable paints.
Adhesion also drives capillary action, which is the upward movement of liquids inside narrow tubes. This movement happens when the liquid is more attracted to the tube than to itself. As a result, the liquid climbs upward against the pull of gravity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key factor | Surface polarity |
| Primary forces | van der Waals forces and hydrogen bonding |
| Contrasts with | Cohesion (attraction within the same substance) |
| Related phenomena | Capillary action and meniscus formation |
| Practical applications | Glues, paints, and protective coatings |
You can observe strong adhesion when you pour plain water into a glass graduated cylinder. Water molecules are highly polar and can form strong hydrogen bonds. The glass surface contains silica (SiO2) molecules, which also feature partial electrical charges. The water molecules are strongly attracted to the glass walls of the cylinder. They stick to the glass and begin to creep slightly up the inner sides. This creeping action forms a curved surface at the top of the liquid called a meniscus. Chemists must always read the volume at the bottom of this meniscus for an accurate measurement.
Frequently asked questions
Why does water stick to glass but not to wax?
Glass is a polar material, so the polar water molecules are highly attracted to it. Wax is nonpolar, so water molecules prefer to stick to each other instead of the wax.
Is adhesion a chemical reaction?
No, it is typically a physical attraction based on intermolecular forces. The two materials stick together but do not change their fundamental chemical identities.
How does adhesion help plants survive?
Adhesion helps water stick to the inside walls of plant xylem tubes. This allows capillary action to pull water from the roots all the way up to the leaves.