All matter interacts through forces that operate at a molecular level, known as intermolecular forces. These attractions dictate many physical properties of substances, especially liquids, and their ability to interact with their environment. Cohesion and adhesion represent two fundamental ways that molecules exert forces on one another. Understanding these forces is foundational to grasping how liquids behave, particularly how water interacts with itself and other materials.
Understanding Cohesion
Cohesion is the attractive force that exists between molecules of the same substance. This force allows a liquid to stick to itself, giving the substance structural integrity and allowing it to resist external pressure. For water, this self-attraction is primarily driven by the formation of hydrogen bonds between adjacent water molecules.
Cohesive forces are responsible for phenomena like surface tension, where molecules at the surface are pulled inward by their neighbors below. This inward pull minimizes the surface area, allowing small insects to walk across the water’s surface or causing water to form spherical droplets.
Understanding Adhesion
Adhesion describes the attractive force that occurs between molecules of two different substances. This force is responsible for a liquid “wetting” a surface, which happens when liquid molecules are more attracted to the surface molecules than they are to each other. The strength of this attraction often depends on the electrical charges present on the surface of the other material.
A common example is when water sticks to a clean glass surface, which is a polar material that strongly attracts the polar water molecules. This principle also explains why materials like glue or paint successfully bond to another object. In biological systems, adhesion allows water to bond to the cellulose in plant cell walls, which is necessary for water uptake.
The Combined Effects of Adhesion and Cohesion
While cohesion and adhesion are distinct, they frequently work together to produce complex and observable effects. The most prominent example of this synergy is capillary action, which allows a liquid to move up a narrow tube against the force of gravity. This process is relevant for the movement of water in plants.
In a plant’s xylem vessels, adhesion causes water molecules to cling to the inner walls of the tube, pulling those edge molecules upward. Simultaneously, cohesive forces among the water molecules ensure that the molecules in the center are pulled along by their neighbors adhering to the wall. The narrower the tube, the greater the surface area-to-volume ratio, which maximizes the upward pull. This continuous chain allows water to be transported from the roots to the leaves.
Another common visual result of this interplay is the formation of the meniscus, the characteristic curve seen at the top surface of a liquid in a container. When water is placed in glass, the adhesive forces attracting water to the glass are stronger than the cohesive forces holding the water together. This stronger adhesion causes the water to climb the sides slightly, resulting in a concave, U-shaped meniscus. Conversely, if cohesive forces were stronger than the adhesive forces, as with mercury in glass, the surface would form a convex shape.