Cohesion is the property of like molecules (of the same substance) to stick to each other due to mutual attraction. Adhesion is the property of different molecules or surfaces to cling to each other. For example, solids have high cohesive properties so they do not stick to the surfaces they come in contact. On the other hand, gases have weak cohesion. Water has both cohesive and adhesive properties. Water molecules stick to each other to form a sphere. This is the result of cohesive forces. When contained in a tube, the water molecules touching the surface of the container are at a higher level (see Meniscus). This is due to the adhesive force between the water molecules and the molecules of the container.
|Constituents||Dissimilar molecules||Similar molecules|
|Effect||Capillary action, meniscus||Surface tension, capillary action and meniscus|
Effects of cohesion and adhesion
Surface tension is the result of cohesive forces between adjacent molecules. The molecules in the bulk of a liquid are pulled equally in all directions by the neighboring molecules. But the surface molecules do not have molecules on all sides. Hence they are pulled inwards causing the liquid to shrink to form a surface with minimum area, a sphere. Hence water droplets are spherical.
Water molecules bead together on a wax paper because surface tension is greater than the adhesive forces between the paper and water molecules.
The surface tension of water allows objects heavier than it to float across it. When water molecules do not stick to the object (non-wettable) and the weight of the object is less than the forces due to surface tension.
The curved surface of a liquid inside a container is the meniscus.
- When the cohesive forces between the liquid molecules are greater than the adhesive forces between the liquid and the wall s of the container, the surface of the liquid is convex. For example, Mercury in a container.
- When the cohesive forces between the liquid are less than the adhesive forces between the liquid and the container, the surface curves up. For example, water in a glass container.
- When both adhesive and cohesive forces are equal, the surface is horizontal. For example, distilled water in a silver vessel.
Capillary action is the result of cohesive and adhesive forces. When a liquid flows through a narrow space, the cohesive and adhesive forces act together to lift it against the natural force of gravity. Wetting of a paper towel, water flowing up from the roots to the tip of a plant are a few examples of capillary action.
Adhesion is used for the proper functioning of glue, paint, tar, cement, ink etc. Adhesive and cohesive forces together induce capillary action which is the principle used in lamp wicks. Synthetic fibers use wicking to remove sweat from the skin.