Capillary action is the phenomenon where a liquid rises in a narrow tube, such as a glass capillary or a plant stem, due to the adhesive and cohesive forces between the liquid and the tube. This phenomenon can be explained by a combination of intermolecular forces, namely adhesion and cohesion.
Adhesion refers to the attractive forces between unlike molecules. In the case of capillary action, the adhesive forces between the liquid and the walls of the capillary tube allow the liquid to "wet" the tube, which means that the liquid molecules adhere to the tube's surface. The strength of the adhesive forces depends on the chemical nature of the liquid and the tube's material. For example, water is attracted to glass due to the presence of polar groups on the glass surface.
Cohesion, on the other hand, refers to the attractive forces between like molecules. The cohesive forces between the liquid molecules cause them to stick together, forming a meniscus, which is the curved surface of the liquid in the tube. The cohesive forces are usually stronger than the adhesive forces, causing the liquid to rise in the tube against the force of gravity.
The height that the liquid rises in the tube depends on the balance between the adhesive and cohesive forces, as well as the diameter of the tube. A narrower tube will have a greater capillary rise than a wider one because the liquid experiences a greater amount of adhesive forces with the tube's surface in a narrower tube.
In summary, capillary action is a result of the interplay between adhesive and cohesive forces, which causes a liquid to rise in a narrow tube. Adhesion allows the liquid to wet the tube, while cohesion causes the liquid molecules to stick together, forming a meniscus that rises in the tube against the force of gravity.