![]() Water molecules buried inside the liquid is then being pulled and pushed evenly in every direction, producing no net pull. ![]() This is the origin of the cohesive forces within the water. Thus, within liquid water, each molecule's partial positive charge is attracted to its neighbor's partial negative charge. Water, for example, is a polar molecule that consists of a partial positive charge on the hydrogens and a partial negative charge on the oxygen. However, before we explain why some liquid have a concave up meniscus while others share a concave down meniscus, we have to understand the adhesive forces at work of surface tension. The meniscus is the curvature of a liquid's surface within a container, such as a graduated cylinder. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the glasses surface. In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests. Similarly, the term "adhesive forces" refers to the attractive forces between unlike substance, such as mechanical forces (sticking together) and electrostatic forces (attraction due to opposing charges). This force tends to unite molecules of a liquid, gathering them into relatively large clusters due to the molecules' dislike for its surroundings. For instance, rain falls in droplets, rather than a fine mist, because water has strong cohesion, which pulls its molecules tightly together, forming droplets. Specifically, these attractive forces exist between molecules of the same substance. ![]() ![]() The term "cohesive forces" is a generic term for the collective intermolecular forces (e.g., hydrogen bonding and van der Waals forces) responsible for the bulk property of liquids resisting separation. ![]()
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