An analysis of the topic of the cohesive forces between liquid molecules
Viscosity and intermolecular forces
Van der Waals gases such as methane , however, have weak cohesion due only to van der Waals forces that operate by induced polarity in non-polar molecules. These forces govern the shape which the liquid takes on. In simple terms, the polarity state in which a molecule is oppositely charged on its poles of water molecules allows them to be attracted towards each other. Credit: Dr. When a liquid comes into contact with a surface such as the walls of a graduated cylinder or a tabletop , both cohesive and adhesive forces will act on it. 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. Adamson, A.
Adhesive Forces Attractive forces between molecules of different types are called adhesive forces. Thus, within liquid water, each molecule's partial positive charge is attracted to its neighbor's partial negative charge.
Intermolecular forces in liquids
If the adhesive forces between a liquid and a surface are stronger, they will pull the liquid down, causing it to wet the surface. As a result, "evaporation of alcohol produces a surface tension gradient driving a thin film up along the side of a wine glass" Adamson. Surfactants are molecules, such as soaps and detergents, that reduce the surface tension of polar liquids like water. In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests. These attractive forces exist between molecules of the same substance. B Such a liquid cannot form strong interactions with the polar Si—OH groups of glass, so the surface of the oil inside the capillary will be lower than the level of the liquid in the beaker. Due to the effects of adhesive forces, liquid on a surface can spread out to form a thin, relatively uniform film over the surface, a process known as wetting.
How does this cohesive force create both a concave up and concave down surface then? What will be the shape of the meniscus convex or concave? Solution A Motor oil is a nonpolar liquid consisting largely of hydrocarbon chains.
Photo used with permission by Greg Emel. What is the common thread?
The model of a liquid surface acting like a stretched elastic sheet can effectively explain surface tension effects. The height to which the water rises depends on the diameter of the tube and the temperature of the water but not on the angle at which the tube enters the water.
In this section we examine effects directly attributable to cohesive and adhesive forces in liquids. However, in the absence of the adhesive force when water reaches the tip of the glassthe cohesive force remains present.
Intermolecular forces in gases
Mercury exhibits more cohesion than adhesion with glass. Contributors Cohesive and adhesive forces are associated with bulk or macroscopic properties and hence the terms are not applicable to discussion of atomic and molecular properties. When adhesive force is stronger than the cohesive force: concave down meniscus, the surfaces is covered by the wetting agent, the last drops of liquid in the bottle always refuse to come out. Will the oil be pulled up into the tube by capillary action or pushed down below the surface of the liquid in the beaker? When a liquid comes into contact with a surface such as the walls of a graduated cylinder or a tabletop , both cohesive and adhesive forces will act on it. Due to a combination of these two effects, long-chain hydrocarbons such as motor oils are highly viscous. Understand capillary action. This results in a relatively strong Coulomb force between molecules. Solution A Motor oil is a nonpolar liquid consisting largely of hydrocarbon chains. The smaller the diameter, the higher the liquid rises.
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