Cohensive Forces Overview
The terms "adhesive" and "cohesive" are not appropriate when discussing atomic and molecular properties because they are connected to bulk (or macroscopic) properties. Both adhesive and cohesive forces act on a liquid when it comes into contact with a surface (like the tabletop or the walls of a graduated cylinder). The liquid's shape is controlled by these forces. Wetting is the process of a liquid on a surface spreading out to create a thin, essentially uniform film over the surface due to the effects of adhesive forces. As an alternative, the liquid can separate into a number of tiny, roughly spherical beads that stand on the surface and make minimal contact with it when strong cohesive forces are present.
What is Cohesive Forces?
Molecules adhering to one another out of mutual attraction is known as cohesion, also known as cohesive attraction or cohesive force. The cohesion force is the attraction between two comparable materials or molecules. Water is a good example of cohesion. Hydrogen bonds are formed between neighboring water molecules. Cohesive force is the result of similar molecules sticking together as a result of their mutual attraction. Surface tension is produced by cohesion, which results in a solid-like state that allows the deposition of light or low-density materials.
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Cohesion Forces in Liquids
Of all the non-metallic liquids, water has the highest degree of cohesion. Because of its cohesive qualities, water is sticky and forms drops, but chemistry and electricity play a more intricate role in this process. Cohesive forces between molecules cause a liquid's surface to constrict to its smallest surface area. The common name for this phenomenon is surface tension. The surface area is decreased by cohesive forces that pull molecules on the surface inward. Molecules inside the liquid experience no net force because they have neighbors on all sides.
Between molecules of the same type, cohesive forces act as an attractive force. Liquids can be contained in open containers because cohesive forces hold the molecules together. The attractive forces that exist between different types of molecules are known as adhesive forces. These forces, for instance, cause liquid drips to stick to glass windows. In this section, we'll examine the effects of cohesive and adhesive forces in liquids.
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Cohesive Forces Example
An excellent example of a cohesive substance is water. Water is made up of molecules of dihydrogen monoxide (HOH), which have two hydrogens and one oxygen. Look at the graphical representation of the chemical make-up of a water molecule. The presence of two charged polar opposites is what is referred to as the molecule's polarity. The partially positive pole is caused by the hydrogens' slightly positive charge, while the partially negative pole is caused by the oxygen's slightly negative charge. Water molecules are attracted to or cling to one another due to their polarity.
Water molecules are held together by a cohesive force. An intermolecular hydrogen bond, a weak or transitory type of chemical connection, is what creates this force. It is created when the hydrogen from two different HOHs reacts. As a result, as they cohere, they create a water drop.
Surface Tension
The attraction of surface particles by the liquid's bulk, which tries to reduce surface area, causes surface tension, which is the tension in a liquid's surface layer. When the liquid's surface is sufficiently robust, surface tension is used. It can support a lot of weight. Another effect of surface tension is the ability of some insects to float above the water or move through it. The three main factors that promote capillarity are adhesion, cohesion, and surface tension. In vascular plants, this is particularly important. Water can ascend a narrow tube against gravity with the help of capillary action. Surface tension causes the liquid's molecules to be drawn inward from the surface, creating the smallest surface area possible. Cohesion then causes the water molecules to stick together.
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Adhesive and Cohesive Forces
The adhesive force is the force that draws two dissimilar molecules together. Take the attraction between the molecules of water and air, for instance. The attraction between two comparable molecules is known as the cohesive force. Take the attraction between water molecules as an example.
Cohesive force is the term used to describe the attraction between molecules of the same type. For instance, the force of cohesion between the molecules of water maintains the substance's molecular bond. Adhesion force is the term used to describe the attraction between various types of molecules. For instance, the molecular adhesion between the water and the glass causes the particles to stay stuck to the glass even after the water has been drained.
The term "adhesive forces" (attraction resulting from opposing charges) refers to the attractive forces between dissimilar substances, such as mechanical forces (sticking together) and electrostatic forces. In the case of a liquid wetting agent, adhesion causes a liquid to stick to the surface it is resting on.
Difference between Cohesion and Adhesion
Cohesive and adhesive forces are identical. Adhesives, such as glue and cement, which can bind specific materials, are substances that can affix two surfaces or objects. The bandage is described as being sticky. Adhesive forces can occasionally be related to how strongly an adhesive is drawn to a substrate. The adhesive sticks to the substrate as a result of the adhesive force. Two or more materials interact with each other to produce this force. In the process, both strong and weak bonds are created. There are two ways that hydrogen can be shared (hydrogen bonding): covalent bonding, where atoms are shared, and ionic bonding, where atoms are exchanged. Contrarily, cohesive forces are primarily governed by interactions between adhesive molecules or mechanical adhesion that takes place within the molecules of the adhesive themselves.
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Effects of Adhesive and Cohesive Forces
It is generally accepted that cohesive and adhesive forces coexist, and that these forces can be detected to varying degrees in a variety of processes and activities. Both adhesion and cohesion are in charge of the meniscus, a liquid surface curvature that is kept in a tube or container. In physics, adhesion describes the force that holds liquid edges to the container wall. The force of attraction between the water molecules known as cohesion is what shapes the liquid surface's central curve. The meniscus's shape is also determined by this force. If the cohesion force between liquid molecules is greater than the adhesion force between the liquid and the inner surface of the tube, the meniscus will have a convex shape. For instance, glass encases mercury. Similar to a glass tube filled with water, the meniscus will be concave if adhesion is greater than cohesion. Imagine that you accidentally spill some water on a surface. The surface will be horizontal if the adhesion and the cohesion are equal. If the adhesive force is strong, water will be quickly absorbed by the surface and will also cause it to become wet. In contrast, if the cohesive forces are strong, the water molecules will be attracted to one another more than they will be to the surface. Less water is absorbed by the surface as a result of this.
Macroscopic Effects of Cohesive and Adhesive Forces
The shape (and whether or not it will wet the surface) that a liquid takes when it is placed on a smooth surface depends on the relative strengths of the cohesive and adhesive forces acting on it. A liquid will be pulled downward and will wet the surface if the adhesive forces between it and the surface are stronger. The liquid will maintain its spherical shape and form beads on the surface, though, if the cohesive forces within the liquid are stronger and resist such adhesion.
Case I: The Meniscus
The meniscus is a liquid's surface curvature inside of a container, like a graduated cylinder. To understand the adhesive forces of surface tension, we must first understand why some liquids have a concave up meniscus while others share a concave down meniscus. A polar molecule like water, for instance, has a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atoms. Each molecule's partial positive charge is drawn to its neighbor's partial negative charge in liquid water as a result. The cohesive forces in the water have their roots in this. The buried water molecules are then uniformly pulled and pushed in all directions, resulting in no net pull. As a result of the absence of pulling forces in the upward direction, the molecules on the liquid's surface encompass a net downward pull.
Then, how does this cohesive force produce a surface that is concave both up and down? The response relates to the forces of adhesion between water molecules and the surface of the container. The liquid convects downward to minimize contact with the surface of the wall when its cohesive force is greater than its adhesive force to the wall. The liquid is more attracted to the wall than its neighbors when the adhesive force of the liquid to the wall is stronger than the cohesive force of the liquid, which results in the upward concavity.
Case II: Tears of Wine
Wine dribbles appear to "float" above the liquid's meniscus in stirred glasses of wine, forming "tears." The forces of surface tension, cohesion, and adhesion cause this venerable phenomenon. Water is less volatile than alcohol. This causes a thin film to rise up the side of a wine glass as a result of alcohol evaporation, according to Adamson. The "solutal Marangoni effect" is the name given to this process. Some water adheres to the glass's walls due to adhesive forces. The "tears" form from the cohesive forces within the water holding it together. Although the surface tension gradient is "the driving force for the motion of the liquid" (Gugliotti), the forces of cohesive and adhesive forces are what cause the tears to form.
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Conclusion
A cohesive or adhesive force is a force of attraction. These forces cause molecules to be drawn to or pushed away from one another. Molecules are attracted to one another by adhesive forces. Molecules of a substance are held together by an adhesive force. Additionally, the forces of adhesion and cohesion reveal a great deal about biological processes, such as the movement of water through xylem tubes. As a general rule, adhesion and cohesion are the properties of molecules of the same substance sticking together, while adhesion refers to molecules of different substances sticking together.