Cohesion (chemistry)
Cohesion in chemistry refers to the attractive intermolecular forces that cause molecules of the same substance to stick together. This phenomenon is essential for various processes and is best exemplified by water, where the hydrogen bonds between molecules create a cohesive force that enables droplets to maintain their shape. Cohesion differs from adhesion, which is the attraction between different substances. Together, these forces contribute to important scientific phenomena like surface tension, which allows liquids to resist external forces, and capillary action, where liquids move through narrow spaces, such as in the stems of plants.
Surface tension, for example, is observable when a glass is overfilled with water, demonstrating how cohesive forces keep the water intact despite external pressure. Additionally, the unique shapes observed in the liquid's surface, called meniscus, occur due to the interplay of cohesion and adhesion. In practical applications, wetting agents can modify these forces, allowing liquids to spread more efficiently across surfaces, enhancing cleaning processes. Understanding cohesion is not only vital in chemistry but also plays a significant role in everyday life and natural processes.
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Cohesion (chemistry)
In chemistry, cohesion is a type of intermolecular force that causes an attraction between molecules of the same type. More simply, cohesion is the force that makes molecules of the same material stick together. Cohesion differs from adhesion, which is a type of intermolecular force that causes an attraction between unlike molecules. Cohesive forces, along with adhesive forces, contribute to scientific phenomena such as surface tension and capillary action. Substances called wetting agents can affect the cohesion and surface tension of liquids.
Background
Cohesion is an intermolecular force, or a force between molecules. More specifically, cohesion is an attractive force between molecules of the same substance. The attraction between these molecules causes them to resist separation. The word cohesion actually comes from a Latin word that means "to stick together."
One of the best examples of cohesion is a drop of water. Cohesion is the force that allows a drop of water to maintain its shape. It makes the molecules within that drop of water stick together. A water molecule is composed of two hydrogen atoms and one oxygen atom. The two hydrogen atoms gravitate toward one side of the molecule, giving that side of the molecule a positive charge and the oxygen side a negative charge. These electrical charges play a significant role in water molecules' attraction to one another, which affects cohesion. Like electrical charges repel each other, while opposite electrical charges attract each other. Therefore, the slightly positive hydrogen side of one water molecule will be attracted to the slightly negative oxygen side of another water molecule. As a result, water molecules will form bonds, called hydrogen bonds, which create the cohesive force that helps water molecules stick together.
Like cohesion, adhesion is an intermolecular force. However, adhesion is an attractive force between molecules of different substances. Consider a drop of water hanging at the tip of a leaf after a rainstorm. While cohesion causes the drop of water to maintain its round shape, adhesion causes the drop of water to cling to the leaf. The water molecules' attraction to the leaf molecules causes them to "stick" to the leaf. Although many discussions about cohesion focus on water, molecules of other substances, such as the element, also have cohesive properties.
Overview
Cohesion, along with adhesion, contributes to several scientific phenomena in everyday life. For example, cohesion is responsible for surface tension, or the ability of a liquid's surface to repel an outside force. Anyone who has overfilled a glass of water has witnessed surface tension. Although the liquid is above the rim of the glass, surface tension prevents it from spilling over the sides. Within the glass, the intermolecular forces among water molecules push and pull evenly in all directions. These molecules experience no net pull. The water molecules at the surface, however, are different. The surface molecules are pulled downward by the water molecules below them, but they have no water molecules above them pulling upward. As a result, the surface molecules experience a net pull in the downward direction. These molecules form stronger bonds with one another and with the water molecules below them. The stronger bonds—which result from cohesion—are responsible for surface tension. They help prevent the water's surface from being broken by external forces. Surface tension not only prevents water from spilling over the sides of an overfilled glass but also allows objects, such as a paper clip, a piece of paper, or a water strider insect, to rest on the surface of water without breaking it.
Both cohesion and adhesion contribute to a process called capillary action. Capillary action is the movement of a liquid through a thin tube or the stem of a plant. Cohesion holds molecules of the liquid together, while adhesion helps the liquid hold on to the walls of the tube or stem. A common experiment demonstrating capillary action involves placing a stalk of celery into water containing food coloring. After a few days, the celery stalk's leaves appear the same color as the dyed water. In this experiment, cohesion causes water molecules to stick together, while adhesion allows water molecules to stick to the walls of the tube or stem. Together, the two forces pull the dyed water upward through the tube-like vessels of the celery stalk. Through capillary action, the water appears to have "climbed" the celery stalk.
When looking eye level at a column of water in a glass graduated cylinder, one may notice that the surface is not perfectly flat. Instead, the surface is curved. The curved surface of a column of liquid is called a meniscus. Cohesion and adhesion are responsible for this shape.
Recall the overfilled glass of water from the example of surface tension. Like the water in that glass, the water in a graduated cylinder experiences intermolecular forces. Deep within the column of water, the forces among water molecules are balanced. The molecules are pushed and pulled evenly in all directions. The surface molecules, however, experience surface tension. A net downward pull from the water molecules below them results from cohesion. At the same time, the surface molecules nearest the walls of the graduated cylinder experience a stronger attraction to the glass than to their neighboring water molecules (adhesion). As a result, the surface of the water is highest where water makes contact with the glass and lowest in the middle. This gives the meniscus a U-shaped appearance.
With mercury, cohesion and adhesion cause the surface to be highest near the middle and lowest near the glass sides of the cylinder, which gives the meniscus an upside-down U shape. The intermolecular forces responsible for cohesion among mercury molecules are stronger than the adhesive forces between mercury molecules and glass. Rather than clinging to the glass walls of the cylinder, mercury molecules cling more closely to one another. This reduces contact between the mercury and the glass, and creates a meniscus shaped like an inverted U.
Materials called wetting agents can decrease cohesion and increase adhesion of liquids. Wetting agents make it easier for a liquid to spread across a surface. For example, imagine trying to rinse a dinner plate covered with oily residue. Cohesion causes the water to form little droplets on the plate. Adding dish detergent, a wetting agent, not only helps dissolve the oily residue but also reduces water's cohesive properties and increases its adhesive properties. As a result, the water is able to spread across the surface of the plate and clean it more thoroughly.
Bibliography
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