[Physics Class Notes] on Cohesion Pdf for Exam

The things that we observe in our daily life are explained by various phenomena. Sometimes we might not observe these phenomena even though they might play a crucial role in our life. Cohesion and adhesion are the two phenomena that occur in our daily life. Although they might sound similar, they are completely different terms. The relationship between adhesion and cohesion is described by surface tension, which is the most important physical property of the water. The cohesion force is caused by the Van der Waal force and the hydrogen bonding that makes the liquids withstand the separation. Whereas the adhesion force is associated with the electrostatic force. The water droplets formed on the surface are done by the strong cohesive force. Let us see what is cohesive force in physics. 

Define Cohesion

The property or action in which the molecules are striking together is called cohesion. It is also known as cohesive attraction or cohesive force. In Latin, cohesion means unity. It is the intrinsic property of the substance that is caused due to the structure and shape of the molecules. When molecules get close to one another it makes the distribution of the surrounding electrons get disturbed. This, in turn, creates the electrical attraction between the electrons and maintains an electronic structure such as a water droplet. In other words, we can say that cohesion creates surface tension that allows the solid-like state on which low-density or lightweight molecules or materials can be placed. 

The best example to represent cohesion force meaning is water, it is strongly cohesive as each of the molecules may create four hydrogens with other water molecules in the form of tetrahedral configuration. In between the molecules, the strong Coulomb force develops. The polarity of the water molecules allows them to attract each other. This polarity is due to the electronegativity of the atom of the molecules. Compared to the atoms of hydrogen, oxygen atoms are more electronegative, thus the electrons that are shared through the covalent bonds are arranged themselves and are closer to oxygen than hydrogen. Hence these are known as polar covalent bonds. These polar covalent bonds become opposite in between the electrons. In water, the oxygen atoms carry negative charges, whereas the hydrogen atoms carry positive charges. This polarization of charge allows the molecules to align with the adjacent molecules with the help of strong intermolecular hydrogen bonds. 

Mercury, which is present in the glass flask is the best example of the effects that are found in the ratio between the adhesive and cohesive force. The mercury does not spread out at the bottom of the flask because of its low adhesive strength to the glass and high cohesive force. And, if enough mercury is placed in the flask to cover the bottom, it forms a convex meniscus but when the water is filled in the glass, it makes a concave meniscus. Unlike water or other liquids, the glass surface is not wet by mercury. And if the glass is tipped, the mercury will start rotating inside. 

Cohesion Vs Adhesion

Conclusion:

The water is a highly cohesive liquid, it clumps together to form drops due to its cohesive properties. The positive and negative charges of the hydrogen and the oxygen molecules make them get attracted to each other. Cohesive force meaning is a generally used term to represent the collective intermolecular forces that are responsible for the bulk property of the liquids. And usually, these forces exist in between the molecules that are present in the same substance.