What Is Cohesion? Understanding Forces Between Molecules
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.
FAQs on Cohesion in Physics: Meaning & Applications
1. What is a simple explanation of cohesive force in Physics?
A cohesive force is the intermolecular force of attraction that exists between molecules of the same substance. Think of it as the 'stickiness' that holds a single substance together, like the force keeping water molecules in a droplet bound to each other.
2. Can you give a common example of cohesion in daily life?
A classic example is water. The cohesive forces between water molecules are very strong, which is why water forms distinct droplets on a surface. This force is also responsible for a property called surface tension, which allows small insects to walk on water. Mercury is another example of a liquid with extremely high cohesive forces.
3. What is the main difference between cohesive and adhesive forces?
The key difference lies in the types of substances involved.
- Cohesive force is the attraction between molecules of the same substance (e.g., a water molecule attracting another water molecule).
- Adhesive force is the attraction between molecules of different substances (e.g., a water molecule sticking to a glass surface).
4. In which state of matter are cohesive forces the strongest?
Cohesive forces vary significantly with the state of matter. They are strongest in solids, where molecules are packed tightly in a fixed lattice structure. They are weaker in liquids, which allows molecules to flow past each other but still keeps the liquid together. In gases, cohesive forces are the weakest and almost negligible, which is why gas molecules spread out to fill any container they occupy.
5. How is the concept of cohesion related to surface tension?
Surface tension is a direct result of cohesion. Inside a liquid, a molecule is pulled equally in all directions by cohesive forces from its neighbours. However, a molecule at the surface is only pulled inwards and sideways by other molecules. This imbalance creates a net inward force, causing the molecules at the surface to pack together tightly, forming a 'skin'. This phenomenon, caused by cohesion, is what we call surface tension.
6. What actually causes cohesive forces to exist between molecules?
Cohesive forces are a type of van der Waals force, which are weak intermolecular attractions. They originate from the way electrons are distributed in molecules. Even in neutral molecules, the random movement of electrons can create temporary, uneven charge distributions (dipoles). These temporary dipoles can then induce dipoles in neighbouring molecules, leading to a weak, short-range attraction that holds the substance together.
7. How does cohesion help plants move water from roots to leaves?
This process is central to the Cohesion-Tension Theory in biology. As water evaporates from the leaves (transpiration), it creates a tension or pull. Because of the strong cohesive forces between water molecules, they stick together, forming a continuous, unbroken column of water in the plant's xylem tubes. When one molecule is pulled up, it tugs the next one along, effectively drawing water all the way from the roots to the top of the plant.
