Cohesion refers to the attraction between molecules of the same substance, whereas adhesion refers to the attraction between different substances.
Think of cohesion as water drops merging together, and adhesion as water sticking to a glass surface.
In other words, cohesion is like water sticking to itself, while adhesion is like water sticking to other things. The interesting question here is which one would be stronger than the other?
The strength of cohesive or adhesive forces comes down to the molecular interactions involved:
- Hydrogen Bonding: Water, for instance, exhibits strong hydrogen bonding, leading to significant cohesive forces. However, when water is in contact with surfaces like glass that can also participate in hydrogen bonding, the adhesive forces can surpass the cohesive ones.
- Van der Waals Forces: These forces can contribute to both cohesion and adhesion, depending on the molecules involved.
- Electrostatic Interactions: Some adhesion can be attributed to electrostatic forces, especially in cases involving charged surfaces.
- Chemical Bonding: In some instances, actual chemical bonds can form at interfaces, leading to exceptionally strong adhesion.
Cohesion vs. Adhesion
| Cohesion | Adhesion |
| Cohesion involves molecules of the same type interacting with each other, like water with water. | Adhesion involves interactions between different types of molecules, such as water and glass. |
| The force in cohesion arises from the attraction among like molecules. | The force in adhesion arises from the attraction between different molecules. |
| An example of cohesion is beads of water forming on a surface. | An example of adhesion is when water spreads out on a leaf. |
| In plants, cohesion helps in forming continuous water columns which are crucial for transport. | Adhesion in plants causes water to climb up plant stems, assisting in capillary action. |
| Cohesion can cause a liquid to form a convex meniscus, like mercury does in a tube. | Adhesion can cause a liquid to form a concave meniscus, like water in a tube. |
| Due to cohesion, liquids tend to form droplets when placed on a solid surface. | Due to adhesion, liquids tend to spread out when in contact with certain solid surfaces. |
| Cohesion is responsible for phenomena like surface tension in cell membranes. | Adhesion plays roles such as helping cells attach to surfaces, aiding processes like inflammation. |
What is Cohesion?
Cohesion refers to the phenomenon where molecules of the same type tend to stick together, thanks to the attractive forces operating between them.
Fundamental Forces Behind Cohesion
The primary force responsible for cohesion in many liquids, including water, is the hydrogen bond. In molecules without hydrogen bonding, other intermolecular forces like van der Waals forces or dipole-dipole interactions can lead to cohesive behavior.
Examples of Cohesion
- Water Drops: When you see water drops on a leaf or on your car window, it’s cohesion in action. The water molecules prefer to stick together, forming droplets.
- Mercury in a Glass Thermometer: Mercury, unlike water, does not wet the glass. It remains cohesive and forms a distinct convex meniscus in a glass tube.
- Surface Tension: It’s the reason small insects like water striders can walk on the surface of water. The cohesive forces create a “film” on the water surface, preventing the insect from sinking.
- Water’s High Boiling Point: Water has a higher boiling point than many other molecules of its size due to the strong cohesive forces from hydrogen bonding.
- Formation of Liquid Beads: When a liquid is spilled, it tends to form beads or droplets. This is especially pronounced for liquids with high cohesive forces.
What is Adhesion?
Adhesion is the phenomenon where molecules of different types attract one another. These forces are usually due to various intermolecular attractions, such as hydrogen bonds, van der Waals forces, or electrostatic forces.
Underlying Forces of Adhesion
The forces behind adhesion vary depending on the molecules involved. Often, it’s a combination of forces, including hydrogen bonding, dipole interactions, and van der Waals forces.
Examples of Adhesion
- Wetted Surfaces: When water spreads on materials like paper or cloth, it’s due to adhesive forces between water molecules and the material’s molecules.
- Capillary Rise: In thin tubes or porous materials, liquids often rise against gravity. Water, for example, climbs up plant roots and stems because of adhesion to the cell walls.
- Paint on a Wall: The reason paint sticks to walls is because of adhesive forces between the paint molecules and the wall molecules.
- Stickers or Glue: The adhesive material can stick to surfaces because of the attractive forces between the adhesive and the surface.
- Liquid Meniscus in Tubes: When you place water in a glass tube, it forms a concave meniscus. This is because the adhesive forces (water-glass) are stronger than the cohesive forces (water-water).
Chemist’s Point of View
From a chemist’s perspective, cohesion and adhesion are essential for understanding various properties of substances, such as solubility, boiling point, and capillarity. Chemists consider the molecular interactions, the nature of the substance, and its behavior in different conditions.
For example, understanding cohesion helps chemists explain why some liquids have high surface tension and why certain substances have higher boiling points than others.
Biologist’s Point of View
From a biologist’s standpoint, cohesion and adhesion are vital for understanding several biological processes.
For example, the transport of water in plants, known as transpiration, is hugely dependent on both cohesion and adhesion. Cohesive forces help pull water columns up against gravity, and adhesive forces enable water to stick to the sides of thin tubes or xylem in plants.
Moreover, adhesion plays a role in cellular interactions, such as when white blood cells adhere to the walls of blood vessels during an inflammatory response.
Key Differences Between Cohesion and Adhesion
Similarities Between Cohesion and Adhesion
- Both cohesion and adhesion result from intermolecular forces. These forces arise from attractions between molecules, whether they’re of the same type (in cohesion) or different types (in adhesion).
- Both phenomena play a significant role in determining the behavior of liquids, especially in contact with surfaces. For instance, they both influence the formation of a liquid’s meniscus in a container.
- Both cohesion and adhesion are essential in biological processes. For example, in plants, cohesion helps maintain a continuous flow of water from the roots to the leaves, while adhesion assists water in moving against gravity through capillaries.
- The strength of both cohesive and adhesive forces can change with temperature. As temperature increases, molecules move more vigorously, which can reduce the effects of these forces.
- Surface tension in liquids arises primarily due to cohesive forces, but adhesion also plays a role, especially when a liquid is in contact with a surface.
