Understanding Intermolecular Forces: Cohesive and Adhesive Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles (atoms, molecules, or ions) of a substance. These forces are weaker than the intramolecular forces that hold atoms together within a molecule, but they play a significant role in determining the physical properties of substances, such as boiling point, melting point, viscosity, and surface tension.

There are different types of intermolecular forces. When considering liquids, two important types of forces relate to the interaction between molecules:

• Forces between molecules of the same substance.
• Forces between molecules of different substances.

Cohesive Force Explained

The force of attraction between molecules of the same substance is specifically called the cohesive force or cohesion. This force is responsible for holding the molecules of a liquid or solid together. For example, the reason why water forms droplets is due to the cohesive forces between the water molecules, which try to pull the molecules inward, minimizing the surface area.

Let's look at the provided options in the context of these forces:

• Viscosity: Viscosity is a measure of a fluid's resistance to flow. While intermolecular forces (including cohesive forces) contribute to viscosity, viscosity itself is a property describing flow resistance, not the force between molecules of the same substance.
• Surface Tension: Surface tension is a property of the surface of a liquid that allows it to resist an external force due to the cohesive forces between liquid molecules. It's a result of cohesive forces, but not the name for the force itself.
• Adhesive Force: The adhesive force (or adhesion) is the force of attraction between molecules of different substances. For example, water molecules sticking to a glass surface is due to adhesive forces between water and glass molecules. This is distinct from the force between water molecules themselves.
• Cohesive Force: As discussed, the cohesive force is the attractive force between molecules of the same substance. This directly matches the definition given in the question.

Therefore, the force of attraction between molecules of the same substance is called the cohesive force.

Comparison Table: Cohesive vs. Adhesive Forces

Revision Table: Key Concepts in Fluid Mechanics

Additional Information on Cohesive Forces

Cohesive forces are essentially attractive intermolecular forces like van der Waals forces (London dispersion forces, dipole-dipole interactions) or hydrogen bonds, acting between identical molecules. The strength of these cohesive forces varies greatly between different substances, affecting their physical states and properties. For instance, substances with strong cohesive forces tend to have higher boiling points and greater surface tension compared to substances with weaker cohesive forces.

Understanding cohesive force is fundamental to studying the behavior of liquids, including phenomena like droplet formation, meniscus formation, and capillary action, where the interplay between cohesive and adhesive forces is crucial.