The question asks why the free surface of a liquid tends to contract into the smallest possible area, behaving like a stretched sheet.

Explaining Liquid Surface Contraction

This behavior is a direct consequence of surface tension. Surface tension is the property of a liquid surface that allows it to resist an external force, causing it to behave like an elastic sheet.

Role of Cohesive Force

The primary reason for surface tension is the force of cohesion. Cohesion refers to the intermolecular attraction between like molecules (molecules of the same substance). Inside the bulk of the liquid, each molecule is pulled equally in all directions by neighboring molecules. However, molecules at the surface have fewer neighbors above them, resulting in a net inward pull.

This net inward pull causes the surface molecules to be held more tightly together, minimizing the surface area. Liquids naturally adopt shapes that minimize their surface area for a given volume (like spheres for droplets), which is a manifestation of this cohesive force at the surface.

Other Forces Considered

• Force of adhesion: This is the attraction between molecules of *different* substances. While adhesion plays a role in how liquids interact with surfaces (e.g., wetting), it doesn't cause the liquid's own surface to contract.
• Force of friction: This force opposes relative motion between surfaces in contact. It is not relevant to the inherent tendency of a liquid surface to contract.
• Force of diffusion: This is the process of particles moving from an area of higher concentration to lower concentration. It is unrelated to surface tension.

Therefore, the tendency of a liquid surface to contract is due to the force of cohesion among its molecules.