Surface Tension and Liquid Surface Contraction

Introduction:
The tendency of a liquid surface to contract is known as surface tension. It is a property that arises due to cohesive forces between the molecules of a liquid. Surface tension is responsible for various phenomena such as capillary rise, droplet formation, and the behavior of liquid surfaces.

Surface Tension:
Surface tension can be defined as the force acting per unit length along the boundary of a liquid surface. It is measured in units of force per unit length (N/m or dyn/cm). The surface tension of a liquid is influenced by the cohesive forces between its molecules. These forces create a net inward force on the molecules at the liquid surface, causing it to contract.

Explanation:
The tendency of a liquid surface to contract is primarily due to surface tension. When a liquid is at rest, the molecules at the surface experience a net inward force due to the imbalance of cohesive forces. This force acts tangentially to the surface, causing it to contract and minimize its surface area.

Comparing Options:
Let's analyze the other options mentioned in the question to understand why they are not the correct answers:

a) Cohesion: Cohesion refers to the attraction between molecules of the same substance. While cohesion plays a role in surface tension, it is not the property directly responsible for liquid surface contraction.

b) Adhesion: Adhesion refers to the attraction between molecules of different substances. While adhesion can influence the shape of a liquid surface, it is not the primary property responsible for surface contraction.

c) Viscosity: Viscosity refers to a liquid's resistance to flow. While viscosity can affect the behavior of a liquid, it is not directly responsible for surface contraction.

d) Surface Tension: Surface tension arises due to cohesive forces between liquid molecules. It is the property directly responsible for the tendency of a liquid surface to contract.

Conclusion:
In summary, the correct answer is option 'D' - surface tension. The contraction of a liquid surface is primarily due to the cohesive forces between its molecules, which create a net inward force and result in surface tension. Surface tension plays a crucial role in various natural phenomena and has significant practical applications in fields like engineering and physics.