Which property describes the cohesive force at the air-water interface, affecting droplet formation?

• A. Shadow Zone
• B. Surface Tension
• C. State
• D. Sound

Answer: (B)

Surface tension is the cohesive force at the air–water interface that creates a tiny, stretched “film” on the surface. Water molecules at the surface are pulled inward by neighboring molecules, producing an inward, cohesive pull along the surface. This makes the surface behave like a flexible skin and sets the energy cost of creating new surface area. Droplets form and detach because surface tension resists the formation of new surface; a balance of forces determines when a blob of water breaks into a droplet—the stronger the surface tension, the more it pulls the liquid into a compact, spherical shape, and the more external force is needed to overcome it. The other terms don’t describe this interfacial cohesive property: a shadow zone refers to signal or light propagation effects, state is about the phase of matter, and sound is a wave phenomenon in a medium.