Which term describes the tightness on the surface of a liquid caused by molecular cohesion?

• A. Surface tension
• B. Capillarity
• C. Viscosity
• D. Pressure

Answer: (A)

Surface tension is the cohesive force at the surface of a liquid that makes the surface behave like a stretched skin. Molecules at the surface experience a net inward pull from their neighbors, but they aren’t pulled equally in all directions, so they stay packed tightly along the surface. This creates a small energetically costly surface area, so liquids tend to minimize surface area and form droplets or keep surfaces drawn together. Water, for example, has a relatively high surface tension—about 72 millinewtons per meter at room temperature—though it decreases as temperature rises. Capillarity involves how liquids rise or fall in small tubes due to the balance of cohesive and adhesive forces and is related to surface effects but is about motion in capillary tubes, not the tight surface itself. Viscosity is the internal resistance to flow, and pressure is the force per area applied to a fluid; neither describes the surface tightness caused by cohesion.