How is surface tension defined?

• A. The force within the bulk liquid away from the surface
• B. The color property of a liquid
• C. The force required to expand or pull apart the surface of the liquid
• D. The tensile strength of the solid

Answer: (C)

Surface tension is the cohesive force at the surface of a liquid that makes it act as if its surface is covered by a stretched film. It can be defined as the force per unit length required to stretch or expand the surface, or equivalently the energy needed to create a new surface area. This is exactly what the option describes—the force required to expand or pull apart the surface of the liquid. The other ideas don’t fit: forces in the bulk of the liquid aren’t about the surface, color has no relation to surface tension, and tensile strength is a property of solids, not liquids.