What term describes the measure of how difficult it is to stretch or break the surface of a liquid?

The measure of how difficult it is to stretch or break the surface of a liquid is referred to as surface tension. This phenomenon arises from the cohesive forces between liquid molecules; at the surface, molecules experience an imbalance of forces since they are not surrounded by similar molecules on all sides. This creates a 'film' at the surface, allowing it to resist external force to some extent.

Surface tension plays a critical role in various biological and physical processes, such as the ability of certain insects to walk on water and the formation of droplets. The other terms relate to different properties of liquids: viscosity refers to a fluid's resistance to flow, density measures mass per unit volume, while fluidity describes the ease with which a fluid can flow. Understanding surface tension is essential in fields ranging from biology to materials science, as it affects behaviors and interactions at the liquid interface.