Q.Define surface energy and surface tension of a liquid. Explain the molecular (intermolecular-force) origin of surface tension, and state the relation between the surface energy per unit area of a liquid surface and its surface tension.

Surface tension, $T$, is defined as the tangential force acting per unit length along an imaginary line drawn on a liquid's free surface, $T = F/l$, with SI unit $\text{N/m}$.

Surface energy is the potential energy stored per unit area of the liquid's free surface.

Molecular origin: a molecule deep inside the liquid is attracted, within its molecular range, symmetrically from all sides by neighbouring liquid molecules, so the net force on it is zero. A molecule sitting exactly at the liquid's free surface, however, has liquid molecules pulling it inward and sideways, but comparatively very few molecules of the much less dense air above pulling it outward -- so the net force on a surface molecule is a resultant pull directed inward, into the bulk of the liquid. This makes the surface behave like a stretched membrane, always tending to contract to the smallest possible area, which is exactly the observed behaviour that surface tension describes. …

Define surface tension and surface energy separately, explain the molecular asymmetry (fewer attracting neighbours above the surface than below) that causes both, and state that the work needed to c …

• Describing surface tension only as "a liquid's tendency to minimise its surface area" without giving the precise force-per-unit-length definition. …