Question

Why potential energy of molecule is greater at surface of liquid than in bulk

Answer 1

because, potential energy is directly proportional to tha mean height of the substance from the surface of earth.so the molecule of liquid hamare PE at the surface than bulk...

Answer 2

Surface effects! The molecules, atoms, nucleons, etc at the edge of a bulk material are exposed to a different environment than those in the interior. Since the bulk material is bound, the interior constituents have a higher binding energy than those at the edge. This makes those constituents near the edge more weakly bound which is equivalent to having a higher energy.