Intermolecular forces

The molecules of a liquid, unlike those of a solid, are not rigidly fixed in place -- they are free to move about relative to one another. The attractive force between LIKE molecules (molecules of the same substance) that holds a liquid together as a coherent body is called the COHESIVE FORCE; when the liquid is in contact with a solid, the attractive force between the molecules of the solid and the molecules of the liquid is called the ADHESIVE FORCE. Both of these intermolecular forces act only over a very short range -- roughly $10^{-9}$ m (10 angstrom) -- called the SPHERE OF INFLUENCE of a molecule; forces from molecules lying outside this sphere are negligible. Consider three molecules A, B, and C at different depths in a liquid. Molecule A, well inside the liquid, has its entire sphere of influence filled with other liquid molecules on every side, so the net force it experiences from all directions cancels out to zero. Molecule B, positioned so that about three-quarters of its sphere of influence lies below the liquid surface and only one-quarter lies in the air above, has more neighbouring molecules pulling it downward than upward, so it experiences a net DOWNWARD force. Molecule C, sitting exactly on the free surface (roughly half its sphere of influence in the liquid, half in the air), experiences the LARGEST net downward force of the three, because it has essentially no upward-pulling neighbours in the air above it to balance the liquid molecules pulling it down. Because EVERY molecule that lies within the surface region of the liquid experiences this same net inward/downward pull, work has to be done AGAINST this cohesive pull to bring any additional molecule up from the interior to the surface -- and this work is stored as extra potential energy in the surface molecules, meaning surface molecules have a higher potential energy than molecules in the bulk of the liquid. Since a system in stable equilibrium always seeks to MINIMIZE its potential energy, a liquid surface always tends to have the smallest possible number of molecules on it, i.e., the liquid tends to occupy the SMALLEST possible surface area for a given volume -- this tendency of a liquid surface to contract to the minimum possible area is exactly what is called SURFACE TENSION. Everyday demonstrations of surface tension include: water striders and other insects walking on top of a water surface, which behaves like a stretched, springy membrane strong enough to support their small weight; the bristles of a wet paintbrush clinging together in a single point, because the thin water film between adjacent bristles contracts to the smallest possible area, pulling every bristle in toward its neighbours; a greased steel needle floating on a water surface (causing a visible dimple) despi …