Q.Distinguish between cohesive and adhesive forces.

Unlike the atoms of a solid, the molecules of a liquid are not rigidly fixed in place -- they remain free to move relative to one another. Two distinct kinds of short-range intermolecular force govern liquid behaviour. The COHESIVE FORCE is the force of attraction between LIKE molecules -- molecules of the same liquid -- and it is what holds a liquid together as a single coherent body (this is why a drop of water stays a drop rather than dispersing into isolated molecules). The ADHESIVE FORCE is the force of attraction between UNLIKE molecules -- specifically, between the molecules of a liquid and the molecules of a solid surface it touches (for instance, between water molecules and glass). Both forces act only over an extremely short range, roughly $10^{-9}$ m (10 angstrom), called the SPHERE OF INFLUENCE of a molecule; any molecule lying outside this tiny range exerts a negligible force. The relative STRENGTH of cohesive versus adhesive force for a given liquid-solid pair decides whether that liquid wets the solid (adhesive stronger than cohesive: water on glass) or beads up and refuses to wet it (cohesive stronger than adhesive: mercury on glass) -- this comparison is exactly what fixes the angle of contact between the two. A useful contrast is tha …