Q. On a clean glass plate a drop of water spreads to form a thin layer whereas a drop of mercury remains almost spherical because?
Answer: Cohesion of mercury is greater than its adhesion with glass
Notes: When liquid is placed on a smooth surface like glass plate, the relative strengths of the cohesive and adhesive forces acting on that liquid determine the shape it will take. If the adhesive forces between a liquid and a surface are stronger, they will pull the liquid down, causing it to wet the surface. However, if they cohesive forces among the liquid itself are stronger, they will resist such adhesion and cause the liquid to retain a spherical shape and bead the surface. Mercury drop remains spherical on a plate of glass because its cohesive force is greater than its adhesive force with glass. In water, the adhesive force are stronger than adhesive forces therefore it spreads on glass plate.