You have learnt that attractive intermolecular forces exist between particles of substances. The attractive forces between like molecules are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces.

The attractive intermolecular forces between unlike particles are said to be adhesive forces. When two objects with unlike particles are brought in contact, the system experiences both adhesive and cohesive forces. Consider a capillary tube (tube of small diamater) dipped vertically into water and mercury as shown in Figure 6.6. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces between the water molecules. This is the reason why we see an upward turning meniscus at the walls of the vessel. This property contributes to capillary action. On the other hand, the adhesive forces between mercury atoms and the walls of a glass tube are weaker than the cohesive forces between mercury molecules.