Definition:

Surface tension is defined as the property of living surface, which enables it to support object denser than the liquid.

It is due to surface tension, that falling drops of rain water are fairly spherical. Surface tension prevents drops of water are from passing through the rain coal or umbrellas. Cleaning detergents like omo reduces surface tension in water and enables the water to pass through small holes or pores.

A molecule in the interior of liquid such as B is attracted equally on each of side, remains in equilibrium. However a molecule on the surface of the liquid such as A has resultant downward pull which is not equalized by upward force. Hence the molecule has a tendency of getting aloes to the interior as possible.

Cohesive and adhesive forces

19. Cohesive Forces: Cohesive forces are forces of attraction between molecules of the same substance. E.g. The attraction between molecules of mercury. Cohesive forces cause water to form small droplets.
19. Adhesive Forces: These are forces of attraction between molecules of different substances for example; the attraction between molecules of water and molecules of oil. Water wet glass because the adhesive forces are stronger than the cohesive forces. Mercury does not wet glass because the cohesive forces are greater than the adhesive forces.

NB: Mercury curve downward due to strong cohesive forces and water curve upward due to strong adhesive forces.

Capillarity of Different Substances

It is the rise and fall of liquid through very tiny pores. This account for the fact that kerosene in a bush lamp will rise up the wick and water also with rise up a tree through the process of capillarity when the forces of cohesion between liquid molecules over greater than the forces of adhesion will have capillarity depression. E.g. Mercury on the other hand, if the forces of adhesion are greater than the forces of cohesion, there will have capillarity rise