Cohesive & Adhesive Forces Page 1 of  1

Cohesive forces:

The forces of attraction between like molecules.

Cohesive forces are directed inwards away from the walls of the container.

The forces of attraction between unlike molecules.

Adhesive forces are directed outwards towards the walls of the container.
Adhesive forces play an important role in the function of adhesive materials such as:

Glue – Cement – Tar – Painting material – Ink.

The shape of the liquid surface inside a container depends on

1.   The cohesive forces.

The angle of contact:

It is the angle between the solid surface & the tangent of the liquid surface at the point of contact measured inside the liquid.

The factors affecting the angle of contact:

1.        The kind of liquid.

2.        The kind of solid.

N.B:

 1. When the cohesive forces F1 are less than adhesive forces  F2:

·          The resultant force is directed out of the liquid & attracts the molecules of the liquid towards the walls of the container.

·          The effect of the adhesive forces will be strong.

·          The liquid surface will rise on (elevated).

·           The free surface will be curved inwards (concave).

·          The angle of contact (q)  < 90.

·          The liquid wets the solid.

·           Ex: in case of water in a glass vessel.

 2.         When the cohesive forces F1 are greater than the adhesive forces F2:

·          The resultant force is directed inward away from the container & attracts the molecules of the liquid inwards.

·          The effect of the cohesive forces will be strong.

·          The liquid surface will be depressed.

·          The free surface will be curved outwards (convex).

·          The angle of contract (q)  > 90.

·          The liquid doesn’t wet the solid.

·          Ex: in the case of mercury in a glass vessel.

3.            When the cohesive forces F1 are equal to the adhesive forces F2:

·          The effect of the cohesive forces is equal to the effect of adhesive forces.

·          The resultant force = zero

The liquid surface takes horizontal level.

·          The liquid surface will be neither elevated nor depressed.

·          The free surface will be plan (horizontal).

·          The angle of contact  (q)  = 90.

·          The liquid doesn’t show capillary.

·          Ex: in the case of pure water in a silver vessel.