Question

There is a depression in the surface of the liquid in a capillary when

• A. The cohesive force is smaller than the adhesive force.
• B. The cohesive force is greater than the adhesive force.
• C. The cohesive and adhesive forces are equal.
• D. None of the above is true.

Answer 1

The correct option is B The cohesive force is greater than the adhesive force.

So, what did we learn in capillary action?
The surface tension of a liquid depends upon the forces between the molecules of the liquid. The attractive forces between the molecules of the same substance are called cohesive forces, while that in the molecules of different substances are called adhesive forces. When water is taken in a glass tube (such as burette), there are cohesive forces between water molecules and adhesive forces between glass and water. In this case, the adhesive forces are stronger than cohesive forces. As a result, water acquires a concave upward surface. Such a curved surface of a liquid is known as meniscus. In case of mercury, cohesive forces are stronger than adhesive forces, and hence mercury has concave downward meniscus. The rise or fall of liquids in capillary tubes (known as capillary action) can also be explained in terms of surface tension.

Depression in the surface takes place when intermolecular attraction force of liquid called cohesive force dominates the force of attraction between the liquid and the capillary called adhesive force.