Question

The angle of contact for the liquid that wets the wall of the vessel is ?

Answer 1

Hint: The angle of contact is defined as the angle made by the surface of the liquid with the outline of the contact surface when a solid and liquid are in contact with each other. This angle depends on the fact that is the liquid able to wet the surface of the solid, that is, wettability of the liquid and wettability of a liquid depends upon the cohesive and adhesive forces.

Complete step by step answer:
We will first understand the cohesive and adhesive forces and then try to answer our question.
The force of attraction between the similar kinds of molecules is known as cohesive force and this phenomenon is known as cohesion. This is the force that acts between the particles of water and is responsible for keeping the water surface in-tact.
The Force of attraction between dissimilar kinds of molecules is known as adhesive force and this phenomenon is known as adhesion. The force acting between the particles of liquid and the particles of water is adhesive force.
Now, for a liquid which can wet a wall, the force of adhesion is greater than the cohesive force acting among the particles of the liquid. And we know that, when adhesive forces are greater than cohesive forces, the angle of contact is acute.
Hence, the angle of contact for the liquid that wets the wall of the vessel is an acute angle, that is, less than 90 degrees.

Note: If the liquid is unable to wet the solid surface, then the force of cohesion is greater than the force of adhesion and in this case the angle of contact is an obtuse angle. When, both the forces are exactly equal, the angle of contact in that special case is equal to a right angle, that is, it is the angle of contact is equal to 90 degrees.