Question

A liquid does not wet the solid surface if the angle of contact is then:
(A) Zero
(B) Obtuse (more than $90^\circ$)
(C) Acute (less than $90^\circ$)
(D) $90^\circ$ (right angle)

Answer 1

This problem can be solved by taking the consideration of surface tension (i.e., a mechanical property of fluids) of liquid. The term surface tension can be defined by taking into account some basic terms such as – Molecular range, Sphere of influence, and Surface film.
-Molecular range: It is a maximum distance up to which a molecule can exert some amount of measurable force of attraction on other molecules.
-Sphere of influence: It is an imaginary sphere drawn with a molecule as centre and molecular range as radius.
-Surface film: It is the top most layer of liquid at rest with thickness equal to the molecular range.

Step-by-step solution:
Step 1: It has been found that for any given volume, a liquid in small quantity (i.e., any liquid drop) at rest, free from all external forces like gravity, always tends to have a spherical shape. Since we know that for a given volume, a sphere is going to have the least surface area.
So, it shows that the free surface of any liquid quantity at rest has a tendency to attain a least surface area.
Step 2: Thus, surface tension can be defined as the property of liquid by presence of that property the free surface of liquid at rest tends to have minimum surface area and so that it behaves as if this liquid is covered with a stretched membrane (i.e. Surface film).
The liquid is going to wet the solid surface or not is defined by another term that is known as angle of contact.
Angle of contact is defined as the angle between a liquid and solid in contact and enclosed between the tangents to the liquid surface at the point of contact and the solid surface inside the liquid.
When angle of contact is obtuse (more than $90^\circ$), then in this condition, the molecules of liquid are attached strongly to themselves and weakly to those of solid, i.e., the force of cohesion of liquid molecules is greater than the force of adhesion of liquid molecules and solid molecules. Because of this, a lot of energy will be required to create a liquid-solid interface. So, liquid does not wet the solid surface when the angle is obtuse.

So, the option (B) is correct.

Note:
-Angle of contact will increase with the increase in the temperature of liquid.
-Angle of contact does not depend upon the inclination of the solid surface to the liquid surface.
-Angle of contact decreases on adding soluble impurity to a liquid.