Question

Water proofing agent changes the angle of contact:
A) From an obtuse to acute value.
B) From an acute to obtuse value.
C) From obtuse to \[\dfrac{\pi }{2}\].
D) From an acute to \[\dfrac{\pi }{2}\].

Answer 1

Hint: We need to understand the phenomena involved in the water proofing and its relation with angle produced between water and the surface to solve this problem. The angle of contact determines whether a material is water-proofing or not.

Complete step by step solution:
We know that every liquid exposed to air having liquid-air, liquid-solid and solid-air interface has some extra energy than the molecules present in the bulk of the system. This is due to the free ends without any interactions with the other molecules. This is the Surface energy of the molecules.
Surface tension is defined as the surface energy per unit area or the force per unit length of the molecule exposed to the other medium. The surface energy is dependent on the particle nature of the liquid under consideration. If the molecules of the liquid themselves are highly attractive, then the solid and the liquid have a lesser contact, making an obtuse angle. In other cases, when the liquid molecules are repulsive to each other when compared to the solid, they stick to the solid causing an acute angle of contact.

The above figure shows the water-sticking and water-proofing agents respectively. The liquid which has the angle as \[\theta <\dfrac{\pi }{2}\] is the water-sticking agent and the one with the angle \[\theta >\dfrac{\pi }{2}\] is water-proofing agent.
The angle of contact is the angle formed between the horizontal of the solid surface and the liquid. A water-proofing agent is the one which is least attracted to the water, this is possible as the molecules in the liquid are attracted to each other more strongly than to the solid, thus forming an obtuse angle with the solid.

The correct answer is option B.

Note:
The angle of contact between a solid surface and the liquid is dependent on the surface tensions between all the interfaces among the three media – air, liquid and the solid. The wax forms a water-proofing agent whereas on paper the water sticks