Question

The main cause of viscosity is:
A. Force of repulsion between molecules.
B. Cohesive forces.
C. Adhesive forces.
D. Both cohesive and adhesive forces.

Answer 1

Viscosity is the quantity that defines the resistance of a fluid to flow. As well as the flow of layers with varying velocities inside them, fluids avoid the relative movement of submerged objects through them. The resistance of a fluid (liquid or gas) to a change in form or to the rotation of the neighbouring portions in relation to each other.

Complete answer: The term 'cohesive forces' is a general term for the intermolecular mutual forces responsible for the bulk property of separation-resistant liquids. Specifically, within molecules of the same material, these attracting forces occur. The word 'adhesive forces' also refers to the attractive forces between various substances, such as mechanical forces (sticking together and electrostatic forces (attraction due to opposing charges). On the other hand, due to strong cohesive forces between the molecules, any layer in a moving fluid tries to drag the adjacent layer to move with an equal speed and thus produces the effect of viscosity. In other words, when the intermolecular forces of attraction inside a fluid are high, it becomes the cause of viscosity.
Hence, option (B) is the correct option.

Note: In this question you are asked to find the main cause of viscosity. In between the given option, we must analyze the concept of viscosity with each given option. Cohesive forces are intermolecular mutual forces responsible for the bulk property of separation-resistant liquids and the main cause of viscosity due to its intermolecular force of attraction capability in the liquid.