Question

The cause of viscosity of liquid is:
A. diffusion
B. adhesive force
C. gravitational force
D. cohesive force

Answer 1

Hint: The viscosity is a property of a liquid. When the various layers of the liquid slide against one another, the frictional forces between the layers prevents the change in shape of the liquid.

Detailed step by step solution:
Frictional forces are resistive forces which come into action whenever two surfaces slide over one another. The surfaces may be solid or fluid. In case of fluids i.e. liquids and gases, we have layers of the material which slide over one another. As a result we have a frictional force between the layers, and the magnitude of this friction decides the ability of that liquid to change its shape. The viscosity of a liquid is the manifestation of this frictional force between the layers.
Greater the frictional force between the layers of the liquid, greater is viscosity because it becomes difficult for such liquid to change its shape. These forces are cohesive in nature which means that the layers cling to each other trying to avoid the deformation of the shape of the liquid. For example, water has less viscosity compared to honey. Water flows easily while honey does not.
Hence, the correct answer is option D.

Note: 1. Diffusion refers to the movement of the molecules of a substance from higher concentration to lower concentration.
2. Adhesive forces are the forces which make one material to bind with another material. The difference between adhesion and cohesion is that cohesion is between molecules of the same substance while adhesion is between the molecules of different substances.
3. The gravitational force is the force exerted by one massive body on another massive body.