Welcome to class! 

In today’s class, we will be talking about viscosity. Enjoy the class!


Illustration Chemistry Viscosity Fluid Measure Resistance Stock Vector (Royalty Free) 1308961747

Viscosity is the property of liquid which opposes the relative motion between different layers. It is the name given to the internal friction that exists between the layers of the molecules of fluids (liquid or gas) in contact with surfaces.

Some liquid like water, alcohol and spirit, are less viscous whereas thick liquid like castor oil, glycerine and coal tar, is more viscous.

Classification of fluid according to their viscous properties

Classification of fluids is based on high and low viscosity depending on the frictional force.

(1) When frictional force is low’ low viscosity, Examples of low viscous fluid are:

(a) Water

(b) Kerosene

(c) Petrol

(d) Ethanol

(2) When frictional force is large’ high viscosity, Examples of high viscous fluid are:

(a) Glue

(b) Syrup

(c) Grease

(d) Glycerine

Effects of viscosity
  1. It is responsible for the different rates of flow of fluids.
  2. It affects the motion of bodies in fluids.
Factors acting on a solid object which is made to fall in a liquid

Three factors are acting on a solid object which is made to fall in a liquid:

  1. The object’s weight (downwards)
  2. The upthrust (Upwards)
  3. The frictional force or viscous drag (Upwards)
Differences between viscosity and friction
S/N Viscosity Friction
1 Viscosity depends on the area of the surface in contact Friction does not depend on the area of the surface in contact
2 Viscosity is not dependent on the normal reaction Friction is dependent on the normal reaction
3 Viscosity take place in liquids and gases (Fluids) Friction occurs in solids
4 Viscosity depends on the relative velocity between layers Frictions do not depend on the relative velocities between tow layers
The similarity between viscosity and friction
  1. Both Forces oppose relative motion between surfaces
  2. Both depend on the nature of the materials in contact.
Application of viscosity
  1. The viscous liquid is used as a lubricant: Oil and, grease, and air are used as lubricants because of their viscosity. Engine oils are used in lubricating engines and other machines so as to keep metal surfaces from rubbing against each other. Water is used as a lubricant because it has a low viscosity.
  2. The knowledge of viscous drag is applied in the design of ships and aircraft.
  3. Particles of a suspension in liquid fall with a steady terminal speed and this can be used to estimate the enlarged size of the particles.


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