Viscosity is the resistance to flow the liquid.All liquid show some viscosity.Some liquid have high resistance to the flow the liquid.
Such liquid is said to have high viscosity.For example glycerin which flows slowly is said to have high viscosity.Water and alcohol flow readily and said to have low viscosity.Viscosity is due to the intermolecular force of attraction between the liquid molecules. The greater the intermolecular forces the greater will be the viscosity of liquid.When a fluid is subjected to external forces, it resists flow due to internal molecular friction, Viscosity is a measure of that internal friction. Viscosity can be referred to as the measurement of a fluid’s resistance to flow. Viscosity can be viewed in two different ways. The first is a fluid’s tendency to flow as is visually indicated. One can think of this as the time it takes to watch a fluid pour out of a container. The term used to describe this is Kinematic Viscosity and it is expressed in units indicating flow volume over a period of time. The most commonly used unit of Kinematic Viscosity is the centistoke.
In order to understand, why different liquid flow with different speed, consider the flow of liquid through a narrow tube or pipe.
We can imagine that the liquid flowing through a tube consists of a large number of concentric molecular layers.A thin layer in immediate
with the wall of tube is almost stationary.Then each succeeding layer moves with gradually increasing speed which becomes maximum at
the centre.
The internal friction or resistance that one layer of a liquid moving with the certain viscosity offers to another adjacent layer moving with a different velocity is called the viscosity of liquid.
