Viscosity is defined as the resistance of one layer to the flow of another layer of a liquid.
Water flowing in the river runs fast in the middle but slowly along the bank. It means liquid flows in the form of layers. Liquid molecule’s layers slide one past the other and they offer resistance to each other. This internal resistance that one layer offers to the flow of the other layer is called viscosity.
Factors affecting Viscosity:
1. Intermolecular forces:
Stronger intermolecular forces among molecules of liquid higher will be the viscosity and vice versa e.g. the viscosity of glycerin is higher than in water because hydrogen bonding in glycerin is stronger than in water.
Higher is the temperature low will be the viscosity and vice versa e.g. it is easy to pour honey from a bottle placed outside as compared to a bottle placed in the refrigerator. In the refrigerator due to the low-temperature viscosity of honey was high.
3. Molecular Size and Mass:
Molecules having bigger sizes or more molecular mass offer more resistance to each other’s flow, so more will be the viscosity e.g. mobile oil, grease etc have high viscosity, and they consist of larger hydrocarbon molecules.
Higher is the density high will be the viscosity and vice versa e.g. viscosity of sulphuric acid is higher than water due to the higher density of sulphuric acid.
5. Molecular Shape:
Irregular-shaped molecules offer more resistance to each other’s flow as compared to regular-shaped molecules e.g. viscosity of liquid Sulphur is higher at 190oC than that at 140 oC.
Sulphur molecules are ring-shaped at 140 oC and offer less resistance to each other’s flow while the molecules at 190 oC are tangled and offer more resistance to flow.
Measurement of Viscosity:
Viscosity is measured in terms of the coefficient of viscosity (η). The coefficient of viscosity is defined as the force required to maintain a difference of velocity of 1m/s between two parallel layers of liquid one metre apart.
S.I Unit is Kg.m-1.s-1
Other units are poise and centipoise
1 poise = 10-1 Kg.m-1.s-1
1 Kg.m-1.s-1 = 10 poise
1 poise = 100 centipoise
Viscosity is usually measured by an Oswald viscometer (See Figure given below).
In this method relative viscosity is measured. For this purpose time for the flow of reference and test liquid between two marks, a & b made on the viscometer is noted. And viscosity is calculated by using the formula:
Water is usually chosen as a reference liquid. The viscosity of water is taken as 1 centipoise at 25oC.