Certain liquids flow faster than others. Water and kerosene oil flow rapidly while honey and castor oil flow slowly. The cause of different rates of flow of liquids may be easily understood by considering flow of liquid in a beaker. If water is stirred in a beaker with a rod and left undisturbed for some time, its swirling motion stops after some time. The faster moving outer layer adjacent to the edge of the beaker comes to stop first. The slower moving layer near the center, which stops last, is pulling it back. Different layers in a liquid move over one another with different speeds in the direction of the flow of the liquid. However, due to intermolecular forces, there is resistance of one layer to the other layer. These internal self-governing forces tend to oppose any free motion. This resistance to the flow of a liquid is termed as viscosity.

'The internal resistance to flow in liquids, which, one layer offers to another layer trying to pass over it is called viscosity'.

The viscosities of liquids are compared in terms of coefficient of viscosity (h). This is defined as the tangential force per unit area required to maintain a unit difference in velocities.

The units of viscosity are poise (P), where 1P = 1 g cm-1 s-1. In S.I. unit, 1P = 0.1 Nsm-2.

Viscosity is also related to the intermolecular forces in the liquid. If the intermolecular forces are large, viscosity will be high. For example, water has higher viscosity than methyl alcohol because intermolecular forces in water are more than that in methyl alcohol.

With rise in temperature viscosity of a liquid decreases. This increases the average kinetic energy and so the intermolecular forces can be easily overcome.

Liquids with extensive hydrogen bonding show higher viscosity due to an increase in size and mass of the molecule.

For example, glycerol and sulphuric acid show higher viscosity than water due to hydrogen bonding.

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