Internal resistance to flow offered by one layer of liquid to another layer is called Viscosity. Viscosity is very much similar to friction of solids but not same. The viscosity of thicker liquids like honey is more than that of thinner liquids like water.

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Points to Remember:

1. Viscosity of liquid is much greater (about 100 times more) than that of gases.
2. Viscosity of gases is independent of pressure but Viscosity of liquid(except water) is directly proportional to pressure.
3. Viscosity depends on the area of the layers of liquid, the relative velocity between the two layers and distance between these  two layers.
4. Viscosity in liquids is because of the cohesive forces among molecules but incase of gases, it is due to diffusion.
5. The viscosity of liquid is inversely proportional to temperature, because the cohesive force between the molecules of liquid decreases when temperature increases.
6. The viscosity of gases is directly proportional to temperature, because diffusion increases with increase in temperature.

Related Concepts:

Strokes Law, Surface Tension