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How does the viscosity of fluids vary with increase in temperature?
1. Increases for both liquids and gases
2. Increases for liquids, decreases for gases
3. Increases for gases, decreases for liquids
4. Decreases for both liquids and gases

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Correct Answer - Option 3 : Increases for gases, decreases for liquids

Concept:

Viscosity: The property that represents the internal resistance of a fluid to motion (i.e. fluidity) is called viscosity.

There are two ways to write viscosity:

  • Dynamic viscosity: This is also termed as absolute viscosity. A common unit of dynamic viscosity is poise

                                  1 poise = 0.1 Pa.s = 0.1 N.s/m2

  • Kinematic Viscosity: The ratio of dynamic viscosity to density appears frequently and this ratio is given by the name kinematic viscosity. Its unit is Stoke or m2/s (1 stoke = 0.0001 m2 /s).
  • Viscosity is affected by temperature.
  • The viscosity of liquids decreases but that of gases increases with increase in temperature.
  • This is due to the reason that in liquids, the shear stress is due to the inter-molecular cohesion which decreases with increase of temperature.
  • In gases, the inter-molecular cohesion is negligible and the shear stress is due to exchange of momentum of the molecules, normal to the direction of motion. 
  • The molecular activities increases with rise in temperature and so the viscosity of gases increases. 

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