•    Temperature affects the viscosity.

•    The viscosity of liquids decreases when the temperature increases, while the viscosity of gases increases with the increase of temperature.

•    This is due to reason that the viscous forces in a fluid are due to cohesive forces and molecular momentum transfer.

•    In liquids the cohesive forces are more important than the molecular momentum transfer, due to closely packed molecules and with the increase in temperature, the cohesive forces decrease with the result of decreasing viscosity.

•    But in gases, molecular momentum transfer is more important than the cohesive forces. Molecular momentum transfer increases with the increase in temperature and hence viscosity of a gas increases.

•    For ordinary pressure, viscosity is independent of pressure and depends upon temperature only.

•    The relation between viscosity and temperature for liquids and gases are:

(i) For liquids,    μ =  μ₀ ( 1/ 1+ At + Bt² )

where  μ = Viscosity of liquids at t⁰C, in poise
        
   μ₀ = Viscosity of liquids at 0⁰C, in poise
   
 A, B = Constants for the liquid

For water, μ₀ = 1.79 x 10^-3 poise, A = 33.68 x 10^-3 and B = 22.1 X 10^-3

The equation shows that with the increase of temperature, the viscosity decreases.

(ii) For gases,    μ = μ₀ + αt - βt²

where for air     μ₀ = 0.17 x 10^-6 , α = 0.056 x 10^-6, β = 0.1189 x 10^-9

The equation shows that with the increase of temperature, the viscosity increases.

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