Effect Of Temperature:

The viscosity of liquid decreases with increase in temperature. No definite relation has been found to exist between viscosity and temperature, but various emperical formulas have been suggested from time to time which are all approximate.

Slotte gave the emperical formula

\( \displaystyle{\eta_t=\frac{\eta_0}{(1+\alpha{t}+\beta{t^2})}} \)

where, \( \eta_0 \) and \( \eta_t \) are the coefficient of viscosity at \( 0^\circ{C} \) and \( t^\circ{C} \) respectively. \( \alpha \) and \( \beta \) are the constants.

This is not very accordant with the experiment, So the modified relation is,

\( \displaystyle{\eta_t=\frac{A}{{(1+Bt)}^C}} \)

where, \( A \), \( B \) and \( C \) are constants.

It is applicable to pure liquids but is not applicable to oils or to any mixture of chemical compounds.

According to Andrae,

\( \eta{V^{\frac{1}{3}}}=Ae^{\frac{c}{VT}} \)

Where, \( V \) is the specific volume of the liquid, \( T \) is the absolute temperature, \( A \) and \( C \) are the constants. This formula holds good for oils but does not satisfy wholly for water, alcohol etc.

Variation of viscosity of liquids with temperature:

The viscosity of liquids increases rapidly with increase in pressure. This varies from liquid to liquid. In case of water at normal temperature, there is decrease in viscosity for the first few hundred atmospheres. For more viscous liquids the increase in viscosity with pressure is much more than in case of fairly mobile liquids.