Ncert Exercises & Solutions

With an increase in temperature, the viscosity of:

(a)	gases decrease.	(b)	liquids increase.
(c)	gases increase.	(d)	liquids decrease.

Which of the following options is/are correct?

1.	(b), (c)	2.	(a), (d)
3.	(c), (d)	4.	(a), (b)

Hint: The viscosity of a liquid depends on the intermolecular forces and the viscosity of the gases depends on the movement of the molecules.

Step 1: Find the variation of viscosity of the liquids with temperature.
For liquids, the coefficient of viscosity,

η_{liquid} \propto \frac{1}{\sqrt{T}}

$\eta_{\text {liquid }} \propto \frac{1}{\sqrt{T}}$

i.e. with an increase in temperature,

η_{liquid}

$\eta_{\text {liquid }}$ decreases.

Step 2: Find the variation of viscosity of the gases with temperature.
For gases, the coefficient of viscosity,

η_{gases} \propto \sqrt{T}

$\eta_{\text {gases }} \propto \sqrt{T}$

i.e. with an increase in temperature,

η_{gases}

$\eta_{\text {gases }}$ increases.
Hence, option (3) is the correct answer.