Question

Which of the following statements correctly gives the variation in the viscosities of a liquid and a gas with change in temperature?

• A. Viscosity increases with increase in temperature for a liquid and decreases with increase in temperature for a gas
• B. Viscosity increases with increase in temperature for a liquid and increases with increase in temperature for a gas
• C. Viscosity decreases with increase in temperature for a liquid and decreases with increase in temperature for a gas
• D. Viscosity decreases with increase in temperature for a liquid and increases with increase in temperature for a gas.

Answer 1

The correct option is D Viscosity decreases with increase in temperature for a liquid and increases with increase in temperature for a gas.

Viscosity of a liquid $\left({\eta }_L\right)$ is due to the cohesion between its molecules. With increase in temperature of a liquid, cohesive forces decrease, because the separation between liquid molecules increases. This leads to the decrease in viscosity of a liquid.

In case of gases, the force of cohesion between molecules is very low, due to the high average speed of gas molecules. Viscosity of a gas $\left({\eta }_g\right)$ is due to the momentum transfer between its molecules. With the increase in the temperature of a gas, molecular motion increases, leading to the rise in viscosity of the gas.