Question

Which of the following correctly states how the viscosities of a liquid and a gas will change with temperature?

(a) Viscosity increases with the increase in temperature of a liquid and decreases with the increase in temperature of a gas

(b) Viscosity increases with the increase in temperature of a liquid and increases with the increase in temperature of a gas

(c) Viscosity decreases with the increase in temperature of a liquid and decreases with the increase in temperature of a gas

(d) Viscosity decreases with the increase in temperature of a liquid and increases with the increase in temperature of a gas

I got this question during an interview.

This intriguing question comes from Viscosity in section Properties of Fluids of Fluid Mechanics

Answer 1

Correct choice is (a) Viscosity increases with the increase in temperature of a liquid and decreases with the increase in temperature of a gas

Explanation: Viscosity of a liquid is due to the cohesion between it’s molecules. With the increase in temperature of a liquid, cohesion increases, leading to the rise in viscosity. Viscosity of a gas is due to the momentum transfer between it’s molecules. With the increase in the temperature of a liquid, molecular motion increases, leading to the fall in viscosity.