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

Answer 1

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

With the increase in temperature intermolecular spacing for the liquid molecules increases leading to decrease in cohesive force. With decrease in cohesive force, viscosity decreases. Hence for liquid with increase in temperature, viscosity decreases.
In case of gases, dominating forces are molecular momentum transfer. With increase in temperature, number of collisions increase and hence the resistance to the flow increases. Hence with increase in temperature, viscosity of gases increases.