Relationship between fluid viscosity and temperature

The distance between molecules in a gas is larger compared to that in a liquid; therefore, the viscosity of a gas is primarily determined by the movement of its molecules. When adjacent layers of gas move relative to each other, gas molecules continuously diffuse from one layer to another during their random thermal motion. Internal friction arises due to the transfer of momentum, which in turn hinders their relative movement. Therefore, the viscosity of gases increases as temperature rises, while the viscosity of liquids also increases with rising temperature; however, the viscosity of liquids decreases significantly as temperature increases. This is because the viscosity of liquids is caused, on the one hand, by the attractive forces between molecules, and on the other hand, by the change in momentum resulting from the irregular thermal motion of those molecules.

Correction: In gases, the distance between molecules is greater than in liquids; therefore, gas viscosity is primarily determined by molecular motion. When adjacent layers of gas move relative to each other, gas molecules continuously diffuse from one layer to another during their random thermal motion. Internal friction arises due to the transfer of momentum, which in turn hinders their relative movement. Therefore, the viscosity of gases increases as temperature rises, whereas the viscosity of liquids decreases significantly with rising temperature. This is because the viscosity of liquids is caused, on the one hand, by the attractive forces between molecules, and on the other hand, by the change in momentum resulting from the irregular thermal motion of those molecules.

This is because the reasons for the viscosity of liquids and gases are fundamentally different; in liquids, intermolecular forces play a major role, while in gases, molecular thermal motion is the key factor