Viscosity, quality of a fluid that tends to prevent it from flowing when subjected to a force. The strength with which a moving layer of fluid drags neighboring layers of fluid along with it defines its viscosity. If a fluid begins to flow under the influence of gravitation, the molecules from the stationary layers of the fluid must cross a boundary to enter the region of flow. Once past the boundary, these molecules receive energy from those in motion and start to move also. The viscosity of a fluid decreases with a decrease in density when the temperature increases. In a less dense fluid, fewer molecules are available per unit volume to transfer the motion from the moving layer to the stationary layer. This lack affects the speed of the different layers. Momentum is transferred less readily between the layers, and the viscosity falls.

Viscosity (HyperPhysics).

Physical chemistry definition [local] of Viscosity.

Teaching Activities

Microscale activity for high school chemistry 'Viscosity of Liquids'.

Activity for calculating the viscosity of lava flow [local] for high school students.