What is the reciprocal of viscosity?

• A. Viscosity index
• B. Density
• C. Fluidity
• D. Permeability

Answer: (C)

Fluidity is the reciprocal of viscosity because viscosity measures how much a fluid resists flow, so taking the inverse shows how easily the fluid can flow. In other words, fluidity equals 1 divided by viscosity. A fluid with low viscosity flows easily and has high fluidity; as viscosity increases, fluidity decreases.

Other properties listed don’t describe this inverse relationship: viscosity index tells how viscosity changes with temperature, density is mass per unit volume, and permeability describes how easily a fluid moves through a porous solid.