Which term describes the resistance to flow of a liquid?

• A. Viscosity
• B. Density
• C. Permeability
• D. Elasticity

Answer: (A)

The main concept here is viscosity—the measure of how much a liquid resists flowing. Viscosity reflects the internal friction between layers of the liquid as they slide past one another when a stress is applied. A liquid with high viscosity, like honey, flows slowly because its molecules grip each other more, while a liquid with low viscosity, like water, flows easily because its internal friction is lower. This resistance to flow is what viscosity quantifies, and it’s typically expressed in dynamic viscosity units (pascal-seconds).

Density, on the other hand, is the mass per unit volume and doesn’t describe how easily a liquid moves. Permeability relates to how easily a fluid can move through a porous material, not to the flow of the liquid itself in a bulk, non-porous context. Elasticity concerns how a material deforms and returns to its original shape, rather than how a liquid flows under stress.