Fluids change volume under external pressure primarily due to which material property? (Assume ordinary liquids and gases in classical fluid mechanics; choose the most appropriate fundamental property.)

Introduction / Context:
In fluid mechanics and engineering, we often need to know whether a fluid's volume will change when external pressure is applied. This determines how we model flow (compressible vs incompressible), affects wave speed, cavitation, and the behavior of hydraulic systems.

Given Data / Assumptions:

• Ordinary fluids (liquids or gases) under external pressure.
• No phase change, chemical reaction, or temperature-driven expansion is considered.
• Small to moderate pressure variations relevant to engineering analysis.

Concept / Approach:

The property that quantifies volume change with pressure is compressibility (or its inverse, bulk modulus). Viscosity quantifies internal friction and shear stress–strain rate relation; plasticity is a solid-material concept describing permanent deformation after yield; elasticity describes stress–strain but for fluids in bulk it collapses to compressibility for volumetric changes.

Step-by-Step Solution:

Verification / Alternative check:

Speed of sound c in a fluid is related to compressibility through c^2 = 1/(ρ * k_eff). If volume could not change, c would be infinite, which is not physical except as an approximation for water at low speeds.

Why Other Options Are Wrong:

Plasticity: pertains to solids and permanent shape change, not bulk volume change in fluids. Viscosity: measures resistance to shear, not compressive volumetric change. “None of these” and “elasticity only (no volume change)” are incorrect because fluids do exhibit finite compressibility.

Common Pitfalls:

Assuming “incompressible” means zero compressibility; it is an approximation valid when density changes are negligible for the problem scale.

Final Answer:

Compressibility