Basic property definition – mass per unit volume at standard conditions The mass per unit volume of a fluid evaluated at a standard temperature and pressure is called:

Introduction / Context:
Fluid properties are often tabulated at standard temperature and pressure (STP). Knowing basic definitions avoids confusion when converting between related quantities used in hydraulics and thermodynamics.

Given Data / Assumptions:

• Mass density ρ is mass per unit volume.
• Specific weight γ is weight per unit volume.
• Specific gravity SG is a ratio relative to a reference density.

Concept / Approach:
Mass density is the fundamental property ρ = m / V. Specific weight uses the gravitational conversion γ = ρ * g. Specific gravity is dimensionless: SG = ρ / ρ_ref (for liquids, commonly water at 4 °C). The question explicitly asks for “mass per unit volume,” which is mass density.

Step-by-Step Solution:
1) Identify the requested definition: mass per unit volume → ρ.2) Distinguish from γ: weight per unit volume = ρ * g.3) Distinguish from SG: ratio of ρ to a reference ρ_ref (dimensionless).4) Choose “mass density.”

Verification / Alternative check:
Dimensional consistency: ρ units are kg/m^3 (S.I.); γ units are N/m^3; SG has no units. This confirms that only “mass density” fits the statement exactly.

Why Other Options Are Wrong:

• Specific weight / weight density: incorporate g and are not mass per volume.
• Specific gravity: a ratio, not an absolute property with units.
• None of these: incorrect because ρ is the standard term.

Common Pitfalls:
Using weight density data when a calculation requires mass density, leading to unit errors by a factor of g.

Final Answer:
mass density