Which statement correctly describes air density and density of air?

• A. They are identical scientific terms.
• B. Density of air is the mass per volume and air density is the weight per volume.
• C. Density of air is the weight per volume and air density is the mass per volume.
• D. Both terms refer to air pressure.

Answer: (C)

Understanding the difference between mass per volume and weight per volume helps here. Air density is the mass of air in a given volume, typically expressed as kilograms per cubic meter (rho). Density of air, in contexts that distinguish terms, is the weight of that air per unit volume, i.e., weight density (gamma), which equals mass density times gravity (gamma = rho × g). So the statement that density of air is the weight per volume and air density is the mass per volume reflects that distinction. For example, at sea level, rho is about 1.225 kg/m^3, and the corresponding weight density is about 12 N/m^3 (1.225 × 9.81). The other options mix up these definitions or confuse density with pressure, which is why they aren’t correct.