What does density measure in a fluid?

• A. The weight per unit volume of the fluid.
• B. The mass per unit volume of the fluid.
• C. The amount of fluid present per unit area.
• D. The ratio of the fluid's density to the density of air.

Answer: (B)

Density measures how much matter is contained in a given volume of a fluid, i.e., mass per unit volume. This tells you how heavy a volume of fluid is for its size: denser fluids have more mass in the same volume. Common units are kilograms per cubic meter (kg/m^3) or grams per cubic centimeter (g/cm^3). It’s important to distinguish density from weight per unit volume; the latter, weight density, is mass density times gravity and is a different quantity. The idea of “amount per unit area” describes a surface quantity rather than density, and a ratio to air describes relative density (how heavy the fluid is compared with air), not the density of the fluid itself.