Which term describes the mass per unit volume?

• A. Mass
• B. Density
• C. Energy
• D. Volume

Answer: (B)

Density is the measure of how much mass is packed into a given volume. It is defined as density = mass divided by volume, so it tells you how tightly matter is arranged in space. Because of this, objects with the same size can feel heavier or lighter depending on how much mass they contain in that space. In everyday terms, density explains why some things float while others sink: a substance with lower density than the surrounding liquid will rise, while a denser substance will sink.

Common practical units are grams per cubic centimeter (g/cm^3) for solids and kilograms per cubic meter (kg/m^3) in larger-scale applications. This concept is distinct from mass (the total amount of matter) and volume (the space an object occupies), and from energy (the ability to do work or produce change). For example, a block with 500 g mass in a 100 cm^3 volume has a density of 5 g/cm^3. Temperature and pressure can change density because they alter how tightly the material is packed.