How is density defined in physical terms?

Density is defined as mass per unit volume, which refers to how much mass is contained within a specific volume of a substance. This property is significant in various scientific fields, including chemistry and physics, as it helps determine how substances behave in different situations, such as whether they will float or sink in a liquid.

Understanding density is crucial for applications in water quality because it influences the distribution of pollutants and organisms in bodies of water. For instance, denser substances tend to settle at the bottom, while less dense materials may remain suspended or float. This principle plays a vital role in environmental assessments and the management of aquatic ecosystems.

The other options do not accurately represent the concept of density: mass per unit area pertains to surface density rather than volume, weight per unit volume mixes concepts of gravitational force with volume, and weight per unit area also doesn't reflect the volumetric nature needed to describe density accurately. This distinction makes mass per unit volume the correct and universally accepted definition of density.