How is density defined in relation to substances?

Density is defined as the measure of how heavy a substance is for its size, which is mathematically expressed as mass per unit volume. In practical terms, density indicates how much matter is contained within a specific volume of a substance. For example, when comparing two substances of the same volume, the one with the greater mass has a higher density. This concept is crucial in various fields, including water treatment, where understanding the density of substances can impact processes such as sedimentation, flotation, and overall fluid dynamics.

The other choices describe different properties: the speed at which a liquid flows relates to viscosity, the temperature at which a substance melts refers to its melting point, and the concentration of nutrients in water pertains to water quality and chemistry but does not define density. Therefore, the definition of density as a measure of mass relative to volume is distinctly captured in the correct choice.