The "relative density" of a substance refers to what?

The correct answer relates to the concept of relative density, which is defined as the ratio of the density of a substance to the density of a reference substance, typically water. This measure allows for the comparison of different materials in terms of their mass-to-volume ratio. Since water has a density of approximately 1 g/cm³ at standard conditions, substances with a relative density greater than 1 will sink in water, while those with a relative density less than 1 will float.

This comparative approach using water is particularly useful in various fields such as engineering, chemistry, and environmental science, where understanding buoyancy and material properties is essential. By focusing on water, which is abundant and universally recognized, relative density provides a straightforward measure that is easy to understand and apply.