Final answer:
Relative density is compared to the density of water to determine specific gravity, which predicts whether a substance will float or sink in water, informing about its buoyancy.
Step-by-step explanation:
The relative density of a substance is determined by comparing its density to the density of water.
This comparison is known as the specific gravity of the substance, which is a dimensionless quantity expressing the ratio of the substance's density to that of water at 4.0 °C.
Water at this temperature and one atmosphere of pressure has a density of 1000 kg/m³, or equivalently, 1 g/cm³.
The specific gravity is calculated by dividing the density of the substance by the density of water, providing a simple way to compare densities without worrying about units.
For example, if a substance has a specific gravity less than 1, then it is less dense than water and will float, such as an ice cube with a relative density of about 0.91.
Conversely, a substance with a specific gravity greater than 1 is denser than water and will sink.
Essentially, the relative density can predict whether a substance will float or sink in water, which is particularly useful in studying buoyancy.