Final answer:
Specific gravity is a dimensionless quantity that compares the density of a substance to the density of water, which helps in easily determining behaviors like buoyancy without worrying about units.
Step-by-step explanation:
The concept being described is specific gravity, which is a comparison of the density of one material to another, typically using water as a reference. Density itself is a physical property that represents the mass per unit volume of a substance. The specific gravity is dimensionless and is defined as the ratio of the density of the material to the density of water at 4.0 °C and one atmosphere of pressure. This makes comparing densities easier as it eliminates the need for unit conversion, simplifying calculations related to buoyancy and other phenomena.
Water is typically used as a reference point because its density at 4 °C and one atmosphere is 1000 kg/m³, which also corresponds to 1 g/cm³.
An example is aluminum, which has a density of 2.7 g/cm³ or 2700 kg/m³, but a specific gravity of 2.7, making it nearly 2.7 times denser than water. Knowing the density enables us to predict behaviors like sinking or floating when an object is placed in a fluid.