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Relative density is the ratio of the density of a substance to the density of a reference substance, while absolute density is the mass per unit volume of a substance.
Relative density, also known as specific gravity, is a dimensionless quantity as it is the ratio of two densities and hence the units cancel out. It is usually compared to the density of a reference substance, typically water for liquids and solids, and air for gases. For example, if a substance has a relative density of 2, it means it is twice as dense as the reference substance. Relative density is useful in various scientific applications where it's necessary to compare the densities of different substances without referring to their actual densities.
On the other hand, absolute density, often just referred to as 'density', is the mass of a substance divided by its volume. It is usually measured in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³). Absolute density is an intrinsic property of a substance, meaning it does not depend on the amount of substance present. It can change with temperature and pressure, especially for gases. Absolute density is used in many scientific and engineering calculations, such as determining the buoyancy of an object or the pressure at a certain depth in a fluid.
In summary, while both relative and absolute density are measures of how much mass is contained in a certain volume, they are used in different contexts. Relative density is a comparison to a reference substance, while absolute density is a specific measurement of a substance's mass per unit volume.
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