Density (absolute)

It is defined by the ratio of the mass of a substance to the volume it occupies:

r = m / V

r is thus a quantity which has a dimension (mL^-3).

Density (relative)

Relative density of a substance is the ratio of its density, r, and of the density of another substance taken as a reference, r_R.

d = r / r_R

The relative density is thus a dimensionless quantity.

Examples: the relative density of a liquid compared to water and the relative density of a gas compared to air.

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