# relativ density

The relative density  d , also specific density , describes the quotient of two densities as the quantity of the dimension number .

## definition

${\ displaystyle d = {\ frac {\ rho} {\ rho _ {0}}}}$

With

• the density ρ of a substance under consideration (defined in DIN 1306 as mass per volume ).

The symbol  ρ 0 stands for the reference variable to which the density under consideration is related ( see following chapter ):

• either the density of a normal state of the substance under consideration
• or (more often) the reference density of an upholstery fabric .

## Use and reference values

### With solids

In mineralogy and engineering , density data are mostly based on the density of pure water in the normal state at 3.98 ° C. Use is useful when a material - e.g. B. as a result of different temperatures - exists in different structures, e.g. B. with larger or smaller porosity .

### With liquids

The relative density is the ratio of the mass of a certain volume of a liquid at temperature T1 to the mass of the same volume of water at temperature T2: ${\ displaystyle d_ {T2} ^ {T1}}$

• A common relative density is , this describes the density of a liquid at 20 ° C in (z. B. 2 g / cm³) in relation to the density of water at 20 ° C (approx. 1 g / cm³). The relative density would then be 2, so the liquid, at 20 ° C, is twice as dense as water at 20 ° C.${\ displaystyle d_ {20} ^ {20}}$
• Another example is , this describes the density of a liquid at 20 ° C in relation to the density of water at 4 ° C, more precisely at 3.98 ° C.${\ displaystyle d_ {4} ^ {20}}$

### With gases

In the case of gases , the relative density is usually used to represent the “gravity ratio” to dry air . See DIN 1871 (May 1999), in which the relative density of a gas is defined as the quotient of the density of a gas and the density of dry air at the same pressure and temperature :

• a gas with a relative density <1.0 is lighter than air, so it rises upwards: e.g. B. natural gas with .${\ displaystyle d = 0 {,} 55 \ dots 0 {,} 75}$
• a gas with a relative density> 1.0 is heavier than air and thus falls downwards: e.g. B. LPG or propane with .${\ displaystyle d \ approx 1 {,} 55}$

Other common standard reference densities ( normals ) for gases are the densities of dry air:

• under normal conditions ( air pressure 1013.25 mbar and 0 ° C): or${\ displaystyle \ rho _ {0} = 1 {,} 2931 \, \ mathrm {\ frac {kg} {m ^ {3}}}}$
• under standard conditions (1013.25 mbar and 20 ° C or 25 ° C).

## literature

• DIN 1306 density; Terms, information
• DIN 1871 Gaseous fuels and other gases - density and other volumetric quantities
• Bergmann, Schaefer: Textbook of Experimental Physics , Volume 1, 11th Edition

## Individual evidence

1. European Pharmacopoeia , Deutscher Apotheker Verlag Stuttgart, 6th edition, 2008, pp. 33–34, ISBN 978-3-7692-3962-1 .
2. The International pharmacopoeia, Volume 1 in the Google book search.