Editing Relative density (section)

{{Short description|Ratio of two densities}}
{{redirect-distinguish-for|Specific gravity|specific weight|the album by Joe McPhee and Joe Giardullo|Specific Gravity (album)}}
{{Infobox physical quantity
| name = Specific gravity
| unit = Unitless
| symbols = ''SG''
| derivations = <math> SG_\mathrm{true} = \frac {\rho_\mathrm{sample}}{\rho_\mathrm{H_2O}}</math>
}}

[[File:US Navy 111005-N-ZN781-031 Aviation Boatswain's Mate (Fuel) 3rd Class Rolando Calilung tests for a specific gravity test on JP-5 fuel.jpg|right|thumb|A [[United States Navy]] [[Aviation boatswain's mate]] tests the specific gravity of [[JP-5]] fuel]]

'''Relative density''', also called '''specific gravity''',<ref name=specificgravity1>{{cite book| last = Dana| first = Edward Salisbury
|title = A text-book of mineralogy: with an extended treatise on crystallography...
|publisher = John Wiley and Sons| date = 1922| location = New York, London(Chapman Hall)
|pages = 195–200, 316
|url = https://books.google.com/books?id=rCwaAAAAYAAJ&pg=PA156}}</ref><ref name=specificgravity2>{{cite book
|last = Schetz| first = Joseph A.|author2=Allen E. Fuhs
 |title = Fundamentals of fluid mechanics| publisher =Wiley, John & Sons, Incorporated
|date =1999-02-05|pages = 111, 142, 144, 147, 109, 155, 157, 160, 175
|url =https://books.google.com/books?id=YCSSolzuu9IC&pg=PP1|isbn =0-471-34856-2}}</ref> is a [[dimensionless quantity]] defined as the [[ratio]] of the [[density]] (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nearly always measured with respect to [[water]] at its densest (at {{cvt|4|°C|°F|1|disp=or}}); for gases, the reference is air at [[room temperature]] ({{cvt|20|°C|°F|disp=or}}). The term "relative density" (abbreviated '''r.d.''' or '''RD''') is preferred in [[SI]], whereas the term "specific gravity" is gradually being abandoned.<ref>{{cite book |author1=United States Bureau of Reclamation |title=Metric Manual |date=1978 |publisher=U.S. Department of the Interior, Bureau of Reclamation |url=https://books.google.com/books?id=E6BLn01jRz8C&pg=PA37|page=37 |language=en}}</ref>

If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass.  If the reference material is water, then a substance with a relative density (or specific gravity) less than 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density greater than 1 will sink.

Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 [[Atmosphere (unit)|atm]] (101.325 [[Pascal (unit)|kPa]]). Where it is not, it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice, the specific gravity, as specified above, is multiplied by 1000.<ref name=briggs /> Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as [[brine]]s, [[must weight]] ([[syrup]]s, juices, honeys, brewers [[wort]], [[must]], etc.) and acids.