1.4 Specific Gravity

Specific gravity is defined as the ratio of the density of the given substance to that of the standard substance. It is denoted by the symbol SG. It should be noted that the standard substances are water for solids and liquids whereas dry air is taken as standard for gaseous fluids. Therefore Specific Gravity can be written as

For Solids or liquids:

SG_{solids or fluids} = (Density_{solid or fluid} / Density _water)

For Gaseous Substances:
 
SG_Fluid = (Density_Fluid / Density _air)
   
_{It can be seen that as Specific Gravity is the ratio of two densities there will be no units. 
  Note:}

1. _{Remember that the density of water is 1000 kg/m3, and that of air is 1.28 kg/m3.}
2. _{Thus the specific gravity of mercury becomes 13.6, as density of mercury is 13600 kg/m3.}
3. _{Specific Gravity is an important topic in the concept of buoyancy.}
4. _{Specific gravity is also called as relative density.}

 <sub>Video:

 
Example problem:
Find the specific gravity of gold whose density is measured to be 19,300kg/m3.(Take density of water to be 1000kg/m3).
Solution:
We know that specific gravity is the ratio of the density of the substance to that of the water for solid substances.
Therefore 
SGGold = 19300/1000
=>19.3
Therefore the specific gravity of gold is found to be 19.3(No units).</sub>

Ignore the program and continue to the next topic if you don't know programming in python. This is just for the sake of programmers.

Python Program

def specific_gravity_solid(density):
 """Specific Gravity for solid or liquid
 density = The density of the substance"""
 sg = density/1000
 return sg

def specific_gravity_gas(density):
 """Specific Gravity for gaseous substances
 density = The density of the gas"""
 sg = density/1.28
 return sg

#Find the density of carbon di-oxide

print specific_gravity_gas(1.98) #Prints 1.546875

Output:

1.546875

References:

http://www.britannica.com/EBchecked/topic/558700/specific-gravity
http://dictionary.reference.com/browse/specific+gravity
http://en.wikipedia.org/wiki/Specific_gravity