Mass Density
Copper Cube in Water and Mercury


The mass density (ρ) of a fluid is defined as the ratio of the mass of the fluid (m) to its volume (V). That is,
The SI unit for the mass density is kg/m^{3} while the English
unit is slugs/ft^{3}.
For liquids, mass density is not a strong function of temperature
and pressure. Hence, density is generally assumed to be independent of
the temperature and pressure for liquids. On the other hand, for gases,
density varies with both temperature
and
pressure.
The relation between ρ, T and P for gases
is given by the
ideal gas law, which will be presented in a later section.
The density of different liquids has a wide range. For example, the
density of water at 16^{o}C is 999 kg/m^{3} while the
density of mercury at 20^{o}C is 13,550 kg/m^{3}. In
order for an object to float in a liquid, the density of the object must
be less than
that
of the liquid.
For example,
a copper cube (ρ = 9,000 kg/m^{3})
will sink in water but it will float on mercury.

20°C (68°F)
1 atm 
Density, ρ 
(kg/m^{3}) 
(slug/ft^{3}) 
Water, pure 
998 
1.936 
Water, sea 
1,025 
1.989 
Ammonia 
608 
1.180 
Benzene 
881 
1.709 
Carbon
Tetrachloride 
1,590 
3.085 
Ethanol 
789 
1.531 
Freon 12, liquid 
1,327 
2.575 
Gasoline 
680 
1.319 
Glycerin 
1,260 
2.445 
Kerosene 
804 
1.560 
Mercury 
13,550 
26.29 
Methanol 
791 
1.535 
SAE 10W Oil 
870 
1.688 
SAE 30W Oil 
891 
1.729 
SAE 50W Oil 
902 
1.750 


For gases, a similar phenomenon can be observed. Consider the balloons,
as shown in the figure. The density of helium inside the
balloons
is less than the surrounding
air for a given temperature and pressure, hence the balloons rise in
air. For more information on this subject, readers are referred to
the discussion of buoyancy. 

The specific weight, γ, of a fluid is defined
as
where g is the gravitational acceleration. Basically, the specific weight
represents weight per unit volume. Sometimes it is also referred
to as the weight density. The SI unit for the specific weight is N/m^{3} while
the English unit is lb/ft^{3}. 