| The density of a sample is its mass divided by its volume: | If you fill 500cm3 of a sample into a glass beaker and the mass weighs 1000g, then the density would be: |
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| As the density is very much dependent on temperature, this temperature must be recorded and controlled very precisley. For instance, water shows the following temperature dependency: |
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Usually the density is measured at a reference temperature of 20°C. The density of water at 20° is:
Water d20 = 0.99821 g/cm3
Density units
As describe above, the density is usually given in g/cm3. Sometimes the density is also given in kg/m3. Thus the conversion formula is:
1kg/m3 = 1000 x 1g/cm3
For instance:
Water d20 = 0.99821 g/cm3
= 998.21 kg/m3
A second unit used is the relative density (or specific gravity: S.G), where the measured density is divided by a reference value, the density of water at the same temperature.
Sometimes a third unit is also used, the same as above but with the reference density of water at 4°C.
(*) Note: density of water at 4°C is 1.00000 g/cm3.
Examples
Every sample has a well defined density (at a specified temperature). For example, at 20°C the following samples have following densities:
d20 water = 0.9982 g/cm3
d20 ethanol = 0.7893 g/cm3
(*) Note: d20/4 is equal to d20, only without unit.
Density of solutions:
The above described values of density are only valid for pure samples. What about dissolved solutions? Let us take a solution of Ethanol/Water as an example.
Concentration of Ethanol in Water
As we have seen before, pure Water has a density of d20 = 0.9982 g/cm3 and pure Ethanol d20 = 0.7893 g/cm3. A solution of Ethanol/Water will have a density value which depends on the concentration.
Density at 20°C
(g/cm3)
0.9982
0.00
0.9963
1.00
0.9945
2.00
0.9927
3.00
0.9910
4.00
0.9893
5.00
0.9878
6.00
0.9862
7.00
0.9847
8.00
0.9833
9.00
0.9819
10.00
0.9687
20.00
0.9539
30.00
0.9352
40.00
0.9139
50.00
0.8911
60.00
0.8676
70.00
0.8436
80.00
0.8180
90.00
0.7893
100.00
A density measurement of this solution will enable us to determine the concentration quickly and precisely.
Example:
d20 = 0.9687 g/cm3 => Ethanol conc. = 20%