What is specific gravity?
The specific gravity of a fluid, which is represented by the letter S , is defined as the ratio between the mass of a given volume of that fluid and the mass of an equal volume of water at 4 ° C. This is an intensive property of matter which means that it depends only on its composition and not on the amount of matter present.
The formula that defines specific gravity is:
However, since we know that the density of any substance is given by the relationship between its mass and its volume, from which it is obtained that the mass is equal to the density times the volume ( m = ρ.V ), this equation can be rewrite in terms of the densities of both fluids as follows:
This means that specific gravity actually measures the relative density of a fluid as a function of the density of water at 4 ° C. The reason why water at 4 ° C is used as a standard is that, at this temperature, the water acquires its maximum density.
How is specific gravity measured?
Specific gravity is measured with the use of a hydrometer. This consists of a glass tube that contains a lead or mercury weight at the bottom and a graduated scale at the top straight.
The hydrometer works based on Archimedes’ principle, according to which a body floating on a liquid displaces a volume of liquid equivalent to its own weight. When introducing the tube into a liquid, the float level of the hydrometer determines its specific gravity.
If the hydrometer floats higher, then the liquid will have a specific gravity greater than 1, while, if it is submerged deeper, it means that the fluid is less dense than water, so the hydrometer will mark a value less than 1.
For this reason, different hydrometers are usually designed for cases in which you want to measure the specific gravity of fluids denser than water and those in which you want to measure that of less dense fluids.
What is the purpose of measuring specific gravity?
Specific gravity is an important measure of the density of a fluid for a very simple reason: it is a measure of dimensionless density, that is, it has no units. This is very useful, since it allows you to compare the densities of different fluids regardless of the system of units in which the measurements are carried out.
For example, a very common application of specific gravity is that it serves as an indirect measure of urine concentration. Therefore, doctors use it to get an idea of how well our kidneys are working .
It is also used very frequently to determine the concentration of battery acid, so that you know if it is necessary to add more water or more concentrated acid.
Specific gravity of some common fluids
Fluid |
Temperature (° C) |
Specific gravity (S) |
Methane |
-164 |
0.466 |
Liquid propane |
-40 |
0.585 |
Hexane |
25 |
0.657 |
Hexene |
25 |
0.673 |
Ethylamine |
16 |
0.683 |
Octane |
25 |
0.701 |
Petroleum ether |
25 |
0.716 |
Vehicular gasoline |
60 ° F |
0.739 |
Pentane |
25 |
0.755 |
Ethyl alcohol |
25 |
0.787 |
Methanol |
25 |
0.791 |
Kerosene |
60 ° F |
0.820 |
Toluene |
25 |
0.865 |
Olive oil |
fifteen |
0.915 |
Peanut oil |
15.5 |
0.920 |
Palm oil |
15.5 |
0.923 |
Coconut oil |
fifteen |
0.925 |
Sunflower oil |
15.5 |
0.925 |
Soy oil |
15.5 |
0.926 |
Examples of the use of specific gravity
For the density calculation
Specific gravity allows the quick calculation of the density of a liquid or other substance from the value of the density of water at 4 ° C. This density is presented in the following table in different units:
Unit system |
Density units |
Water density at 4 ° C |
International System (SI) |
kg / m3 |
1000 kg / m3 |
Cgs system |
g / cm3 |
1.00 g / cm3 |
Other system |
g / mL |
1.00 g / mL |
Fps system |
lb / ft3 |
62.4 lb / ft3 |
Example
Suppose we want to determine the density of glycerin at 25 ° C in units of pounds per cubic foot. Then, from the specific gravity formula we solve for the density of the fluid:
From the first table we obtain the value of the specific gravity of glycerin, which is 1.263, while from the second table we obtain the density of water at 4 ° C in units of pounds per cubic foot, which is 62.4 lb / ft ^{3} and we substitute in the formula:
Thus, in a very simple way we obtain the density in the desired units.
Calculation of mixture densities
Suppose we have a solution with a specific gravity of 1.26 and we want to dilute it 2L of this solution by adding 1 L of pure water. Specific gravity can be used to calculate, at least roughly, the specific gravity and then the density of the new solution.
The process is very simple. The mixture will have a total volume of 3L, so the original solution will make up 2/3 of the final solution and the added pure water will make up the other missing 1/3. Each component of the mixture contributes to its specific gravity based on these proportions, so:
Then, from this value, the density can be calculated as in the previous example. In this case, it will be calculated in units of the SI system: