In textbooks and charts you will often see specific gravity included in data tables for different substances. This might lead you to believe that it is a unique property of a substance, but in reality the specific gravity is a relative density metric.
What is Specific Gravity?
Every substance has a density at a given set of temperature and pressure conditions. Density is a measure of a substance’s mass divided by the volume that it takes up; this is shown by the units of measurement, for example, grams per millilitre. Note that the bulk density of a substance is the mass per total volume, which is usually the same thing. However, certain substances can have varying bulk densities based on their containers or surroundings, but we’ll ignore those differences for now.
Now how does density tie into specific gravity? The ratio of the density of the substance to the density of a reference substance is how specific gravity is calculated. The reference substance has to be at the same set of conditions and water is usually selected, although other substances can be used as well.
Why is it Useful?
There are two main reasons that knowing the specific gravity of a substance can be useful. First of all it allows you to easily calculate the density of a substance as long as you know the density of the reference substance.
Secondly, it is very commonly used in buoyancy situations. If the specific gravity is above one, it will sink in the reference substance, while if it is less than one it will float. When there are spills in water this allows cleanup crews to determine what will float and what will sink to the bottom by a simple visual inspection of the specific gravity of the different substances involved.
A closely related term to density is specific weight. One such use of the specific gravity is to first calculate the density of the substance and then use that to determine the specific weight. Instead of mass per volume, specific weight is the weight per volume of that substance. To calculate it you simply have to multiply the density of the substance by the force of gravity.