Density is the relationship between the mass of a certain portion of the substance and the volume of that portion. It is expressed by the Greek letter d (delta lowercase). But the volume of a substance (gaseous, liquid or solid) depending on its temperature, so it is necessary to know this information at the time of declaring its density. For example, the metallic element osmium has a density of 22.5 g/cm3 at 0 ° C. Substances whose density is less than a float in the water (gasoline, oil). Specific gravity is the ratio between the weight of a substance and the volume of that portion. He expressed the Greek letter r (ro) or by the acronym Pe, and is the ratio between the weight and volume, weight being the product of mass and gravity.
The unit most often used to express the specific gravity of a body is gram-force to cm3.
Among the high density substances are the platinum, 21.50 g/cm3, the gold, 19 g/cm3; mercury, 13.60 g/cm3, the lead of 11.50 g/cm3, and copper of 8.90 g/cm3.
Among the compounds that have low densities are naphtha, 0.70 g/cm3, the alcohol and oil of 0.80 g/cm3, and oil of 0.92 g/cm3.