Density and Specific Gravity
Density (d) is a measure of mass per unit volume of a substance, it can be calculated using the formula:
$$Density = {Mass \over Volume} $$
Thus, if 10 ml of water weighs 10 grams, density of water is:
$$Density = {10 \ grams \over 10 \ ml} = 1 g/ml $$
Specific gravity is a dimensionless unit, defined as a ratio of density of a substance to the density of water.
$$Specific \ gravity = {Density \ of \ substance \over Density \ of \ water} $$
Density of water is one, if the weight of a particular volume of substance is known, its specific gravity can be calculated by dividing its weight by its volume.
$$Specific \ gravity = {{Weight \ of \ substance(g) \over volume \ of \ substance(ml)} \over {1 g/ml(density \ of \ water)}}$$
The above equation is similar to the equation for density, except that specific gravity has no units. If the specific gravity of a solution is known, interconversions between the volume of a substance and its weight are possible using the following equations:
$$Weight \ of \ substance (g) = {Volume \ of \ substance (ml) \times Specific \ gravity} $$
$$Volume \ of \ substance (ml) = {Weight \ of\ substance (g) \over Specific \ gravity} $$
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Solved Problem: 20 ml of a liquid weighs 35 g. What is the density of this liquid? $$Density = {35 \ grams \over 20 \ ml} = 1.75g/ml $$ Answer: 1.75 g/ml |
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Solved Problem: 20 ml of a liquid weighs 35 g. What is the specific gravity of this liquid? $$Specific \ gravity ={ {35 \ g \over 20 \ ml} \over 1g/ml(density \ of \ water) } $$ Answer: 1.75 |
Thus, in practice the main difference between specific gravity and density is the lack of units in specific gravity.