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Relative density
Beginner
Density compared with water.
Relative density (specific gravity) is the ratio ρ/ρ_water; it has no units. Values below 1 float on water.
Worked example
Ice 0.92, mercury 13.6.
Common mistake
A number, not a density in kg/m³.
In more detail
Relative density is the ratio of a substance's density to the density of water. It is a dimensionless number, so a value of 2.5 means 2.5 times as dense as water.
Real-world uses
- Mineralogists identify minerals by their relative density.
- Batteries are checked by the relative density of their electrolyte.
- Miners and chemists use relative density to compare liquids quickly, such as checking the strength of a battery acid against water.
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Frequently asked questions
Is relative density the same as specific gravity?
Yes, the terms are used interchangeably for liquids and solids.
What is the relative density of gold?
About 19.3.
Does relative density have units?
No. It is a ratio of two densities, so the units cancel and it is a pure number.
Related topics in Fluids & Elasticity
States of matter · Density · Pressure · Units of pressure · Pressure in a fluid