Difference between Density and Relative Density

Density

Density is the mass per unit volume.

So if we divide the mass of a substance with its volume then we get the density of that substance.

Here we get another definition of mass.
Mass=volume X density.

Unit of density in SI is Kg/meter3.

So if we say density of ice is 916 kg/meter3 in SI means if we take a slab of ice of unit volume (length=1 meter, breadth = 1 meter and height = 1 meter), and measure its mass then its mass will be 916 kg.

Similarly density of water is 1000 kg/m3 means water of 1 cubic meter volume have a mass of 1000 kg. Now we will discuss about Relative density or specific gravity.

Relative Density or specific gravity

Relative density or specific gravity of a substance is the ratio of its own density and the density of water.

It’s a comparative parameter calculated with respect to water’s density.
It’s a ratio of two densities and a number only without any unit.

Relative density of water is certainly 1.
[Relative density of water = density of water/density of water = 1]

And relative density of ice = density of ice / density of water = 916/1000 = 0.916

A substance with relative density more than 1 will sink in water and a substance with that value less than 1 will float in water.
So from above example, we can say an ice cube will float in water.

So hopefully, the above discussion clarifies the difference between relative density and density.

Why density of water used as reference?

We calculate relative density with respect to water. Why?

Generally density of water at 4 °C (the temperature at which density of water reaches its maximum value) is used as the reference to measure relative density.

In SI units, the density of water is approximately 1000 kg/m3 and in CGS units it’s 1 g/cm3 which makes relative density calculations pretty convenient.

The density of the material only needs to be divided by 1000 or 1, depending on the units.

Relative density: Examples

Water: 1
Iron: 7.87
Table Salt: 2.17
Mercury:13.56

Suggested Study: Mole vs Molecule

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