### How does Density differ from Relative Density?

##### April 11, 2017

# 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/meter^{3}. So if we say **density of ice** is 916 kg/meter^{3 }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/m^{3 }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/m ^{3}** and in CGS units it’s

**1 g/cm**which makes

^{3}**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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