**Density**

One important characteristic of a substance is density. Density is the ratio of mass to volume. Mathematically written:

ρ = m / V

ρ (rho) is a Greek letter commonly used to express density, m is mass and V is volume. The density of the homogeneous fluid is different from the density of a homogeneous solid. Iron, for example, have the same density in each part. In the Earth’s atmosphere, the higher the atmosphere from the surface of Earth, the smaller its density, while for seawater, the deeper the seawater, the greater its density. The density of a homogeneous fluid can depend on environmental factors such as temperature and pressure.

The International System Unit for density is kilograms per cubic meter (kg/m^{3}). For CGS units, the unit of mass density is expressed in grams per cubic centimeter (gr/cm^{3}). Following is the density data of several substances.

The density of the substances stated in the table above is the density of materials at 0 ^{o}C and pressure of 1 atm (atmosphere, atm = unit of pressure).

**Specific gravity**

Specific gravity is the ratio of the density of a substance to the density of water. The specific gravity of a material can be obtained by dividing the density by 10^{3} kg/m^{3} (10^{3} kg/m^{3 }= density of water). Specific gravity has no dimensions.

If the density of an object is smaller than the density of water, then the object will float. The specific gravity of the floating objects is lower than 1. On the contrary, if the density of an object is higher than the density of water, the specific gravity is higher than 1. In this case, the object will sink.