Matter exists in three states solid, liquid and gas. Any state of matter that can flow is a fluid. Liquids and gases are therefore, referred to as fluids.

__Density :__

Density of a substance is the mass per unit volume. Homogenous bodies have uniform density:

ρ = M/V

The unit of density is kg/m3 in S.I. system. Dimension of density is given by [ρ] = = [M] [L^{-3}].

__Relative Density or Specific Gravity :__

Relative density of a substance is the ratio of the density of the substance to the density of pure water (ρ_{w} = 1000 kg/m3) at 4°C.

ρ_{r} = ρ/ρ_{w}.

Relative density is dimensionless

**Illustration 1:** A hollow metallic sphere has inner and outer radii respectively as 5 cm and 10 cm. If the mass of the sphere is 2.5 kg, find (a) density of the material (b) relative density of the material of the sphere.

**Solution:** The volume of the material of the sphere is

V = 4/3 π(r_{2}^{3} – r_{1}^{3})

= (4/3)x 3.14 x 0.000875 m^{3}

= 0.00367 m^{3}

(a) Therefore, density of the material of the sphere is

ρ= M/V = 2.5/0.00367 kg/m^{3}

= 681.2 kg/m^{3}

(b) ρ_{r} = 681.2/1000 = 0.6812