03 1.2 Density (ρ)
Density is a basic property of every material: it is the mass of a unit volume of the substance.
ρ = m / Vol
- SI unit: kg/m³ (also g/cm³; 1 g/cm³ = 1000 kg/m³).
- Dimensional formula: ML⁻³.
Why does density differ from one material to another?
- The atoms or molecules have different masses.
- The spaces between the atoms or molecules (inter-atomic or intermolecular spaces) differ. (In gases the spaces are huge, so gas densities are small.)
Denser materials sink; objects of lower density float on liquids of higher density.
Typical values: water 1000 kg/m³, sea water ≈ 1030, oil ≈ 800–900, mercury 13 600, aluminium 2700, iron ≈ 7800, gold 19 320, air at room temperature ≈ 1.2 kg/m³.