Densities

Density (density symbol ρ) is a physical property defined as mass per unit volume. It is an intrinsic property of a material but depends on temperature and pressure. The SI unit is kilogram per cubic meter (kg/m^3); common non-SI units include grams per cubic centimeter (g/cm^3) and kilograms per liter (kg/L).

For gases, density is determined by the ambient pressure and temperature. From the ideal gas law, ρ = PM/RT, where M is the molar mass and R is the gas constant. Consequently, increasing pressure or lowering temperature increases density; real gases deviate from the ideal model at high pressure or low temperature.

In liquids and solids, density varies less with pressure, but temperature changes can still be significant. Water has a density of about 1,000 kg/m^3 at 4°C; ice is about 0.916 g/cm^3 at 0°C. Common metals range from roughly 7 to 20 g/cm^3 (aluminum about 2.7 g/cm^3; iron about 7.87 g/cm^3; gold about 19.3 g/cm^3). Air at sea level and 15°C has a density of about 1.225 kg/m^3.

Density can be measured by methods such as Archimedes’ principle (submerging an object to compare buoyant force), using a pycnometer, hydrometer, oscillating U-tube densitometer, or X-ray density measurements.

Related concepts include specific gravity, the ratio of a material’s density to that of water, and apparent versus true density in porous media, where pore space lowers bulk density. Temperature and pressure charts allow correction to standard conditions.