Density

Density is a measure of mass per unit volume of a material or object. It is an intrinsic property that relates the amount of matter in a given space. In scientific use, density is typically denoted by the symbol ρ (rho) and expressed in SI units of kilograms per cubic meter (kg/m^3). For solids and liquids, density is roughly constant over moderate ranges of pressure, but it changes with temperature as materials expand when heated, which lowers density. For gases, density depends strongly on temperature and pressure and is described by the ideal gas law in its common form ρ = PM/RT, where P is pressure, T is temperature, M is molar mass, and R is the gas constant.

Common reference values include the density of water, about 1,000 kg/m^3 at 4°C, which is often used as a benchmark. Specific gravity compares density to a reference density, typically that of water at 4°C, and is a dimensionless quantity.

Density can be measured directly as mass divided by volume or inferred from displacement for irregular objects. It is a fundamental parameter in materials science, chemistry, physics, and engineering, influencing porosity, phase changes, and material performance. In fluids, density differences drive buoyancy and convection; Archimedes’ principle states that a submerged object experiences an upward buoyant force equal to the weight of the displaced fluid.

Beyond mass density, related concepts include number density, charge density, and energy density, which describe how much of a given quantity is contained per unit volume in various contexts.