massetætheder

Massetætheder (Danish plural of massetæthed) refers to the mass per unit volume of a substance, commonly called mass density in English. It quantifies how much mass is contained in a given volume and is denoted by the Greek letter ρ, with the relation ρ = m/V, where m is mass and V is volume. The SI unit is kilogram per cubic meter (kg/m³); common reference values include water at about 1000 kg/m³, air near 1.225 kg/m³ at sea level, steel around 7850 kg/m³, and aluminium about 2700 kg/m³.

Massetætheder vary with temperature and pressure: they typically decrease with increasing temperature for solids and liquids and change markedly for gases. The concept is distinct from but related to specific gravity (the ratio of a substance’s density to that of a reference, usually water) and to number density (particles per unit volume).

Measurement methods include direct determination of mass and displaced volume, Archimedes’ principle, pycnometry, hydrometry, and modern techniques such as computed tomography for complex shapes. Massetætheder play a central role in fluid mechanics, material selection, buoyancy calculations, geology, and quality control. Accurate density data aid in identifying materials, designing structures, and predicting behavior under varying environmental conditions.