tætheder

Tætheder is the plural form of tæthed, a Danish term that denotes the degree of compactness of matter or other quantities. In scientific and everyday language, tætheder often refer to how much of something is packed into a given volume, but the concept can also describe concentrations per unit area or per unit volume in various contexts.

In physics and chemistry, mass density is the most common sense of tætheder. Mass density, denoted ρ, equals mass divided by volume (ρ = m/V) and is measured in units such as kilogram per cubic meter (kg/m^3) or gram per cubic centimeter (g/cm^3). The density of a substance depends on temperature and pressure; for gases, the ideal gas law gives ρ = PM/RT, where P is pressure, M is molar mass, T is temperature, and R is the gas constant. Solids and liquids generally have higher densities than gases, and densities can vary widely between substances (for example, water ≈ 1000 kg/m^3 at 4°C).

Besides mass density, tætheder is used for other types of densities, including population density (people per square kilometer), charge density (coulombs per cubic meter), and energy density (joules per cubic meter). In probability theory, a density function describes a distribution of probability over a continuous variable, integrating to one.

Relativ tæthed, or relative density, is the ratio of a substance’s density to a reference density (typically water at 4°C). This concept is used to compare how heavy substances feel relative to water. Measurement methods include pycnometry and hydrometry, depending on the context.