densitys

Density is a measure of how much mass is contained in a given volume. It is defined by the formula ρ = m/V, where ρ is density, m is mass, and V is volume. In the SI system, density is measured in kilograms per cubic meter (kg/m^3); for many solids, grams per cubic centimeter (g/cm^3) is also common. For gases, density is typically much smaller and can be calculated from the ideal gas law: ρ = pM/RT, where p is pressure, T is temperature, M is the molar mass, and R is the gas constant.

Density depends on temperature and pressure. Most materials expand as they warm, reducing density. Water is notable for its anomalous density behavior: it reaches maximum density near 4 °C, which explains its buoyancy characteristics in lakes and oceans.

Different notions of density exist. Mass density (simply ρ) uses mass per unit volume. Number density (n) is the number of particles per unit volume. Electron density describes the number of electrons per unit volume in a material and is important in fields such as solid-state physics and crystallography.

Measurement methods vary by substance. Archimedes’ principle is used to determine the density of solids by buoyant force in a liquid. Pycnometry and densitometry measure density by volume displacement or refractive and acoustic properties. For heterogeneous or porous materials, specialized techniques such as X-ray or neutron densitometry may be employed.

Density has wide applications across science and engineering. It governs buoyancy, material selection, and mixture behavior, and it is a fundamental property used to characterize substances, compare materials, and model physical processes in disciplines from geology to meteorology to astrophysics.