En física y química, la densidad (del latín densĭtas, -ātis) es una magnitud escalar referida a la cantidad de masa en un determinado volumen de una sustancia. Usualmente se simboliza mediante la letra rho ρ del alfabeto griego. La densidad media es la razón entre la masa de un cuerpo y el volumen que ocupa. Si un cuerpo no tiene una distribución uniforme de la masa en todos sus puntos, la densidad alrededor de un punto dado puede diferir de la densidad media. Si se considera una sucesión de pequeños volúmenes decrecientes (convergiendo hacia un volumen muy pequeño) centrados alrededor de un punto, siendo la masa contenida en cada uno de los volúmenes anteriores, la densidad en el punto común a todos estos volúmenes es: La unidad es kg/m³ en el SI. Como ejemplo, un objeto de plomo es más denso que otro de corcho, con independencia del tamaño y masa.

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The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho), although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume: where ρ is the density, m is the mass, and V is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more specifically called specific weight. For a pure substance the density has the same numerical value as its mass concentration.Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material, usually water. Thus a relative density less than one means that the substance floats in water. The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance (with a few exceptions) decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid. This causes it to rise relative to more dense unheated material. The reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density; rather it increases its mass.

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