Density

Density (volumetric mass density or specific mass) is the ratio of a substance's <a href="/facts/Mass/HHdc8XmF">mass</a> to its <a href="/facts/Volume/aeAXCBUd">volume</a>. The symbol most often used for density is ρ (the lower case <a href="/facts/Greek_language/pzDd6eIz">Greek</a> letter <a href="/facts/Rho/gGezeq5Q">rho</a>), although the Latin letter D can also be used:

ρ
        =
        
          
            m
            V
          
        
        ,
      
    
    {\displaystyle \rho ={\frac {m}{V}},}

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 <a href="/facts/Weight/9NMHCsQd">weight</a> per unit <a href="/facts/Volume/aeAXCBUd">volume</a>, although this is scientifically inaccurate – this quantity is more specifically called <a href="/facts/Specific_weight/NRhILXxs">specific weight</a>.
For a pure substance, the density is equal to its <a href="/facts/Mass_concentration_(chemistry)/FfIuxa7T">mass concentration</a>.
Different materials usually have different densities, and density may be relevant to <a href="/facts/Buoyancy/9dIQwlrh">buoyancy</a>, purity and <a href="/facts/Packaging/QGRXJLjq">packaging</a>. <a href="/facts/Osmium/1TjXD69n">Osmium</a> is the densest known element at <a href="/facts/Standard_conditions_for_temperature_and_pressure/JhJv9KFJ">standard conditions for temperature and pressure</a>.
To simplify comparisons of density across different systems of units, it is sometimes replaced by the <a href="/facts/Dimensionless/TINrpQIq">dimensionless</a> quantity "<a href="/facts/Relative_density/ySquGd2x">relative density</a>" or "<a href="/facts/Specific_gravity/ySquGd2x">specific gravity</a>", 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 relative to water 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 while maintaining a constant pressure decreases its density by increasing its volume (with a few exceptions). In most fluids, heating the bottom of the fluid results in <a href="/facts/Convection/E8vsCPhu">convection</a> due to the decrease in the density of the heated fluid, which causes it to rise relative to denser unheated material.
The reciprocal of the density of a substance is occasionally called its <a href="/facts/Specific_volume/ut49njFh">specific volume</a>, a term sometimes used in <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>. Density is an <a href="/facts/Intensive_property/UcgSh8EG">intensive property</a> in that increasing the amount of a substance does not increase its density; rather it increases its mass.
Other conceptually comparable quantities or ratios include <a href="/facts/Specific_density/ySquGd2x">specific density</a>, <a href="/facts/Relative_density/ySquGd2x">relative density (specific gravity)</a>, and <a href="/facts/Specific_weight/NRhILXxs">specific weight</a>.

Density open-in-new

Density