Editing Density (section)

== Changes of density ==
{{Main article|Compressibility|Thermal expansivity}}

In general, density can be changed by changing either the [[pressure]] or the [[temperature]]. Increasing the pressure always increases the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions to this generalization. For example, the density of [[water]] increases between its melting point at 0&nbsp;°C and 4&nbsp;°C; similar behavior is observed in [[silicon]] at low temperatures.

The effect of pressure and temperature on the densities of liquids and solids is small. The [[compressibility]] for a typical liquid or solid is 10<sup>−6</sup>&nbsp;[[bar (unit)|bar]]<sup>−1</sup> (1&nbsp;bar = 0.1&nbsp;MPa) and a typical [[thermal expansivity]] is 10<sup>−5</sup>&nbsp;[[Kelvin|K]]<sup>−1</sup>. This roughly translates into needing around ten thousand times atmospheric pressure to reduce the volume of a substance by one percent. (Although the pressures needed may be around a thousand times smaller for sandy soil and some clays.) A one percent expansion of volume typically requires a temperature increase on the order of thousands of degrees [[Celsius]].

In contrast, the density of gases is strongly affected by pressure. The density of an [[ideal gas]] is
<math display="block">\rho = \frac {MP}{RT},</math>
where {{math|''M''}} is the [[molar mass]], {{math|''P''}} is the pressure, {{math|''R''}} is the [[Gas constant|universal gas constant]], and {{math|''T''}} is the [[absolute temperature]]. This means that the density of an ideal gas can be doubled by doubling the pressure, or by halving the absolute temperature.

In the case of volumic thermal expansion at constant pressure and small intervals of temperature the temperature dependence of density is
<math display="block">\rho = \frac{\rho_{T_0}}{1 + \alpha \cdot \Delta T},</math>
where <math>\rho_{T_0}</math> is the density at a reference temperature, <math>\alpha</math> is the thermal expansion coefficient of the material at temperatures close to <math>T_0</math>.