Maximum density

The maximum density of a substance is the highest attainable density of the substance under given conditions.

Attaining maximum density
Almost all known substances undergo thermal expansion in response to heating, meaning that a given mass of substance contracts to a low volume at low temperatures, when little thermal energy is present. Substances, especially fluids in which intermolecular forces are weak, also undergo compression upon the application of pressure. Nearly all substances therefore reach a density maximum at very low temperatures and very high pressures, characteristic properties of the solid state of matter.

Irregular maximum densities
A few substances break the typical thermal expansion rules, instead contracting in response to increased temperature. An example of such a substance is zirconium tungstate in the cubic phase, which contracts continuously up to 1050 K. Cubic zirconium tungstate's maximum density would be reached at the temperature where either contraction stops (that is, where the sample's volume stops decreasing) or the substance decomposes.

Maximum density of water
An especially notable irregular maximum density is that of water, which reaches a density peak at 3.98 °C under standard conditions of pressure because of the low-density crystal structure of typical ice. This has important ramifications in Earth's ecosystem; see Water (molecule).