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.
This has important ramifications in Earth's ecosystem.
This physical chemistry-related article is a stub.