<P> The maximum density of a substance is the highest attainable density of the substance under given conditions . </P> <P> 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 . </P> <P> An especially notable irregular maximum density is that of water, which reaches a density peak at 3.98 ° C (39.16 ° F). This has important ramifications in Earth's ecosystem . </P>

Density of water is maximum at which temperature