Editing Water (section)

====As a scientific standard====
On 7 April 1795, the gram was defined in France to be equal to "the absolute weight of a volume of pure water equal to a cube of one-hundredth of a meter, and at the temperature of melting ice".<ref>[http://smdsi.quartier-rural.org/histoire/18germ_3.htm "Décret relatif aux poids et aux mesures"] [Decree relating to weights and measures] (in French). 18 [[French revolutionary calendar|germinal]] an 3 (7 April 1795). {{Webarchive|url=https://web.archive.org/web/20130225163152/http://smdsi.quartier-rural.org/histoire/18germ_3.htm |date=25 February 2013 }}. quartier-rural.org</ref> For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely the mass of one liter of water. In spite of the fact that the decreed definition of the gram specified water at {{convert|0|C}}—a highly reproducible ''temperature''—the scientists chose to redefine the standard and to perform their measurements at the temperature of highest water ''density'', which was measured at the time as {{convert|4|C}}.<ref>[http://histoire.du.metre.free.fr/fr/index.htm here "L'Histoire Du Mètre, La Détermination De L'Unité De Poids"] {{Webarchive|url=https://web.archive.org/web/20130725163108/http://histoire.du.metre.free.fr/fr/index.htm |date=25 July 2013 }}. histoire.du.metre.free.fr</ref>

The [[Kelvin temperature scale]] of the [[International System of Units|SI]] system was based on the [[triple point]] of water, defined as exactly {{convert|273.16|K|C F}}, but as of May 2019 is based on the [[Boltzmann constant]] instead. The scale is an [[absolute temperature]] scale with the same increment as the Celsius temperature scale, which was originally defined according to the [[boiling point]] (set to {{convert|100|C}}) and [[melting point]] (set to {{convert|0|C}}) of water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is also a small quantity of heavier isotopes oxygen-18, oxygen-17, and hydrogen-2 ([[deuterium]]). The percentage of the heavier isotopes is very small, but it still affects the properties of water. Water from rivers and lakes tends to contain less heavy isotopes than seawater. Therefore, standard water is defined in the [[Vienna Standard Mean Ocean Water]] specification.