Editing Water (section)

==Effects on human civilization==

Civilization has historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, was situated between the major rivers Tigris and Euphrates; the ancient society of the Egyptians depended entirely upon the Nile. Large metropolises like Rotterdam, London, Montreal, Paris, New York City, Shanghai, Tokyo, Chicago, and Hong Kong owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore, have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.

===Health and pollution===

Water fit for human consumption is called drinking water or potable water. Water that is not potable can be made potable by filtration or distillation (heating it until it becomes water vapor, and then capturing the vapor without any of the impurities it leaves behind), or by other methods (chemical or heat treatment that kills bacteria). Sometimes the term safe water is applied to potable water of a lower quality threshold (i.e., it is used effectively for nutrition in humans that have weak access to water cleaning processes, and does more good than harm). Water that is not fit for drinking but is not harmful for humans when used for swimming or bathing is called by various names other than potable or drinking water, and is sometimes called safe water, or "safe for bathing". Chlorine is a skin and mucous membrane irritant that is used to make water safe for bathing or drinking. Its use is highly technical and is usually monitored by government regulations (typically 1 part per million (ppm) for drinking water, and 1–2 ppm of chlorine not yet reacted with impurities for bathing water).

===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 to the temperature of the melting ice. For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely how massive one liter of water was. In spite of the fact that the decreed definition of the gram specified water at 0&nbsp;°C—a highly stable ''temperature'' point—the scientists chose to redefine the standard and to perform their measurements at the most stable ''density'' point: the temperature at which water reaches maximum density, which was measured at the time as 4&nbsp;°C

The Kelvin temperature scale of the SI system is based on the triple point of water, defined as exactly 273.16 K or 0.01 °C. The scale is a more accurate development of the Celsius temperature scale, which is defined by the boiling point (100 °C) and melting point (0 °C) of water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is also small quantity of heavier isotopes such as hydrogen-2 (deuterium). The amount of deuterium oxides or heavy water is very small, but it still affects the properties of water. Water from rivers and lakes tends to contain less deuterium than seawater. Therefore, a standard water called Vienna Standard Mean Ocean Water is defined as the standard water.

===As a heat transfer fluid===

Water and steam are used as heat transfer fluids in diverse heat exchange systems, due to its availability and high heat capacity, both as a coolant and for heating. Cool water may even be naturally available from a lake or the sea. Condensing steam is a particularly efficient heating fluid because of the large heat of vaporization. A disadvantage is that water and steam are somewhat corrosive. In almost all electric power plants, water is the coolant, which vaporizes and drives steam turbines to drive generators.

In the nuclear industry, water can also be used as a neutron moderator. In a pressurized water reactor, water is both a coolant and a moderator. This provides a passive safety measure, as removing the water from the reactor also slows the nuclear reaction down.

===Chemical uses===
Organic reactions are usually quenched with water or a water solution of a suitable acid, base or buffer. Water is generally effective in removing inorganic salts. In inorganic reactions, water is a common solvent. In organic reactions, it is usually not used as a reaction solvent, because it does not dissolve the reactants well and is amphoteric (acidic ''and'' basic) and nucleophilic. Nevertheless, these properties are sometimes desirable. Also, acceleration of Diels-Alder reactions by water has been observed. Supercritical water has recently been a topic of research. Oxygen-saturated supercritical water combusts organic pollutants efficiently.