"H2O" and "HOH" redirect here. For other uses, see H2O (disambiguation)[->0] and HOH (disambiguation)[->1]. This article is about general aspects of water. For a detailed discussion of its chemical properties, see Properties of water[->2]. For other uses, see Water (disambiguation)[->3]. [->4][->5]
[->6]Water in three states: liquid, solid (ice[->7]), and (invisible) water vapor[->8] in the air. Clouds[->9] are accumulations of water droplets, condensed[->10] from vapor-saturated air. Water is a chemical compound[->11] with the chemical formula[->12] H[->13]2O[->14]. A water molecule[->15] contains one oxygen[->16] and two hydrogen[->17] atoms[->18] connected by covalent[->19] bonds. Water is a liquid[->20] at standard ambient temperature and pressure[->21], but it often co-exists on Earth[->22] with its solid[->23] state, ice[->24], and gaseous[->25] state (water vapor[->26] or steam[->27]). Water also exists in a liquid crystal[->28] state near hydrophilic[->29] surfaces.HYPERLINK "http://en.wikipedia.org/wiki/Water" \l "cite_note-2"[->30] Water covers 71% of the Earth's surface, and is vital for all known forms of life[->31]. On Earth, 96.5% of the planet's water is found in oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air[->32] as vapor[->33], clouds[->34] (formed of solid and liquid water particles suspended in air), and precipitation[->35].HYPERLINK "http://en.wikipedia.org/wiki/Water" \l "cite_note-6"[->36] Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products. Water on Earth moves continually through the hydrological cycle[->37] of evaporation[->38] and transpiration[->39] (evapotranspiration[->40]), condensation[->41], precipitation, and runoff[->42], usually reaching the sea[->43]. Evaporation and transpiration contribute to the precipitation over land. Safe drinking water[->44] is essential to humans[->45] and other lifeforms even though it provides no calories[->46] or organic[->47] nutrients[->48]. Access to safe drinking water has improved over the last decades in almost every part of the world, but approximately one billion people still lack access to safe water and over 2.5 billion lack access to adequate sanitation. There is a clear correlation between access to safe water and GDP[->49] per capita. However, some observers have estimated that by 2025 more than half of the world population[->50] will be facing water-based vulnerability. A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%. Water plays an important role in the world economy[->51], as it functions as a solvent[->52] for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of the fresh water[->53] used by humans goes to agriculture[->54].
Chemical and physical properties
Water is the chemical substance[->55] with chemical formula[->56] H2O: one molecule[->57] of water has two hydrogen[->58] atoms[->59] covalently[->60] bonded[->61] to a single oxygen[->62] atom. Water appears in nature in all three common states of matter (solid, liquid, and gas) and may take many different forms on Earth: water vapor and clouds in the sky; seawater[->63] in the oceans; icebergs[->64] in the polar oceans; glaciers[->65] and rivers[->66] in the mountains[->67]; and the liquid in aquifers in the ground. The major chemical and physical properties of water are:
·Water is a liquid at standard temperature and pressure[->68]. It is tasteless and odorless. The intrinsic colour of water[->69] and ice is a very slight...