Properties of Water

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Water (H2O, HOH) is the most abundant molecule on Earth's surface, constituting about 70% of the planet's surface. In nature it exists in liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at standard temperature and pressure. At room temperature, it is a nearly colorless with a hint of blue, tasteless, and odorless liquid. Many substances dissolve in water and it is commonly referred to as the universal solvent. Because of this, water in nature and in use is rarely pure and some of its properties may vary slightly from those of the pure substance. However, there are many compounds that are essentially, if not completely, insoluble in water. Water is the only common substance found naturally in all three common states of matter—for other substances, see Chemical properties. Water is essential for life on Earth. Water usually makes up 55% to 78% of the human body.

Forms of water
Like many substances, water can take numerous forms that are broadly categorized by phase of matter. The liquid phase is the most common among water's phases and is the form that's generally denoted by the word "water." The solid phase of water is known as ice and commonly takes the structure of hard, amalgamated crystals, such as ice cubes, or loosely accumulated granular crystals, like snow. For a list of the many different crystalline and amorphous forms of solid H2O, see the article, ice. The gaseous phase of water is known as water vapor (or steam), and it's characterized by water assuming the configuration of a transparent cloud. The fourth state of water, that of a supercritical fluid, is much less common than the other three and only rarely occurs in nature. When water achieves a specific critical temperature and a specific critical pressure (647 K and 22.064 MPa), liquid and gas phase merge to one homogenous fluid phase, with properties of both gas and liquid. Since water only becomes supercritical under extreme temperatures or pressures, it almost never occurs naturally. One example of naturally occurring supercritical water is in the hottest parts of deep water hydrothermal vents, in which water is heated to the critical temperature by scalding volcanic plumes and achieves the critical pressure because of the crushing weight of the ocean at the extreme depths at which the vents are located.

In natural water, almost all of the hydrogen atoms are of the isotope protium,1H. Heavy water is water in which the hydrogen is replaced by its heavier isotope, deuterium,2H. It is chemically almost identical to normal water. Heavy water is used in the nuclear reactor industry to moderate (slow down) neutrons. By contrast, the term light water designates water containing the protium isotope, in contexts when such distinction is needed. An example is the term light water reactor to emphasize that the reactor type uses light water.

Physics and chemistry
Water is the chemical substance with chemical formula H2O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom. Water is a tasteless, odorless liquid at ambient temperature and pressure, and appears colorless in small quantities, although it has its own intrinsic very light blue hue. Ice also appears colorless, and water vapor is essentially invisible as a gas.

Water is primarily a liquid under standard conditions, which is not predicted from its relationship to other analogous hydrides of the oxygen family in the periodic table, which are gases such as hydrogen sulfide. Also the elements surrounding oxygen in the periodic table, nitrogen, fluorine, phosphorus, sulfur and chlorine, all combine with hydrogen to produce gases under standard conditions. The reason that water forms a liquid is that oxygen is more electronegative than all of these elements with the exception of fluorine. Oxygen attracts electrons much more strongly than hydrogen, resulting in a net positive charge on the hydrogen atoms, and a net negative charge on the oxygen atom.

The presence of a charge on each of these atoms gives each water molecule a net dipole moment. Electrical attraction between water molecules due to this dipole pulls individual molecules closer together, making it more difficult to separate the molecules and therefore raising the boiling point. This attraction is known as hydrogen bonding. The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at the timescales faster than 200 femtoseconds. However, this bond is strong enough to create many of the peculiar properties of water described in this article, such as the ones that make it integral to life. Water can be described as a polar liquid that slightly dissociates disproportionately into the hydronium ion (H3O+(aq)) and an associated hydroxide ion (OH−(aq)).

2 H2O (l) H3O+ (aq) + OH− (aq)
The dissociation constant for this dissociation is commonly symbolized as Kw and has a value of about 10−14 at 25°C; see "Water (data page)" and "Self-ionization of water" for more information.

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