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Terrestrial Planet

Terrestrial planet

A terrestrial planet or telluric planet is a planet which is primarily composed of silicate rocks. The term is derived from the Latin word for Earth, "Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which may not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes. Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. At one time there were probably many more terrestrials, but most have been ejected from the solar system or otherwise destroyed. Only one terrestrial planet, Earth, is known to have an active hydrosphere. NASA is considering a proposed project called the Terrestrial Planet Finder, which will be capable of detecting terrestrial planets outside of our solar system (orbiting other stars). The smallest extrasolar planet discovered to date is one of three known planets orbiting around the red dwarf star Gliese 876d. The planet has a mass between six and nine times that of earth and is almost certainly a terrestrial planet. Theoretically, there are two types of terrestrial or rocky planets, one dominated by silicon, as Earth is, and another dominated by carbon, like carbonaceous chondrite asteroids. These are the silicon/silicate planets and carbide/carbon/diamond planets, respectively.

Telluric

The adjective telluric is derived from a Latin word for earth or Mother Earth, "Tellus", and is used in terms related to the Earth such as:
- telluric planet (a planet which is Earth-like in the sense that it is primarily composed of silicate rocks)
- telluric current (an extremely low frequency electrical current that occurs naturally over large underground areas at or near the surface of the Earth).
- telluric contamination (contamination of a spectrum in astronomical spectroscopy by absorption or emission happening in the atmosphere of the Earth). However, it is sometimes also used in relation to tellurium, which is a relatively rare element, in the same chemical family as oxygen, sulfur, selenium, and polonium, e.g. telluric acid.

Silicate

In chemistry, a silicate is a compound consisting of silicon and oxygen (Si_xO_y), one or more metals, and possibly hydrogen. It is also used to denote the salts of silica or of one of the silicic acids. In common conditions, the most stable form is silicon dioxide, SiO₂, often called quartz, and similar species. This always has, in equilibrium, a minute amount of silicic acid, H₄SiO₄. Chemists consider quartz as insoluble, but it moves around at longer timescales. Also, in basic conditions, we find H₂SiO₄^2-. Silicate minerals are noted for their tetrahedral form. Sometimes the tetrahedra are joined in chains, double chains, sheets, and three-dimensional frameworks. They are subclassified into groups based on the degree of polymerization of the tetrahedra, such as nesosilicates, cyclosilicates, and so forth. In geology and astronomy, the term silicate is used to denote a type of rock that consists of silicon and oxygen (usually as SiO₂ or SiO₄), one or more metals, and possibly hydrogen. Such rocks range from granite to gabbro. Most of the Earth's crust is made up of silicate rocks, as are the crusts of other terrestrial planets. Mineralogically, silicate minerals are divided according to their molecular structure into the following groups:
- Olivine (single tetrahedron) - Nesosilicates
- Epidote (double tetrahedra) - Sorosilicates
- Tourmaline (rings of tetrahedra) - Cyclosilicates
- Pyroxene (single chain) - Inosilicates
- Amphibole (double chain) - Inosilicates
- Micas and clays (sheet) - Phyllosilicates
- Feldspars (framework) - Tectosilicates
- Quartz (SiO₂ framework) Silicate was also the name given to the bone-sucking monsters in the British horror movie Island of Terror (aka Night of the Silicates). These were silicon-based organisms created by cancer research gone wrong, which consumed the calcium phosphate in the bones of carbon-based lifeforms.

Rock (geology)

, plutonic, metamorphic rock types of North America. ]] Rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks are classified by mineral and chemical composition; the texture of the constituent particles; and also by the processes that formed them. These indicators separate rocks into igneous, sedimentary, and metamorphic. Igneous rocks are formed from molten magma, and are divided into two main categories: Plutonic rock and Volcanic rock. Plutonic rocks result when the magma cools and crystallises slowly within the Earth's crust, while Volcanic rocks result from the magma reaching the surface either as lava or fragmental ejecta. Sedimentary rocks are formed by deposition of either detrital or organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation. The latter can occur at or near the earth's surface, especially in the case of carbonate-rich sediments. Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the earth's surface, and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). The transformation of one rock type to another is described by the geological model called the rock cycle. The Earth's crust (including the lithosphere) and mantle are formed of rock.

External links

- [http://www.geol.lsu.edu/henry/Geology3041/2IgneousClassify/IgneousClassFlow.htm Classification of Igneous Rocks] Category:Geology Category:Rocks ja:岩石 ms:Batu th:หิน

Gas giant

:This article refers to a astronomical phenomenon. For the rock band, see Gas Giants A gas giant is a large planet that is not composed mostly of rock or other solid matter. Gas giants may still have a rocky or metallic core—in fact, it is expected that such a core is probably required for a gas giant to form—but the majority of its mass is in the form of gas (or gas compressed into a liquid state). Unlike rocky planets, gas giants do not have a well-defined surface. Terms like diameter, surface area, volume, surface temperature and surface density may refer to the outermost layer visible from outside, e.g. from the Earth. gas There are four gas giants in our solar system: Jupiter, Saturn, Uranus, and Neptune. These are also known as the Jovian planets. Uranus and Neptune have been referred by scientists in the past as a separate subclass of giant planets, ice giants, or Uranian planets due to their structure made mostly of ice and rock and gas, which differs from the "traditional" gas giant such as Jupiter or Saturn. This is because their proportion in hydrogen and helium is much lower than the latter's, mostly because of their greater distance from the Sun.

Common structure

The four solar system gas giants share a number of features. All have atmospheres that are mostly hydrogen and helium, and that blend into the liquid interior at pressures greater than the critical pressure, so that there is no clear boundary between atmosphere and body. They have very hot interiors, ranging from about 5000 K for Neptune to over 20,000 K for Jupiter. This great heat means that, beneath their atmospheres, the planets are most likely entirely liquid. When discussions refer to a "rocky core", one should not picture a ball of solid granite, or even, at 20,000 K, liquid granite. Rather, what is meant is a region in which the concentration of heavier elements such as iron and silicon is greater than that in the rest of the planet. All four planets rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent in Jupiter, muted in Saturn and Neptune, and barely detectable at all in Uranus. Finally, all four are accompanied by elaborate systems of rings and moons. Saturn's rings are the most spectacular, and the only ones known before the 1970s. As of 2004, Jupiter was thought to have the most moons, with more than sixty found.

Jupiter and Saturn

Jupiter and Saturn consist almost entirely of hydrogen and helium, and they are so large that this is true even though both are thought to have several Earth masses of heavier elements. Their deep interiors consist of liquid metallic hydrogen, a form of hydrogen distinguished by the fact that it conducts electricity. Both planets have magnetic fields oriented fairly close to their axes of rotation.

Uranus and Neptune

Uranus and Neptune have distinctly different interior compositions, with the bulk of their interiors thought to consist of a mixture (or layered assortment) of rock, water, methane, and ammonia. Both have magnetic fields that are sharply inclined to their axes of rotation.

Terminology

The term was coined by the science fiction writer James Blish. Arguably it is a misnomer, since all of these planets are primarily liquid and not gaseous. In fact, for Neptune and Uranus, the gaseous atmospheres are quite thin compared to the planetary radii -- only extending perhaps one percent of the way to the center. However, at least for Jupiter and Saturn, the name is defensible because their compositions are dominated by hydrogen and helium, which are gases in the outer solar system when not under pressure. Planetary scientists often use 'rock', 'gas', and 'ice' as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase they appear in. In the outer solar system, hydrogen and helium are "gases"; water, methane, and ammonia are "ices"; and silicates are rock. When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean oxygen and carbon, by "rock" they mean silicon, and by "gas" they mean hydrogen and helium. With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants", to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition.

Extrasolar gas giants

Because of the techniques currently available to detect extrasolar planets, all of those found to date have been of a scale associated, in the Solar system, with gas giants. The smallest found as of September 2004 is comparable in mass to Neptune, and many have masses several times that of Jupiter. Many of the extrasolar planets are much closer to their parent stars and hence much hotter than gas giants in the solar system, making it possible that some of those planets are a type not observed in our solar system. Considering the relative abundances of the elements in the universe (approximately 90% hydrogen), it would be surprising to find a predominantly rocky planet more massive than Jupiter. On the other hand, previous models of planetary system formation suggested that gas giants would be inhibited from forming as close to their stars as have many of the new planets that have been observed. extrasolar planet The upper mass limit of a gas giant planet is approximately 70 times that of Jupiter (around 0.08 times the mass of the Sun). Above this point, the intense heat and pressure at the planet's core begin to induce nuclear fusion and the planet ignites to become a red dwarf. Interestingly there appears to be a mass gap between the heaviest gas giant planets detected (about 10 times the mass of Jupiter) and the lightest red dwarfs. This has led to suggestions that the formation process for planets and binary stars may be fundamentally different.

Hydrogen

|- | Critical temperature || 32.19 K |- | Critical pressure || 1.315 MPa |- | Critical density || 30.12 g/L (Bohr radius) Hydrogen (Latin: hydrogenium, from Greek: hydro: water, genes: forming) is a chemical element in the periodic table that has the symbol H and atomic number 1. At standard temperature and pressure it is a colorless, odorless, nonmetallic, univalent, highly flammable diatomic gas. Hydrogen is the lightest and most abundant element in the universe. It is present in water, all organic compounds (rare exceptions exist, like buckminsterfullerene) and in all living organisms. Hydrogen is able to react chemically with most other elements. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. The element is used in ammonia production, as a lifting gas, as an alternative fuel, and more recently as a power source of fuel cells. Despite its ubiquity in the universe, hydrogen is surprisingly hard to produce in large quantities on the Earth. In the laboratory, the element is prepared by the reaction of acids on metals such as zinc. The electrolysis of water is a simple method of producing hydrogen, but is economically inefficient for mass production. Large-scale production is usually achieved by steam reforming natural gas. Scientists are now researching new methods for hydrogen production; if they succeed in developing a cost-efficient method of large-scale production, hydrogen may become a viable alternative to greenhouse-gas-producing fossil fuels. One of the methods under investigation involves use of green algae; another promising method involves the conversion of biomass derivatives such as glucose or sorbitol at low temperatures using a catalyst. Yet another method is the "steaming" of Carbon, whereby hydrocarbons are broken down with heat to release hydrogen.

Basic features

Hydrogen is the lightest chemical element; its most common isotope comprises just one negatively charged electron, distributed around a positively charged proton (the nucleus of the atom). The electron is bound to the proton by the Coulomb force, the electrical force that one stationary, electrically charged nanoparticle exerts on another. The hydrogen atom has special significance in quantum mechanics as a simple physical system for which there is an exact solution to the Schrödinger equation; from that equation, the experimentally observed frequencies and intensities of the hydrogen's spectral lines can be calculated. Spectral lines are dark or bright lines in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. At standard temperature and pressure, hydrogen forms a diatomic gas, H₂, with a boiling point of only 20.27 K and a melting point of 14.02 K. Under extreme pressures, such as those at the center of gas giants, the molecules lose their identity and the hydrogen becomes a liquid metal. Under the extremely low pressure in space—virtually a vacuum—the element tends to exist as individual atoms, simply because there is no way for them to combine. However, clouds of H₂ and singular hydrogen atoms are said to form in H I and H II regions and are associated with star formation, however the existence of singular hydrogen atoms is disputed.. Hydrogen plays a vital role in powering stars through the proton–proton and carbon–nitrogen cycle. These are nuclear fusion processes, which release huge amounts of energy in stars and other hot celestial bodies as hydrogen atoms combine into helium atoms. H₂ is highly soluble in water, alcohol, and ether. It has a high capacity for adsorption, in which it is attached to and held to the surface of some substances. It is an odorless, tasteless, colorless, and highly flammable gas that burns at concentrations as low as 4% H₂ in air. It reacts violently with chlorine and fluorine, forming hydrohalic acids that can damage the lungs and other tissues. When mixed with oxygen, hydrogen explodes on ignition. A unique property of hydrogen is that its flame is completely invisible in air. This makes it difficult to tell if a leak is burning, and carries the added risk that it is easy to walk into a hydrogen fire inadvertently. See also: hydrogen atom.

Applications

Large quantities of hydrogen are needed in the chemical and petrolium industries, notably in the Haber process for the production of ammonia, which by mass ranks as the world's fifth most highly produced industrial compound. Hydrogen is used in the hydrogenation of fats and oils (into items such as margarine), and in the production of methanol. Hydrogen is used in hydrodealkylation, hydrodesulfurization, and hydrocracking. The element has several other important uses.
- The element is used in the manufacture of hydrochloric acid, in welding processes, and in the reduction of metallic ores.
- It is an ingredient in rocket fuels.
- It is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas.
- Liquid hydrogen is used in cryogenic research, including superconductivity studies.
- Since hydrogen is 14.5 times lighter than air, it was once widely used as a lifting agent in balloons and airships. However, this use was curtailed when the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
- Deuterium, an isotope of hydrogen (hydrogen-2), is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions. Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.
- Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs, as an isotopic label in the biosciences, and as a radiation source in luminous paints. There are no "hydrogen wells" or "hydrogen mines" on Earth, so hydrogen cannot be considered a primary energy source like fossil fuels or uranium. Hydrogen can however be burned in internal combustion engines, an approach advocated by BMW's experimental hydrogen car. However, it is currently difficult and dangerous to store and handle in sufficient quantity for motor fuel use. Hydrogen fuel cells are being investigated as mobile power sources with lower emissions than hydrogen-burning internal combustion engines. The low emissions of hydrogen in internal combustion engines and fuel cells are currently offset by the pollution created by hydrogen production. This may change if the substantial amounts of electricity required for water electrolysis can be generated primarily from low pollution sources such as nuclear energy or wind. Research is being conducted on hydrogen as a replacement for fossil fuels. It could become the link between a range of energy sources, carriers and storage. Hydrogen can be converted to and from electricity (solving the electricity storage and transport issues), from bio-fuels, and from and into natural gas and diesel fuel. All of this can theoretically be achieved with zero emissions of CO₂ and toxic pollutants.

History

Hydrogen was first produced by Theophratus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus—by mixing metals with acids. He was unaware that the explosive gas produced by this chemical reaction was hydrogen. In 1671, Robert Boyle described the reaction between two iron fillings and dilute acids, which results in the production of gaseous hydrogen. In 1766, Henry Cavendish was the first to recognize hydrogen as a discrete substance, by identifying the gas from this reaction as "inflammable" and finding that the gas produces water when burned in air. Cavendish stumbled on hydrogen when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a compound of mercury—and not of the acid—he was still able to accurately describe several key properties of hydrogen. Antoine Lavoisier gave the element its name and proved that water is composed of hydrogen and oxygen. One of the first uses of the element was for balloons. The hydrogen was obtained by mixing sulfuric acid and iron. Harold C. Urey discovered Deuterium, an isotope of hydrogen, by repeated distilling the same sample of water. For this discovery, Urey received the Nobel prize for in 1934. In the same year, the third isotope, tritium, was discovered. Because of its relatively simple structure, hydrogen has often been used in models of how an atom works.

Electron energy levels

The ground state energy level of the electron in a Hydrogen atom is 13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm. With the Bohr Model the energy levels of Hydrogen can be calculated fairly accurately. This is done by modeling the electron as revolving around the proton, much like the earth revolving around the sun. Except the sun holds earth in orbit with the force of gravity, but the proton holds the electron in orbit with the force of electromagnetism. Another difference between the Earth-Sun system and the Electron-Proton system is that, in this model, due to quantum mechanics the electron is allowed to only be at very specific distances from the proton. Modeling the hydrogen atom in this fashion yields the correct energy levels and spectrum.

Occurrence

quantum mechanics.]] Hydrogen is the most abundant element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. This element is found in great abundance in stars and gas giant planets. It is very rare in the Earth's atmosphere (1 ppm by volume), because being the lightest gas causes it to escape Earth's gravity, though when compounds are considered, it is the tenth most abundant element on Earth. The most common source for this element on Earth is water, which is composed two parts hydrogen to one part oxygen (H₂O). Other sources include most forms of organic matter (currently all known life forms) including coal, natural gas, and other fossil fuels. Methane (CH₄) is an increasingly important source of hydrogen. Throughout the Universe, hydrogen is mostly found in the plasma state whose properties are quite different to molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionised. The charged particles are highly influenced by magnetic and electric fields, for example, in the Solar Wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. Hydrogen can be prepared in several different ways: steam on heated carbon, hydrocarbon decomposition with heat, reaction of a strong base in an aqueous solution with aluminium, water electrolysis, or displacement from acids with certain metals. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700–1100 °C), steam reacts with methane to yield carbon monoxide and hydrogen. :CH₄ + H₂O → CO + 3 H₂ Additional hydrogen can be recovered from the carbon monoxide through the water-gas shift reaction: :CO + H₂O → CO₂ + H₂

Compounds

The lightest of all gases, hydrogen combines with most other elements to form compounds. Hydrogen has an electronegativity of 2.2, so it forms compounds where it is the more nonmetallic and where it is the more metallic element. The former are called hydrides, where hydrogen either exists as H^- ions or just as a solute within the other element (as in palladium hydride). The latter tend to be covalent, since the H⁺ ion would be a bare nucleus and so has a strong tendency to pull electrons to itself. These both form acids. Thus even in an acidic solution one sees ions like hydronium (H₃O⁺) as the protons latch on to something. Although exotic on earth, one of the most common ions in the universe is the H₃⁺ ion. Hydrogen combines with oxygen to form water, H₂O, and releases a lot of energy in doing so, burning explosively in air. Deuterium oxide, or D₂O, is commonly referred to as heavy water. Hydrogen also forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and the study of the properties of these compounds is called organic chemistry. organic chemistry

Forms

Under normal conditions, hydrogen gas is a mix of two different kinds of molecules which differ from one another by the relative spin of the nuclei. These two forms are known as ortho- and para-hydrogen (this is different from isotopes, see below). In ortho-hydrogen the nuclear spins are parallel (form a triplet), while in para they are antiparallel (form a singlet). At standard conditions hydrogen is composed of about 25% of the para form and 75% of the ortho form (the so-called "normal" form). The equilibrium ratio of these two forms depends on temperature, but since the ortho form has higher energy (is an excited state), it cannot be stable in its pure form. In low temperatures (around boiling point), the equilibrium state is comprised almost entirely of the para form. The conversion process between the forms is slow, and if hydrogen is cooled down and condensed rapidly, it contains large quantities of the ortho form. It is important in preparation and storage of liquid hydrogen, since the ortho-para conversion produces more heat than the heat of its evaporation, and a lot of hydrogen can be lost by evaporation in this way during several days after liquefying. Therefore, some catalysts of the ortho-para conversion process are used during hydrogen cooling. The two forms have also slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 K lower than of the "normal" form.

Isotopes

Hydrogen is the only element that has different names for its isotopes. (During the early study of radioactivity, various heavy radioactive isotopes were given names, but such names are no longer used, although one element, radon, has a name that originally applied to only one of its isotopes.) The symbols D and T (instead of ²H and ³H) are sometimes used for deuterium and tritium, although this is not officially sanctioned. (The symbol P is already in use for phosphorus and is not available for protium.) ;¹H The most common isotope of hydrogen, this stable isotope has a nucleus consisting of a single proton; hence the descriptive, although rarely used, name protium. The spin of a protium atom is 1/2+. ;²H The other stable isotope is deuterium, with an extra neutron in the nucleus. Deuterium comprises 0.0184%–0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. The spin of a deuterium atom is 1+. ;³H The third naturally occurring hydrogen isotope is the radioactive tritium. The tritium nucleus contains two neutrons in addition to the proton. It decays through beta decay and has a half-life of 12.32 years. Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. Like ordinary hydrogen, tritium reacts with the oxygen in the atmosphere to form T₂O. This radioactive "water" molecule constantly enters the Earth's seas and lakes in the form of slightly radioactive rain, but its half-life is short enough to prevent a buildup of hazardous radioactivity. The spin of a tritium atom is 1/2+. ;⁴H Hydrogen-4 was synthesized by bombarding tritium with fast-moving deuterium nuclei. It decays through neutron emission and has a half-life of 9.93696x10^-23 seconds. The spin of a hydrogen-4 atom is 2-. ;⁵H In 2001 scientists detected hydrogen-5 by bombarding a hydrogen target with heavy ions. It decays through neutron emission and has a half-life of 8.01930x10^-23 seconds. ;⁶H Hydrogen-6 decays through triple neutron emission and has a half-life of 3.26500^-22 seconds. ;⁷H In 2003 hydrogen-7 was created ([http://physicsweb.org/articles/news/7/3/3 article]) at the RIKEN laboratory in Japan by colliding a high-energy beam of helium-8 atoms with a cryogenic hydrogen target and detecting tritons—the nuclei of tritium atoms—and neutrons from the breakup of hydrogen-7, the same method used to produce and detect hydrogen-5.

References

# # # # # #
- [http://www.riken.go.jp/engn/r-world/research/lab/wako/ribeam/ RIKEN Beam Science Laboratory, Japan - Heavy hydrogen research]
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes] Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989 ;Book references:
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External links

- [http://www.hydropole.ch/Hydropole/Intro/Phasediag.gif Hydrogen phase diagram.]
- [http://www.compchemwiki.org/index.php?title=Hydrogen Computational Chemistry Wiki] Category:Nonmetals Category:Fuels Category:Chemical elements ko:수소 ms:Hidrogen ja:水素 simple:Hydrogen th:ไฮโดรเจน

Helium

|- | colspan="6" align="center" |
- Atmospheric value, abundance may differ elsewhere. :This page is about the chemical element helium. For the American indie rock band Helium see Helium (band). Helium (He) is a colorless, odorless, tasteless, non-toxic, nearly inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. Its most abundant stable isotope is helium-4 and its rare stable isotope is helium-3. The behavior of liquid helium-4's two varieties—helium I and helium II—is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and those looking at the effects that near absolute zero temperatures have on matter (such as superconductivity). Helium is the second most abundant and second lightest element in the periodic table. In the modern Universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth it is created by the radioactive decay of much heavier elements (alpha particles are helium-4 nuclei produced by alpha-decay). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation. In 1868 the French astronomer Pierre Janssen first detected helium as an unknown yellow spectral line signature in light from a solar eclipse. Since then large reserves of helium have been found in the natural gas fields of the United States, which is by far the largest supplier of the gas. Helium is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). Inhaling a small volume of the gas temporarily changes the quality of one's voice.

Electron energy levels

Depending on the spin orientation of the two electrons in the Helium atom, one speaks of parahelium for two anti-parallel spins (S=0) and of orthohelium for two parallel spins (S=1). For the orthohelium one of the electrons does not sit in the ground orbital (1s). [http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/helium.html]

Applications

orthohelium Pressurized helium is commercially available. Helium is used for many purposes that require one or more of its unique properties; low boiling point, low density, low solubility, high thermal conductivity, or its inertness. Airships and balloons (toy, weather, and research) are inflated with helium because it is lighter than air (1 m³ of helium will lift 1 kg). Helium is currently preferred to hydrogen in airships because, while it is more expensive, it is not flammable and has 92.64% of the lifting power of hydrogen. Trimix, a mixture of helium, oxygen, and nitrogen, is used in deep-sea breathing gas systems to reduce the risk of nitrogen narcosis (high pressure nitrogen having a narcotic effect on the brain) and oxygen toxicity at high pressures. Higher pressures require a greater proportion of helium and reduced amounts of nitrogen and oxygen (every ten-meter increase in depth yields a one atmosphere increase of pressure). Heliox, a mixture of helium and oxygen, and heliair, a mixture of air and helium, is also used in this way. Below 130 meters (430 ft) a mixture of hydrogen, helium, and oxygen called hydreliox is used to help prevent high pressure nervous syndrome. All these uses rely on helium's very low solubility in water (the major component of blood). The extremely low boiling point makes helium useful as a coolant in magnetic resonance imaging, superconducting magnets, cryogenics, and to remove thermal noise from detectors used in astronomy. The extreme coldness of liquid helium is also used to produce superconductivity in some ordinary metals such as lead (lead becomes superconductive at 7.3 K), allowing for a completely free flow of electrons in the metal. Other uses:
- Because of its high thermal conductivity and inertness, helium is used as a coolant in some nuclear reactors (for example, pebble-bed reactors) and in arc welding air-sensitive metals that require heavy welds.
- The high thermal conductivity and sound velocity of helium is also desirable in thermoacoustic refrigeration. The inertness of helium adds to the environmental advantage of this technology over conventional refrigeration systems which may contribute to ozone depleting and global warming effects.
- Its inertness makes it useful as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, protecting important historical documents, and in gas chromatography. This property also makes it useful in pressurizing liquid fuel rockets (see below) and in supersonic wind tunnels.
- The gain medium of the helium-neon laser (the first gas laser) most commonly used to scan bar codes is a mixture of helium and neon.
- This gas' rate of diffusion through solids is three times that of normal air, making it an excellent component in leak detection in high-vacuum equipment and high pressure containers.
- In rocketry helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to precool liquid hydrogen in space vehicles. For example, the Saturn 5 booster used in the Apollo program needed about 13 million ft³ (370,000 m³) of helium to launch.
- Physics researchers use alpha particles (helium nuclei) in particle accelerators and nuclear reaction experiments.
- Helium gas is used to fill the space between lenses in some solar telescopes because its extremely low index of refraction reduces the distorting effect of temperature variations in the gas filling the telescope (some telescopes are filled with vacuum instead).
- Radioactive decay of uranium and thorium produces alpha particles that quickly become helium. This happens at a known constant rate so if the containing rock or mineral can retain its helium then the ratio of helium to its radioactive parent atoms indicates its age. Alternatively, if the helium is not well-retained, the ratio of helium-3 to helium-4 contains some of the same information, since only helium-4 is produced by radioactive decay. Use of helium in this way is called helium dating.

History

Discoveries

Helium was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nm in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in India. Janssen was at first ridiculed since no element had ever been detected in space before being found on Earth. October 20th the same year, English astronomer Norman Lockyer also observed the same yellow line in the solar spectrum and concluded that it was caused by an unknown element after unsuccessfully testing to see if it were some new type of hydrogen. Since it was near the Fraunhofer D line he later named the new line D₃, distinguishing it from the nearby D₁ and D₂ double lines of sodium. He and English chemist Edward Frankland named the element after the Greek word for the Sun god, Helios, and, assuming it was a metal, gave it an -ium ending (a mistake that was never corrected). British chemist William Ramsay isolated helium on March 26, 1895 by treating cleveite (now known to be uraninite) with mineral acids. Ramsay was looking for argon but noticed the yellow D₃ line after he removed nitrogen and oxygen from the gas liberated by the sulfuric acid he put on the cleveite sample. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite the same year by Swedish chemists Per Teodor Cleve and Abraham Langlet in Uppsala in Sweden. They collected enough of the gas to accurately determine its atomic weight. An oil drilling operation in Dexter, Kansas created a gas geyser in 1903 that contained 12% by volume of an unidentified gas. American chemists Hamilton Cady and David McFarland of the University of Kansas discovered it was helium and published a paper in 1907 saying that helium could be extracted from natural gas. Also in 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle is a helium nucleus. Helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes in 1908 in Leiden by cooling the gas to less than one kelvin. He tried to solidify it by reducing the temperature to 0.8 K but failed because helium does not have a triple point temperature where the solid, liquid and gas phases are at equilibrium. It was first solidified in 1926 by his student Willem Hendrik Keesom who subjected helium to a similar amount of cooling as Kamerlingh Onnes but at 25 standard atmospheres of pressure. In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that liquid helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. In 1972, the same phenomenon was observed in liquid helium-3 by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.

Production and use

Great quantities of helium were found in the natural gas fields of the American Great Plains, putting the United States in a very good position to become the leading world supplier. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium production plants during World War I. The goal was to supply barrage balloons with the non-flammable lifting gas. A total of 200,000 ft³ (5700 m³) of 92% helium was produced in the program even though only a few cubic feet (less than 100 liters) of the gas had previously been obtained. Some of this gas was used in the world's first helium-filled airship, the U.S. Navy's C-7, which flew its maiden voyage from Hampton Roads, Virginia to Bolling Field in Washington, D.C. on December 7, 1921. Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding. Helium was also vital in the atomic bomb Manhattan Project. The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas with the goal of supplying military airships in time of war and commercial airships in peacetime. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption. After the "Helium Acts Amendments of 1960" (Public Law 86-777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this helium conservation program, the Bureau built a 425-mile pipeline from Bushton, Kansas to connect those plants with the government's Cliffside partially depleted gasfield, near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gasfield until needed, when it then was further purified. By 1995 32 billion ft³ (1 billion m³) of the gas had been collected and the reserve was US$ 1.4 billion in debt, prompting the Congress of the United States to phase out the reserve starting the next year. The resulting "Helium Privatization Act of 1996" (P.L. 104-273) directed the United States Department of the Interior to start liquidating the reserve by 2005. Helium produced before 1945 was about 98% pure (2% nitrogen), which was adequate for airships. In 1945 a small amount of 99.9% helium was produced for welding use. By 1949 commercial quantities of Grade A 99.995% helium were available. For many years the United States produced over 90% of commercially usable helium in the world. Extraction plants created in Canada, Poland, Russia, and other nations produced the remaining helium. In the early 2000s, Algeria and Qatar were added as well. Algeria quickly became the second leading producer of helium (16% of total in 2002). Through this time helium consumption has increased, as well as costs.

Occurrence and production

Abundance

Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of all elemental matter measured by mass even though there are 8 times as many hydrogen atoms as helium ('elemental matter' does not include dark matter or dark energy, which together may account for 96% of the Universe). It is concentrated in stars, where it is formed from hydrogen by the nuclear fusion of the proton-proton chain reaction and CNO cycle. This so-called 'hydrogen burning' process provides the energy stars need to shine. According to the Big Bang model of the early development of the Universe, the vast majority of helium was formed in the first three minutes after the Big Bang. Its widespread abundance is seen as part of the evidence that supports this theory. However, in the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million at sea level and up to 15 miles (24 km), largely because most helium in the Earth's atmosphere escapes into space due to its inertness and low mass. There is a layer in the heterosphere (a part of the Earth's upper atmosphere) at 600 miles (about 1000 km) where helium is the dominant gas (although the total pressure is very low). Helium is the 71st most abundant element in the Earth's crust where it is found in 8 parts per billion (10⁹). Helium only makes up 4 parts per trillion (10¹²) in seawater. Essentially all helium on Earth is a result of radioactive decay of elements such as uranium and radon. A type of radiation called alpha particles are made of two protons and two neutrons, which also makes them helium-4 nuclei. These +2 positive ions easily gain the two electrons needed to make complete helium atoms. In this way an estimated 0.5 ft³ of helium is produced from every cubic mile of the Earth's crust (3.4 L/km³) per year . This decay product is found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl. There are also small amounts in mineral springs, volcanic gas and meteoric iron.

Production

Helium in the crust is produced by the radioactive decay of uranium and thorium which are present in varying concentrations throughout the crust, but helium migrates and can collect in certain areas when conditions are right. Thus the greatest concentrations (trace amounts up to 7% by volume) of helium on the planet are in natural gas fields, from which most commercial helium is derived. As of 2002 over 100 million m³ (3.5 billion ft³) were produced annually with 80% of production from the United States, 16% from Algeria, and most of the rest from Russia. The principal source for U.S. production is the natural gas wells of the U.S. states of Texas, Oklahoma, Arizona and Kansas. Helium is also produced in Canada, Poland, the People's Republic of China, and Qatar. Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and hydrocarbons such as methane) from natural gas in order to extract gaseous helium (the general process is called fractional distillation). The resulting crude helium gas is subjected to a process of purification in which almost all of the remaining nitrogen and other gases are precipitated out of the mixture through successive exposures to lowering temperatures. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure Grade A helium. The principal impurity in Grade A helium is neon. Diffusion of crude natural gas through special semi-permeable membranes and other barriers is another method to recover and/or purify helium. Helium can also be synthesized by bombardment of lithium-6 or boron with high-velocity neutrons in a nuclear reactor to produce He-4 and tritium. The tritium decays with a half life of 12.5 years to produce He-3. This method of production, however, is not economically viable—at least for making normal commercial-grade helium. Fusion in exploding hydrogen bombs creates helium as well.

Isotopes

Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one He-3 atom for every million He-4. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks. The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis, and its abundance serves as a test of cosmological models. Equal mixtures of liquid He-3 and He-4 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: He-4 atoms are bosons while He-3 atoms are fermions). There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, helium-3 is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth's mantle. It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived isotope is helium-5 with a half-life of 7.6×10⁻²² second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.

Precautions

The voice of a person who has inhaled helium temporarily sounds high-pitched, resembling those of the cartoon characters Alvin and the Chipmunks (although their voices were produced by shifting the pitch of normal voices). This is because the speed of sound in helium is nearly three times that in air. As a result, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. The higher perceived pitch is only due to a different frequency shaping of the voice; the fundamental frequency of the vocal cords remains more or less the same. Although the vocal effect of inhaling helium may be amusing, it can be dangerous if done to excess. The reason is not due to toxicity or any property of helium but simply due to it displacing oxygen needed for normal respiration. One must be aware that in mammals (with the notable exception of seals) the breathing reflex is not triggered by insufficient oxygen but rather excess of carbon dioxide. Unconsciousness, brain damage and even asphyxiation followed by death may result in extreme cases. Also, if helium is inhaled directly from pressurized cylinders the high flow rate can fatally rupture lung tissue. Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. At high pressures, a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome; a small proportion of nitrogen can alleviate the problem. Containers of helium gas at 5 to 10 K should be treated as if they have liquid inside. This is due to the rapid and large increases in pressure and, if allowed, volume that occur when helium gas at that temperature is warmed to room temperature. Sulfur hexafluoride has the opposite effect on the speed of sound as helium. It slows down the speed of sound to about one third of the speed of sound in air. It is also non-toxic, like helium.

References

;Prose Specific references are indicated by comments in the article source
- The Encyclopedia of the Chemical Elements, edited by Cifford A. Hampel, "Helium" entry by L. W. Brandt (New York; Reinhold Book Corporation; 1968; pages 256-267) Library of Congress Catalog Card Number: 68-29938
- Nature's Building Blocks: An A-Z Guide to the Elements, by John Emsley (New York; Oxford University Press; 2001; pages 175-179) ISBN 0-19-850340-7
- Los Alamos National Laboratory (LANL.gov): Periodic Table, "[http://periodic.lanl.gov/elements/2.html Helium]" (viewed 10 October 2002 and 25 March 2005)
- Guide to the Elements: Revised Edition, by Albert Stwertka (New York; Oxford University Press; 1998; pages 22-24) ISBN 0-19-512708-0
- The Elements: Third Edition, by John Emsley (New York; Oxford University Press; 1998; pages 94-95) ISBN 0-19-855818-X
- United States Geological Survey (usgs.gov): [http://minerals.usgs.gov/minerals/pubs/commodity/helium/heliumcs04.pdf Mineral Information for Helium] (PDF) (viewed 31 March 2005)
- [http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html The thermosphere: a part of the heterosphere], by J. Vercheval (viewed 1 Apr 2005)
- Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements, Zastenker G.N. et al., [http://www.ingentaconnect.com/content/klu/asys/2002/00000045/00000002/00378626], published in [http://www.ingentaconnect.com/content/klu/asys Astrophysics], April 2002, vol. 45, no. 2, pp. 131-142(12)
- [http://www3.interscience.wiley.com/cgi-bin/abstract/105558571/ABSTRACT Dynamic and thermodynamic properties of solid helium in the reduced all-neighbours approximation of the self-consistent phonon theory], C. Malinowska-Adamska, P. Sŀoma, J. Tomaszewski, physica status solidi (b), Volume 240, Issue 1 , Pages 55 - 67; Published Online: 19 Sep 2003
- [http://www.yutopian.com/Yuan/TFM.html The Two Fluid Model of Superfluid Helium], S. Yuan, (viewed 4 Apr 2005)
- Rollin Film Rates in Liquid Helium, Henry A. Fairbank and C. T. Lane, Phys. Rev. 76, 1209–1211 (1949), [http://prola.aps.org/abstract/PR/v76/i8/p1209_1 from the online archive]
- [http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html Introduction to Liquid Helium], at the NASA Goddard Space Flight Center (viewed 4 Apr 2005)
- [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1983ApOpt..22...10E&db_key=AST Tests of vacuum VS helium in a solar telescope], Engvold, O.; Dunn, R. B.; Smartt, R. N.; Livingston, W. C.. Applied Optics, vol. 22, Jan. 1, 1983, p. 10-12.
-
- [http://www.mantleplumes.org/HeliumFundamentals.html Helium: Fundamental models], Don L. Anderson, G. R. Foulger & Anders Meibom (viewed 5 Apr 2005)
- [http://www.scuba-doc.com/HPNS.html High Pressure Nervous Syndrome], Diving Medicine Online (viewed 5 Apr 2005) ;Table
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes] Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989
- WebElements.com and EnvironmentalChemistry.com per the guidelines at [http://en.wikipedia.org/wiki/Wikipedia:WikiProject_Elements Wikipedia's WikiProject Elements] (viewed 10 October 2002)
External links
;General
- [http://www.webelements.com/webelements/elements/text/He/key.html WebElements: Helium]
- [http://education.jlab.org/itselemental/ele002.html It's Elemental – Helium]
- [http://theodoregray.com/PeriodicTable/Elements/002/ Photos and applications of Helium] ;More detail
- [http://boojum.hut.fi/research/theory/helium.html Helium] at the Helsinki University of Technology; includes pressure-temperature phase diagrams for helium-3 and helium-4. ;Miscellaneous
- [http://www.cganet.com/N2O/helium_safety.asp Helium Safety] regarding inhalation
- [http://www.phys.unsw.edu.au/PHYSICS_!/SPEECH_HELIUM/speech.html Physics in Speech] with audio samples that demonstrate the unchanged voice pitch
- [http://www.du.edu/~jcalvert/phys/helium.htm Article about Helium and other noble gases]
- [http://www.fluidmech.net/msc/super/super-f.htm this article contains phase diagram for helium] Category:Noble gases ko:헬륨 ms:Helium ja:ヘリウム simple:Helium th:ฮีเลียม

Water

:This article focuses on water as it is experienced in everyday life. See water (molecule) for information on the chemical and physical properties of pure water (H₂O, hydrogen oxide). Water (from the Old English word wæter; c.f German "Wasser", from PIE
- wod-or, "water") is a tasteless, odorless, and nearly colorless (it has a slight hint of blue) substance in its pure form that is essential to all known forms of life and is known also as the most universal solvent. Water is an abundant substance on Earth. It exists in many places and forms. It appears mostly in the oceans and polar ice caps, but also as clouds, rain water, rivers, freshwater aquifers, and sea ice. On the planet, water is continuously moving through the cycle involving evaporation, precipitation, and runoff to the sea. Water fit for human consumption is called potable water. This natural resource is becoming more scarce in certain places as human population in those places increases, and its availability is a major social and economic concern.

Molecular properties

Forms of water

potable water] Water takes many different shapes on earth: water vapor and clouds in the sky, waves and icebergs in the sea, glaciers in the mountain, aquifers in the ground, to name but a few. Through evaporation, precipitation, and runoff, water is continuously flowing from one form to another, in what is called the water cycle. Because of the importance of precipitation to agriculture, and to mankind in general, different names are given to its various forms: while rain is common in most countries, other phenomena are quite surprising when seen for the first time. Hail, snow, fog or dew are examples. When appropriately lit, water drops in the air can refract sunlight to produce rainbows. Similarly, water runoffs have played major roles in human history as rivers and irrigation brought the water needed for agriculture. Rivers and seas offered opportunity for travel and commerce. Through erosion, runoffs played a major part in shaping the environment providing river valleys and deltas which provide rich soil and level ground for the establishment of population centers. Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells. Because water can contain many different substances, it can taste or smell very differently. In fact, humans and other animals have developed their senses to be able to evaluate the drinkability of water: animals generally dislike the taste of salty sea water and the putrid swamps and favor the purer water of a mountain spring or aquifer.

Water in biology

From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. Water carries out this role by allowing organic compounds to react in ways that ultimately allows replication. It is a good solvent and has a high surface tension, and thus allows organic compounds and living things to be transported in it. Fresh water has its greatest density at 4°C, then becoming less dense as it freezes or heats up from this point. As a stable, polar molecule prevalent in the atmosphere, it plays an important atmospheric role as an absorber of infrared radiation, crucial in the atmospheric greenhouse effect without of which, the average surface temperature would be −18° Celsius. Water also has an unusually high specific heat, which plays many roles in regulating global and regional climate, such as the Gulf Stream climate, allowing life to survive. Water is a very good solvent, chemically not unlike ammonia, and dissolves many types of substances, such as various salts and sugar, and facilitates their chemical interaction, which aids complex metabolisms. Some substances, however, do not mix well with water, including oils and other hydrophobic substances. Cell membranes, composed of lipids and proteins, take advantage of this property to carefully control interactions between their contents and external chemicals. This is facilitated somewhat by the surface tension of water. Water drops are stable due to the high surface tension of water caused by the strong intermolecular forces called cohesive forces. This can be seen when small quantities of water are put onto a nonsoluble surface such as polythene: the water stays together as drops. On extremely clean glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces. This property plays a key role in plant transpiration. A simple but environmentally important and unique property of water is that its common solid form, ice, floats on the liquid. This solid phase is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances and for all other 11 uncommon phases of water ice except ice-XI, the solid form is more dense than the liquid form. Fresh water is most dense at 4°C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4°C. This effectively insulates a lake floor from the cold. While this behavior may seem obvious, even intuitive, it should be noted that almost all other chemicals are denser as solids than they are as liquids, and freeze from the bottom up. Life on earth has evolved with and adapted itself to the important features of water. The existence of abundant liquid, vapor and solid forms of water on Earth has been an important factor in the abundant colonization of Earth's various environments by life-forms adapted to those varying and often extreme conditions. Civilizations have historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, is situated between two major rivers. Large metropolises like London, Paris, New York, and Tokyo owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore and Hong Kong, have flourished for precisely this reason. In places such as North Africa and the Middle East, where water is scarcer, access to clean drinking water was and is a major factor in human development.

Astronomical position of Earth and impact on its water

Mesopotamia The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to the origin, evolution, and continued existence of life on Earth. However, if the Earth's location in the solar system were even marginally closer or further from the Sun (ie, a million miles or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist. Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provides a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were less massive, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars). According to the solar nebula model of the solar system's formation, Earth's mass may be largely due to its distance from the Sun. The distance between Earth and the Sun and the combination of solar radiation received and the greenhouse effect of the atmosphere ensures that its surface is neither too cold nor too hot for liquid water. If Earth were more distant, most water would be frozen. If Earth were nearer to the Sun, its higher surface temperature would limit the formation of ice caps, or cause water to exist only as vapor. In the former case, the low albedo of oceans would cause Earth to absorb more solar energy. In the second case, a runaway greenhouse effect and inhospitable conditions similar to Venus would result. It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying solar flux, indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.

Human uses of water

Gaia hypothesis All known forms of life depend on water. Water is a vital part of many metabolic processes within the body. Significant quantities of water are used during the digestion of food. (Note however that some bacteria and plant seeds can enter a cryptobiotic state for an indefinite period when dehydrated, and come back to life when returned to a wet environment) About 72% of the fat free mass of the human body is made of water. To function properly the body requires between one and seven litres of water per day to avoid dehydration, the precise amount depending on the level of activity, temperature, humidity, and other factors. It is not clear how much water intake is needed by healthy people. However, for those who do not have kidney problems, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) dangerous to drink too little. People do often drink far more water than necessary while exercising, however, putting them at risk of water intoxication, which is frequently fatal. The "fact" that a person should consume eight glasses of water per day cannot be traced back to a scientific source. However, leading dieticians and nutritionists will tell you that this is the RDI (Recommended Daily Intake) of water. [http://ajpregu.physiology.org/cgi/content/full/283/5/R993]. The latest dietary reference intake report by the National Research Council recommended 2.7 liters of water total (including food sources) for women and 3.7 liters for men[http://www.iom.edu/report.asp?id=18495]. Water is lost from the body in urine and feces, through sweating, and by exhalation of water vapor in the breath. Humans require water that does not contain too much salt or other impurities. Common impurities include chemicals and/or harmful bacteria, such as crypto sporidium. Some solutes are acceptable and even desirable for perceived taste enhancement and to provide needed electrolytes.

Water as a precious resource

:See water resources for information about fresh water supplies. fresh water Because of the growth of world population and other factors, the availability of drinking water per capita is shrinking. The issue of water shortage can be solved through more production, better distribution and less waste of it. For this reason, water is a strategic resource for many countries. Many battles and wars, such as the Six-Day War in the Middle East, have been fought to gain access to it. Experts predict more trouble ahead because of the world's growing population, increasing contamination through pollution, and global warming. UNESCO's World Water Development Report (WWDR, 2003) from its World Water Assessment Program indicates that, in the next 20 years, the quantity of water available to everyone is predicted to decrease by 30%. 40% of the world's inhabitants currently have insufficient fresh water for minimal hygiene. More than 2.2 million people died in 2000 from diseases related to the consumption of contaminated water or drought. In 2004, the UK charity WaterAid reported that a child dies every 15 seconds due to easily preventable water-related diseases. Some have predicted that clean water will become the "next oil", making Canada, with this resource in abundance, possibly the richest country in the world.

Regulating water distribution

Drinking water is often collected at springs or extracted from artificial borings in the ground, or wells. Building more wells in adequate places is thus a possible way to produce more water assuming the aquifers can supply an adequate flow. Other water sources are the rainwater and river or lake water. This surface water, however, must be purified for human consumption. This may involve removal of undissolved substances, dissolved substances and harmful microbes. Popular methods are filtering with sand which only removes undissolved material while chlorination and boiling kill harmful microbes. Distillation does all three functions. More advanced techniques exist, such as reverse osmosis. Desalination of abundant ocean or seawater is a more expensive solution used in coastal arid climates. The distribution of drinking water is done through municipal water systems or as bottled water. Governments in many countries have programs to distribute water to the needy at no charge. Others argue that the market mechanism and free enterprise are best to manage this rare resource, and to finance the boring of wells or the construction of dams and reservoirs. Reducing waste, that is using drinking water only for human consumption, is another option. In some cities, such as Hong Kong, sea water is extensively used for flushing toilets citywide in order to conserve fresh water resources. Polluting water may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the pollutor. Pharmaceuticals consumed by humans often end up in the waterways and can have detrimental effects on aquatic life if they bioaccumulate and if they are not biodegradable.

The impact of water on human culture

Water is considered a purifier in most religions, including Christianity, Islam, Judaism, and Shinto. For instance, baptism in Christian churches is done with water. In addition, a ritual bath in pure water is performed for the dead in many religions including Judaism and Islam. In Islam, the daily Salah can only be done after ablution (Wodoo), that is, washing parts of the body in clean water. In Shinto, water is used in almost all rituals to cleanse a person or an area. Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganga is also personified as a goddess. Alternatively, gods can be patrons of particular springs, river or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids. The Greek philosopher Empedocles held that water is one of the four classical elements along with fire, earth and air, and was regarded as the ylem, or basic stuff of the universe. Water was considered cold and moist. In the theory of the four bodily humours, water was associated with phlegm. Water was also one of the Five Elements in traditional Chinese philosophy, along with earth, fire, wood, and metal. A common misconception about water is that it is a powerful conductor of electricity. Any electrical properties observable in water are due to the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize (two water molecules become one hydroxide anion and one hydronium cation), but only at a very slight, almost immeasurable level. Pure water can also be electrolized into oxygen and hydrogen gases but without any dissolved ions, this is a very slow process and thus very little current is conducted. Many bottled water companies exploit another common misconception, advertising both purity and