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Helium (Element): Liquid Helium
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Helium is a colorless, odorless, tasteless gas. It has a number of unusual properties. For example, it has the lowest boiling point of any element, -268.9°C (-452.0°F). The boiling point for a gas is the temperature at which the gas changes to a liquid. The freezing point of helium is -272.2°C (-458.0°F). Helium is the only gas that cannot be made into a solid simply by lowering the temperature.
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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 ... needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. It has a second, rare, stable isotope which is called helium-3. The behavior of liquid helium-4's two fluid phases- 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).
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Helium has the lowest melting and boiling point of any element. Liquid Helium is called a "quantum fluid" as it displays atomic properties on a macroscopic scale. The viscosity of liquid helium is 25 micropoises (water has a viscosity of 10,000 micropoises). As helium is cooled below its transition point, it has an unusual property of superfluidity with a viscosity approaching zero micropoises. In addition, liquid helium has extremelyhigh thermal conductivity.
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Helium has other peculiar properties: It is the only liquid that cannot be solidified by lowering the temperature. It remains liquid down to absolute zero at ordinary pressures, but will readily solidify by increasing the pressure. Solid 3He and 4He are unusual in that both can be changed in volume by more than 30% by applying pressure.
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Helium II ... exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, B. V. Rollin.[7][8] As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate.
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Helium solidifies only under great pressure. The resulting colorless, almost invisible solid is highly compressible; applying pressure in the laboratory can decrease its volume by more than 30%.[2] With a bulk modulus on the order of 5×107 Pa[3] it is 50 times more compressible than water. Unlike any other element, helium will fail to solidify and remain a liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure.[4] It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure.
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Liquid Helium