Helium exists in liquid form only at very low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... A liquid will assume the shape of its container. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid. ... Chemistry In chemistry, a critical point is the conditions ( temperature, pressure) at which the liquid state of the matter ceases to exist. ... Isotopes are forms of a chemical element whose nuclei have the same atomic number, Z, but different atomic masses, A. The word isotope, meaning at the same place, comes from the fact that all isotopes of an element are located at the same place on the periodic table. ...

Helium-4 was first liquified in 1908 by Kamerlingh Onnes. Liquid helium-4 is commercially important today because of its use as a cryogenic refrigerants for conventional superconducting magnets such as those used in MRI, NMR, quantum dots, and amorphous solid waters. It is liquified using the Hampson-Linde cycle. Heike Kamerlingh Onnes (September 21, 1853 – February 21, 1926) was a Dutch physicist. ... Cryogenics is the study of very low temperatures or the production of the same, and is often confused with cryobiology, the study of the effect of low temperatures on organisms, or the study of cryopreservation. ... Refrigeration (from the Latin frigus, frost) is generally the cooling of a body by the transfer of a portion of its heat away from it. ... Superconducting magnets are electromagnets that are partially built from superconducting materials and therefore reach much higher magnetic field intensity. ... For other meanings see Magnetic Resonance Imaging (MRI). ... NMR may refer to: Nuclear magnetic resonance, a phenomenon involving the interaction of atomic nuclei and external magnetic fields Nielsen Media Research, a U.S. company which measures TV, radio and newspaper audiences This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the... A quantum dot is a potential well that confines electrons in three dimensions to a region of the order of the electrons de Broglie wavelength in size, a few nanometers in a semiconductor. ... When water is cooled below its normal freezing point, it normally freezes to form hexagonal ice, or Ice I, though it can exist in other solid forms. ...

The temperatures required to liquify helium are low because of the weakness of the attraction between helium atoms. The interatomic forces are weak in the first place because helium is a noble gas. But the interatomic attraction is reduced even further by quantum effects, which are important in helium because of its low atomic mass. The zero point energy of the liquid is less if the atoms are less confined by their neighbors; thus the liquid can lower its ground state energy by increasing the interatomic distance. But at this greater distance, the effect of interatomic forces is even weaker. Properties For alternative meanings see atom (disambiguation). ... Intermolecular forces are electromagnetic forces which act between molecules or between widely separated regions of a macromolecule. ... The noble gases are the chemical elements in group 18 (old-style Group 0) of the periodic table. ... The word quantum, pl. ... The atomic mass of an element (also known as the relative atomic mass or average atomic mass or atomic weight) is the average atomic mass of all the chemical elements isotopes as found in a particular environment, weighted by isotopic abundance. ... In a quantum mechanical system such as the particle in a box or the quantum harmonic oscillator, the lowest possible energy is called the zero-point energy. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ...

Because of the weak interatomic forces, helium remains liquid down to absolute zero; it does not solidify at any pressure below the melting pressure (see table below). At sufficiently low temperature, both helium-3 and helium-4 undergo a transition to a superfluid phase (see table below). In physics, absolute zero is a fundamental lower bound on the temperature of a macroscopic system. ... A solid is a state of matter, characterized by a definite volume and a definite shape (i. ... Pressure is the application of force to a surface, and the concentration of that force in a given area. ... Physics In physics, melting is the process of heating a solid substance to a point (called melting point) where it turns liquid. ... Superfluidity is a phase of matter characterised by the complete absence of viscosity. ...

Liquid helium-3 and helium-4 are not completely miscible below 0.9 K at the saturated vapor pressure. Below this temperature a mixture of the two isotopes undergoes phase separation into a lighter normal fluid that is mostly helium-3, and a denser superfluid that is mostly helium-4. (This occurs because the system can lower its entropy by separating.) At low temperatures, the helium-4 rich phase may contain up to 6% of helium-3 in solution, which makes possible the existence of the dilution refrigerator, capable of reaching temperatures of a few mK above absolute zero. The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor. ... The thermodynamic entropy S, often simply called the entropy in the context of thermodynamics, can be defined in two ways: 1. ... A dilution refrigerator is a cryogenic device first proposed by Heinz London. ... In physics, absolute zero is a fundamental lower bound on the temperature of a macroscopic system. ...

Helium-4 is a non-radioactive and light isotope of helium. ... Helium-3 is a non-radioactive and light isotope of helium. ... The critical temperature of a material is the temperature above which distinct liquid and gas phases do not exist. ... The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid. ... Physics In physics, melting is the process of heating a solid substance to a point (called melting point) where it turns liquid. ... Pressure is the application of force to a surface, and the concentration of that force in a given area. ... Superfluidity is a phase of matter characterised by the complete absence of viscosity. ... The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor. ... Current flowing through a wire produces a magnetic field (M) around the wire. ...

References

• The Properties of Liquid and Solid Helium, J. Wilks (1987). Oxford: Clarendon Press. ISBN 0198512457.
• http://boojum.hut.fi/research/theory/he3.html

See Also

• Liquid nitrogen
• Liquid hydrogen
• Superfluid
• Liquid air
• Cryogen