<P> Helium is the least water - soluble monatomic gas, and one of the least water - soluble of any gas (CF, SF, and C F have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x / 10, respectively, versus helium's 0.70797 x / 10), and helium's index of refraction is closer to unity than that of any other gas . Helium has a negative Joule--Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand . Only below its Joule--Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion . Once precooled below this temperature, helium can be liquefied through expansion cooling . </P> <P> Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium . In a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized . The charged particles are highly influenced by magnetic and electric fields . For example, in the solar wind together with ionized hydrogen, the particles interact with the Earth's magnetosphere, giving rise to Birkeland currents and the aurora . </P> <P> Unlike any other element, helium will remain 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) at about 25 bar (2.5 MPa) of pressure . 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, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30% . With a bulk modulus of about 27 MPa it is ~ 100 times more compressible than water . Solid helium has a density of 0.214 ± 0.006 g / cm at 1.15 K and 66 atm; the projected density at 0 K and 25 bar (2.5 MPa) is 0.187 ± 0.009 g / cm . At higher temperatures, helium will solidify with sufficient pressure . At room temperature, this requires about 114,000 atm . </P> <P> Below its boiling point of 4.22 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium - 4 exists in a normal colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered . Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further . </P>

Where is helium found and in what form