HELIUM

 

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Helium is a colorless, odorless, tasteless chemical element. It is the most unreactive of the noble gases and therefore the least chemically-active chemical element on the periodic table. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. At temperatures near absolute zero, it is a superfluid, a nearly frictionless phase of matter with unusual properties.

 

After hydrogen, helium is the second lightest element and the second most abundant element in the universe, created during big bang nucleosynthesis and to a lesser extent from nuclear fusion of hydrogen in stars. On Earth, helium is primarily a product of the radioactive decay of much heavier elements, which emit helium nuclei called alpha particles; it is found in significant amounts only in natural gas, from which it is extracted at low temperatures by fractional distillation.

 

First detected in 1868 by French astronomer Pierre Janssen as an unknown yellow spectral line signature in the light of a solar eclipse, helium was separately identified as a new element later that year by English astronomer Norman Lockyer. Its presence in natural gas in large, useable amounts was identified in 1905. Helium is used in cryogenics, as a deep-sea breathing gas, for inflating balloons and airships, and as a protective gas for many industrial purposes, such as arc welding. Inhaling a small amount of the gas temporarily changes the frequency of a person's voice; however, caution must be exercised as helium is an asphyxiant.

 

 

Helium Ballon or Airship

 

Because of its low density, helium is the gas of choice

to fill airships such as the Holden Airship

 

 

Applications

 

Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. Pressurized helium is commercially available in large quantities.

 

  • Because it is lighter than air, airships and balloons are inflated with helium for lift. In airships, helium is preferred over hydrogen because it is not flammable and has 92.64% of the lifting power of the alternative hydrogen.

  • For its low solubility in water, the major part of human blood, air mixtures of helium with oxygen and nitrogen (Trimix), with oxygen only (Heliox), with common air (heliair), and with hydrogen and oxygen (hydreliox), are used in deep-sea breathing systems to reduce the high-pressure risk of nitrogen narcosis, decompression sickness, and oxygen toxicity.

  • At extremely low temperatures, liquid helium is used to cool certain metals to produce superconductivity, such as in superconducting magnets used in magnetic resonance imaging. Helium at low temperatures is also used in cryogenics.

  • For its inertness and high thermal conductivity, helium is used as a coolant in some nuclear reactors, such as pebble-bed reactors, and in arc welding air-sensitive metals.

  • Because it is inert, helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, in gas chromatography, and as an atmosphere for protecting historical documents. This property also makes it useful in supersonic wind tunnels.

  • 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 pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 13 million cubic feet (370,000 m) of helium to launch.

  • The gain medium of the helium-non laser is a mixture of helium and neon.

  • Because it diffuses through solids at a rate three times that of air, helium is used to detect leaks in high-vacuum equipment and high-pressure containers.

  • Because of its extremely low index of refraction, the use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes.

  • The age of rocks and minerals that contain uranium and thorium, radioactive elements that emit helium nuclei called alpha particles, can be discovered by measuring the level of helium with a process known as helium dating.

  • Because helium alone is less dense than atmospheric air, it will change the timbre (not pitch ) of a person's voice when inhaled. However, inhaling it from a typical commercial source, such as that used to fill balloons, can be dangerous due to the number of contaminants that may be present. These could include trace amount of other gases, in addition to aerosolized lubricating oil.

  • 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.

 

 

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