An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to as the inert gases. Inert gases are used generally to avoid unwanted chemical reactions degrading a sample. These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in air. The term inert gas is context-dependent because several of the noble gases can be made to react under certain conditions. Purified argon gas is the most commonly used inert gas due to its high natural abundance (78.3% N2, 1% Ar in air) and low relative cost. Unlike noble gases, an inert gas is not necessarily elemental and is often a compound gas. Like the noble gases, the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases. This is a tendency, not a rule, as noble gases and other "inert" gases can react to form compounds. Need/Necessity The inert gases are obtained by fractional distillation of air, with the exception of helium which is separated from a few natural gas sources rich in this element, through cryogenic distillation or membrane separation. For specialized applications, purified inert gas shall be produced by specialized generators on-site. They are often used by chemical tankers and product carriers (smaller vessels). Benchtop specialized generators are also available for laboratories. Applications Because of the non-reactive properties of inert gases, they are often useful to prevent undesirable chemical reactions from taking place. Food is packed in an inert gas to remove oxygen gas. This prevents bacteria from growing. It also prevents chemical oxidation by oxygen in normal air. An example is the rancidification (caused by oxidation) of edible oils. In food packaging, inert gases are used as a passive preservative, in contrast to active preservatives like sodium benzoate (an antimicrobial) or BHT (an antioxidant). Historical documents may also be stored under inert gas to avoid degradation. For example, the original documents of the U.S. Constitution are stored under humidified argon. Helium was previously used, but it was less suitable because it diffuses out of the case more quickly than argon. Inert gases are often used in the chemical industry. In a chemical manufacturing plant, reactions can be conducted under inert gas to minimize fire