An inert gas is a gas which does not undergo chemical reactions under a set of given conditions. The noble gases and nitrogen often do not react with many substances. 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 nitrogen gas and several of the noble gases can be made to react under certain conditions.
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.
The inert gases are obtained by fractional distillation of air. For specialized applications, purified nitrogen gas may be produced by specialized generators on-site. They are often used aboard chemical tankers and product carriers (smaller vessels). Benchtop nitrogen generators are also available for laboratories.
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 nitrogen to remove oxygen gas. This prevents bacteria from growing. Chemical oxidation by oxygen in air is avoided. An example is the rancidification of oil. 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Historical documents may also be stored under an inert gas to avoid degradation. For example, the U.S. Constitution is stored under humidified argon. Helium was previously used, but it was less suitable because it diffuses out of the case more quickly than argon.
Nitrogen is often used as an inert gas in the chemical industry. In a chemical manufacturing plant, reactions are routinely conducted under nitrogen even if they are not sensitive to air, to minimize fire hazards. In such plants and in oil refineries, transfer lines and vessels are purged with inert gas, to avoid residual solvents or process fluids catching fire. At the bench scale, chemists perform experiments on air-sensitive compounds using air-free techniques developed to handle them under inert gas.
Inert gas systems on ships 
Inert gas is produced on board crude oil carriers (above 20000 tonnes) by using either a flue gas system or by burning kerosene in a dedicated inert gas generator. The inert gas system is used to prevent the atmosphere in cargo tanks or bunkers from coming into the explosive range. IG keeps the oxygen content of the tank atmosphere below 8% (on crude carriers, less for product carriers and gas tankers), thus making any air/hydrocarbon gas mixture in the tank too lean to ignite. IG is most important during discharging and during the ballast voyage when more hydrocarbon vapour is likely to be present in the tank atmosphere. Inert gas can also be used to purge the tank of the volatile atmosphere in preparation for gas freeing - replacing the atmosphere with breathable air - or vice versa.
The flue gas system uses the boiler exhaust as its source, so it is important that the fuel/air ratio in the boiler burners is properly regulated to ensure that high quality inert gas is produced. Too much air would result in an oxygen content exceeding 5%, too much fuel oil would result in carryover of dangerous hydrocarbon gas. The flue gas is cleaned and cooled by the scrubber tower. Various safety devices prevent overpressure, return of hydrocarbon gas to the engine room, or supply of IG with too high oxygen content.
Gas tankers and product carriers cannot rely on flue gas systems (because they require IG with O2 content of 1% or less) and so use inert gas generators instead. The inert gas generator consists of a combustion chamber and scrubber unit supplied by fans and a refrigeration unit which cools the gas. A drier in series with the system removes moisture from the gas before it is supplied to the deck. Cargo tanks on gas carriers are not inerted, but the hold space around them is. This arrangement allows the tanks to be kept cool using a small heel of cargo while the vessel is in ballast while retaining the explosion protection provided by the inert gas.
In gas tungsten arc welding (GTAW), inert gases are used to shield the tungsten from contamination. It also shields the fluid metal (created from the arc) from the reactive gases in air which can cause porosity in the solidified weld puddle. Inert gases are also used in gas metal arc welding (GMAW) for welding non-ferrous metals. Some gases which are not usually considered inert but which behave like inert gases in all the circumstances likely to be encountered in some use can often be used as a substitute for an inert gas. This is useful when an appropriate pseudo-inert gas can be found which is inexpensive and common. For example, carbon dioxide is sometimes used in gas mixtures for GMAW because it is not reactive to the weld pool created by arc welding. But it is reactive to the arc. The more carbon dioxide that is added to the inert gas, such as argon, will increase your penetration. The amount of carbon dioxide is often determined by what kind of transfer you will be using in GMAW. The most common is spray arc transfer, and the most commonly used gas mixture for spray arc transfer is 85% argon and 15% carbon dioxide. (Listed as many different names depending on the gas supplier).
In underwater diving an inert gas is a component of the breathing mixture which is not metabolically active, and serves to dilute the gas mixture. The inert gas may have effects on the diver, but these are thought to be mostly physical effects, such as tissue damage caused by bubbles in decompression sickness. The most common inert gases used in breathing gases for diving are nitrogen and helium.
See also 
- inerting system for aircraft
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