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Why is air considered as a mixture?

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Hint: A mixture is formed when two or more different substances combine together such that their individual characteristics are retained. Mixing of substances does not involve any chemical reaction and it is usually possible to separate the individual components of a mixture by physical separation methods.

Complete answer:
> Air is a homogenous and colourless mixture consisting of gaseous nitrogen (\[{{N}_{2}}\]), oxygen (\[{{O}_{2}}\]), carbon dioxide (\[C{{O}_{2}}\]), argon (Ag) and some trace amounts of water vapour, methane (\[C{{H}_{4}}\]), ammonia (\[N{{H}_{3}}\]), etc. Air is considered as a mixture because of the following reasons:
> It is possible to separate air into its constituent gases by means of physical methods like fractional distillation. The boiling point of nitrogen (77.3 K) is less than that of oxygen (90 K). So, when liquefied air is distilled, nitrogen is separated out first and oxygen is left in the mixture. No energy or heat is either absorbed or evolved while mixing gases.
> The identities of the constituent gases in air are retained. That is why air shows the chemical and physical properties of all the gases present in it. For example, \[C{{O}_{2}}\] in air can slowly turn the lime water \[Ca{{(OH)}_{2}}\] dissolved in water) milky like pure \[C{{O}_{2}}\] does following the chemical reaction:

\[Ca{{(OH)}_{2}}(aq)+C{{O}_{2}}(g)\to CaC{{O}_{3}}(s)\downarrow +{{H}_{2}}O(l)\]

> The solution appears milky because of the insoluble calcium carbonate. Oxygen supports combustion and air also is a supporter of combustion.
> The composition of air is not fixed but varies with time and place. Therefore, it cannot be defined by a fixed chemical formula. This is the property of a mixture.
> Liquid air does not boil at a particular temperature. And mixtures do not have constant boiling or melting points.
> Based on the above arguments, air possesses all the properties of a mixture. Thus, air is regarded as a mixture.

Note: Air is a mixture and not a compound. Different substances react chemically form a compound which has properties different than that of the individual components. Also, a compound has a definite formula and cannot be broken into its constituents by physical methods of separation.
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