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And if so, does it come in a liquid form? Thank you for your time

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closed as too broad by Jan, Wildcat, M.A.R. ಠ_ಠ, Todd Minehardt, ron Oct 13 '15 at 14:09

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    $\begingroup$ Need it be reversible? $\endgroup$ – ron Oct 13 '15 at 0:06
  • $\begingroup$ It's too broad, there are lots of things that react with oxygen, changing color. $\endgroup$ – Mithoron Oct 13 '15 at 11:02
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Many compounds change colour when exposed to 'air', however, this is usually due to the presence of either water or oxygen, rather than 'air'.

To give a few examples:

*when distilling a solvent such as THF over sodium/potassium, benzophenone is used as an indicator. In the absence of air, the indicator leaves the solution a deep purple colour, however if an excess of air is present, the indicator turns a clear/brown colour.

*drierite is an indicator often used in inert gas drying tubes. In its dry form, it is either deep blue or orange, however when wet it turns white/pale orange.

Several other indicators exist, however its worth noting that air is merely a mixture of multiple things, and as such any indicator will indicate for a specific component of air rather than air as a whole- with water and oxygen being the most common things to indicate for.

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