Explain the following : Concentrated nitric acid appears yellow w…

Question

Explain the following :
Concentrated nitric acid appears yellow when it is left standing in a glass bottle.

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Answer

Although pure concentrated nitric acid is colourless, it appears yellow when left standing in a glass bottle due to the dissolution of reddish brown nitrogen dioxide gas in the acid. Nitrogen dioxide is produced because of the thermal decomposition of a portion of nitric acid.
$4HNO_3 → 2H_2O + 4NO_2 + O_2$

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