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Question
Give reason why concentrated nitric acid prepared in the laboratory is yellow in colour. State how the colour of the acid is removed.
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Solution
The acid prepared in the laboratory decomposes slightly due to heat, releasing brown nitrogen dioxide (NO2) gas. This gas dissolves back into the acid, giving it a yellow colour.
\[\ce{4HNO3 ->[\Delta] 2H2O + 4NO2 + O2}\]
The yellow colour can be removed by bubbling dry air or carbon dioxide (CO2) gas through the acid to drive out the dissolved NO2, or by adding a little water, which converts the NO2 back into nitric acid.
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