Question

Give reason why concentrated nitric acid prepared in the laboratory is yellow in colour. State how the colour of the acid is removed.

Answer 1

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.