Question

Explain why nitric acid kept in a reagent bottle for a long time turns brown.

Answer 1

• Due to the presence of dissolved nitrogen dioxide nitric acid appears in yellowish-brown colour.
• Nitric acid decomposes slowly even at room temperature in the presence of sunlight.
• $\underset{\mathrm{nitric}\mathrm{acid}}{\underset{}{4{\mathrm{HNO}}_3\left(\mathrm{l}\right)}}\to \underset{\mathrm{nitrogen}\mathrm{dioxide}}{\underset{}{4{\mathrm{NO}}_2\left(\mathrm{g}\right)}}+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\left(\mathrm{g}\right)$
• The liberated nitrogen dioxide dissolves in the acid and gives yellow colour brown vapours are seen in the bottle and the nitric acid turns yellowish in colour.
• Hence,Nitric acid is stored in dark bottles to prevent sunlight from entering the bottle and decomposing the acid.