Anil is a laboratory assistant in a college. He prepared nitric acid but stored it in a transparent container exposed to sunlight. After a few days, brown fumes were observed in the container and the acid turned slightly yellowish in colour.
(a) What was the error made by Anil while storing the acid?
(b) Why were brown fumes observed in the container?
(c) How can the yellowish tinge in the acid be removed?
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(a) The error made by Anil was that he stored the nitric acid in a transparent container exposed to sunlight. Nitric acid should be stored in a dark-coloured (brown) reagent bottle kept in a cool and dark place to prevent decomposition.
(b) Nitric acid is unstable and decomposes on exposure to light and heat to release reddish-brown nitrogen dioxide (NO2) gas, water and oxygen:
4HNO3 $\xrightarrow{\text{light/heat}}$ 4NO2 ↑ + 2H2O + O2 ↑
The brown fumes seen in the container are due to NO2 gas.
(c) The yellowish tinge in nitric acid (caused by dissolved NO2) can be removed by bubbling air or carbon dioxide gas through the acid. This drives out the dissolved nitrogen dioxide and the acid becomes colourless again.