Question

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.

1. What was the error made by Anil while storing the acid?
2. Why were brown fumes observed in the container?
3. How can the yellowish tinge in the acid be removed?

Answer 1

(a) The mistake Anil made was storing nitric acid in a transparent container exposed to sunlight. Nitric acid should be stored in dark-colored (amber) bottles and kept away from light.

(b) The brown fumes were due to the formation of nitrogen dioxide (NO₂). When nitric acid is exposed to sunlight, it decomposes:

\[\ce{4HNO3 -> 2H2O + 4NO2 + O2}\]

The NO₂ gas is brown in color, which is why fumes were observed.

(c) The yellowish tinge in the acid is also due to dissolved NO₂. It can be removed by passing a stream of air through the acid (or gently heating), which drives out the dissolved nitrogen dioxide.