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. 

(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? 
 

Hint:

Nitric acid is a powerful oxidizing agent. Its yellow color is purely due to dissolved NO_2 impurity; pure nitric acid is actually colorless!

Answer 1

(a) Error made by Anil:
Anil stored nitric acid in a transparent container exposed to sunlight.
This is incorrect because nitric acid is photosensitive and decomposes in the presence of light.
It should always be stored in dark coloured (amber) bottles to prevent decomposition.

(b) Reason for brown fumes:
The brown fumes observed are due to the formation of nitrogen dioxide (NO₂) gas.
Nitric acid decomposes in the presence of sunlight as follows:

4HNO₃ → 2H₂O + 4NO₂ + O₂

Nitrogen dioxide is a brown coloured gas, which causes the observed fumes.

(c) Removal of yellowish tinge:
The yellowish colour is due to dissolved NO₂ in nitric acid.
This colour can be removed by bubbling air through the acid.
The air helps to remove dissolved NO₂, restoring the acid to its original colourless state.

Final Answers:
(a) Stored nitric acid in a transparent container exposed to sunlight
(b) Due to decomposition forming brown NO₂ gas
(c) Remove by bubbling air through the acid