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ICSE Class 10 Chemistry Question 14 of 63

Study of Compounds — Nitric Acid — Question 9

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Question

Question 5c

Conc. nitric acid prepared in the laboratory is yellow in colour. Why? How is this colour removed?

Answer

Concentrated nitric acid prepared in the laboratory is yellow in color due to dissolution of reddish brown nitrogen dioxide gas in the acid. This gas is produced due to the thermal decomposition of a portion of nitric acid.

4HNO3 ⟶ 2H2O + 4NO2 + O2

The yellow color of the acid is removed :

  1. If dry air or CO2 is bubbled through the yellow acid, the latter turns colourless because it drives out NO2 gas from warm acid, which is further oxidised to nitric acid.
  2. By adding excess water, nitrogen dioxide gas dissolves in water and thus yellow color of acid is removed.