Question

Give reason for the following:
Commercial concentrated nitric acid is yellow in colour, but when it is dilute with water, it turns colourless.
Give one chemical test for nitric acid.

Answer 1

Commercial nitric acid is yellow in colour because of presence of nitrogen dioxide but when it is dilute with water, it turns colourless because nitrogen dioxide gets dissloved in water.
Brown ring test can be used to test nitric acid:
Brown ring test : Nitric acid in a test tube is taken and then and then freshly prepared ferrous sulphate solution is added. Concentrated H₂SO₄ is then added carefully down the sides of the test tube, dark brown ring is formed at the junction of two layers.
\[\ce{6FeSO4 + 3H2SO4 + 2HNO3 -> 3Fe2(SO4)3 + 2NO + 4H2O}\]

\[\ce{FeSO4 + NO + 5H2O -> \underset{\text{Hydrated nitrosoferrous sulphate(brown ring)}}{[Fe(NO)(H2O)5]SO4}}\]