Question

State one relevant observation for the following:

Concentrated nitric acid appears yellow, when it is left for a while in a glass bottle.

Answer 1

Pure nitric acid [HNO₃] is colourless and unstable and decomposes slightly even at ordinary temperatures and in the presence of sunlight. The decomposition forms reddish-brown nitrogen dioxide [NO₂], which remains dissolved in the acid and imparts a slight yellowish-brown colour.