Explain why

(a) Only all-glass apparatus should be used for the preparation of nitric acid by heating concentrated sulphuric acid and potassium nitrate.

(b) Nitric acid is kept in a reagent bottle for a long time.

(c) Iron is rendered passive with fuming HNO₃. Give reason.

(d) Dil. HNO₃ is generally considered a typical acid but not so in it's reaction with metals.

(e) When it is left standing in a glass bottle, concentrated nitric acid appears yellow.

(a) All glass apparatus is used in the laboratory preparation of nitric acid since the vapours of nitric acid being highly corrosive attack rubber, cork, etc.

(b) When nitric acid is kept in a reagent bottle for a long time, it turns dark yellowish brown in colour.
This is because nitric acid is unstable hence, it decomposes slightly even at ordinary temperatures and in the presence of sunlight. When it is kept for a long time, the decomposition is complete resulting in the formation of reddish brown nitrogen dioxide [NO₂] that dissolves in the acid giving it a darker yellowish brown colour.

4HNO₃ ⟶ O₂ + 2H₂O + 2NO₂

(c) Iron or aluminium are rendered passive on reaction with fuming HNO₃ due to formation of a thin oxide coating on the surface of the metal which prevents further reaction

(d) Dilute nitric acid is generally considered a typical acid except for it's reaction with metals since it generally does not liberate hydrogen on reaction with metals. Nitric oxide on decomposition forms nascent oxygen which oxidizes the hydrogen to water.

(e) Pure nitric acid [HNO₃] is colourless and unstable and decomposes slightly even at ordinary temperatures and in the presence of sunlight.
The decomposition results in formation of reddish brown nitrogen dioxide [NO₂] which remains dissolved in the acid thus imparting a slight yellowish brown colour.