Question

When freshly prepared, $HN{O}_3$ is colourless. On long standing, it turns to yellow colour. Explain.

Answer 1

Freshly prepared $HN{O}_3$ is colorless. But on strong heating, it turns to $N{O}_2$ by the reaction,

$4HN{O}_3\to 4N{O}_2+2H_{2}O+O_2$

Liberated $N{O}_2$ dissolves in remaining acid and gives a brownish yellow color. So it appears like brownish yellow.

[Note: Color of $N{o}_2$ is Brown]