Question

The acidic solution of a salt produced a deep blue colour with starch iodide solution. The salt may be :

• A. chloride
• B. nitrite
• C. acetate
• D. bromide

Answer 1

The correct option is B nitrite

The acidic solution of a salt produced a deep blue colour with starch iodide solution. The salt may be nitrite. Acidic solution of nitrite first forms nitrous acid which decomposes to NO, which is then oxidised to nitrogen dioxide.
$2NaN{O}_2+H_{2}S{O}_4\to N{a}_{2}S{O}_4+2HN{O}_2$
$3HN{O}_2\to HN{O}_3+2NO\uparrow +H_{2}O$
$2NO+O_2\to 2N{O}_2$
Nitrogen dioxide gas decomposes iodide to free iodine which gives blue colour with starch solution.
$2KI+2N{O}_2\to 2KN{O}_2+I_2$
$I_2+\text{starch}\to \text{starch iodide}$