Question

Acidic solution of a salt produced deep blue color with starch and $KI$. The salt is:

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

Answer 1

Answer: (B)

nitrite

Option (B) is correct.
Acidic solution of a salt produced deep blue color with starch and $KI$. The salt is nitrite.
On treating a solid nitrite with $dil.H_{2}S{O}_4$ and warming , reddish brown fumes of $N{O}_2$ gas are evolved. Addition of potassium iodide solution to the salt solution followed by freshly prepared starch solution and acidification with acetic acid produces blue colour. Alternatively, a filter paper moistened with potassium iodide and starch solution and a few drops of acetic acid turns blue on exposure to the gas due to the interaction of liberated iodine with starch.
$2KN{O}_2+H_{2}S{O}_4⟶K_{2}S{O}_4+2HN{O}_2$$2KI+H_{2}S{O}_4⟶K_{2}S{O}_4+2HI$$2HN{O}_2+2HI⟶2H_{2}O+I_2+2NO$$I_2+Starch⟶Bluecolor$