Question

Starch iodide paper is used to test for the presence of:
(A)- Iodine
(B)- Iodide ion
(C)- Oxidising agent
(D)- Reducing agent

Answer 1

Hint Starch iodide paper consists of potassium iodide and starch. It is used for detection of nitrite and free chlorine atoms. Free chlorine and nitrite ion readily accept an electron to gain their stable configuration. As they get reduced, they end up oxidizing the iodide ion present in the form of potassium iodide.

Complete answer:
We know that starch iodide is used to detect nitrite and free chlorine atoms as mentioned in the hint above. They act as oxidising agents. They oxidise the iodine ion present in potassium iodide to form molecular iodine and free chlorine gets reduced to chlorine anion.
The reaction is given below:
$[\text{Cl }\\!\\!]\\!\\!\text{ + }{{\text{e}}^{-}}\text{ }\to \text{ C}{{\text{l}}^{-}}$ (reduction)
$2{{\text{I}}^{-}}\text{ }\to \text{ }{{\text{I}}_{2}}\text{ + 2}{{\text{e}}^{-}}$ (oxidation)
The presence of an oxidising is detected when molecular iodine reacts with starch present in starch iodide paper to give deep purple colouration.
From the above explanation we can conclude that starch iodide paper is used for the detection of oxidising agents like free chlorine, nitrite, peroxide etc.

Therefore, the correct answer is option (C).

Note: Starch iodide paper can be used to detect gaseous oxidisers. The paper is dipped in distilled water or few drops are put on the paper and gaseous oxidiser is passed through the paper. Another use of starch iodide paper is that it can be used to control diazotization reactions. Diazotization reactions include Sandmeyer's reaction, Gattermann reaction etc.