Question

Chlorine turns moist starch iodide paper blue black. Why?

Answer 1

Hint: The answer is based on the fact that iodine is least reactive halogen and also the electropositive and starch is the carbohydrate and the reaction gives you the correct answer.

Complete Solution :
In the chapters of inorganic chemistry, we have studied about the concepts of titrations and other methods that make use of starch. This acts as an indicator in several titrations.

Now, let us see what happens when this starch reacts with iodide paper.
- Iodide paper or starch iodide paper is the paper which is used to mainly detect the presence of oxidising agents like chlorine, nitrous acid, hydrogen peroxide and when this paper is treated with any oxidising agents it changes its colour.
- In starch, amylase which is present in iodine is responsible for the formation of deep blue colour.
- Iodine is not much soluble in liquids and thus this reagent is manufactured by dissolving iodine in liquids in the presence of potassium iodide.
- The reaction that takes place when moist starch iodide paper is introduced into chlorine gas is as shown below:
\[C{{l}_{2}}+2{{I}^{-}}\to {{I}_{2}}+2C{{l}^{-}}\]
\[{{I}_{2}}+starch\to Blue-black~colour\]

- Here, it is clear that chlorine oxidises iodide ions into iodine of the paper and then the starch which acts as an indicator reacts with iodine to indicate blue- black colour.
Thus, here lies the answer to the question of why chlorine changes moist starch iodide paper blue black.

Note: Reactions of iodine and starch are unclear in many concepts but one thing shows that starch – iodine complex is strongly coloured by the diluted iodine which reacts with starch where the iodide ion has positive charge.