Question

State your observation in each of the following cases:
When moist starch iodide paper is introduced into chlorine gas.

Answer 1

Iodine is both the least reactive halogen and the most electropositive, which means that through chemical reactions, it appears to shed electrons to form positive ions. Starch is a carbohydrate which is the storage form of energy in plants. It exists in two forms, including amylose and amylopectin. When starch and iodine come to react, they tend to form a blue colour.

Complete step by step answer:
Let’s first learn about the chemical reaction between starch and iodine;
In starch, amylose in the presence of iodine is responsible for the formation of a deep blue pigment while reacting. Within the amylose coil, the iodine molecule slips within. Iodine is not very soluble in liquids, so the iodine reagent is manufactured in the presence of potassium iodide by dissolving iodine in liquids. This makes the complex of a linear triiodide ion soluble, which slips into the starch coil, causing an intense blue-black colour.
Now, let’s look into the reaction, when a moist starch iodide paper is introduced into chlorine gas;
$\,C{l_2} + 2{I^ - } \to {I_2} + 2C{l^ - }\,$
$\,{I_2} + starch \to \text {Blue black compound}$
So, from the above reaction, it is clear that chlorine converts moist starch iodide paper blue black when the iodide ions on the paper are oxidised to iodine and the starch responds to it to make the compound blue-black.

Note:
Many aspects of the reaction of iodine to starch are unclear, but one theory is clear that a strongly coloured starch-iodine complex arises when a solution of diluted iodine is added to starch. The iodide ion is $\,{I^ + }\,$ which has a positive charge. Iodides are labelled compounds of iodine in formal oxidation state $\, - 1\,$ .