Which process(es) is/are involved in the turning of starch iodide paper blue-black by chlorine gas?

Analysis

The question asks for the chemical mechanism behind the starch-iodide test, which is a classic qualitative test used to detect the presence of oxidizing agents like chlorine ($Cl_2$).

Concept Explanation

To understand this, we need to look at two distinct chemical reactions occurring in sequence:

1. Redox Reaction: Chlorine is a strong oxidizing agent. When it comes into contact with iodide ions ($I^-$) present in the starch-iodide paper, it displaces the iodine. The chlorine is reduced to chloride ions, and the iodide ions are oxidized to elemental iodine ($I_2$). • Equation: $Cl_{2(g)} + 2I^-_{(aq)} \rightarrow 2Cl^-_{(aq)} + I_{2(s)}$
2. Complex Formation: Starch acts as an indicator for iodine. Elemental iodine ($I_2$) reacts with the helical structure of the starch molecule to form a deep blue-black starch-iodine complex. This is a physical-chemical adsorption process.

Step-by-Step Solution

• Step 1: Chlorine gas acts as an oxidizing agent. It reacts with the iodide ions in the paper.
• Step 2: This reaction releases free iodine ($I_2$).
• Step 3: The released iodine then interacts with the starch molecules in the paper.
• Step 4: The interaction between the iodine and the starch creates the characteristic blue-black color.

Looking at the options:

• Option A is incorrect because chlorine does not react directly with starch to produce the color.
• Option B is incomplete; it mentions the iodide ion but ignores the essential role of the starch.
• Option C correctly identifies that chlorine oxidizes the iodide to iodine, which then reacts with the starch.
• Option D is partially correct but misses the initial step of the chlorine gas acting as the oxidizing agent to liberate the iodine.

Conclusion

The process requires both the oxidation of iodide to iodine by the chlorine and the subsequent reaction of that iodine with the starch. Therefore, option C provides the most complete and accurate scientific explanation.

Exam Tip: Always remember that the starch-iodide test is specifically a test for oxidizing agents. If you see "starch-iodide" in a question, immediately think: Oxidation of iodide $\rightarrow$ liberation of iodine $\rightarrow$ blue-black complex with starch.