Chlorine gas turns damp starch-iodide paper

1. Analyze the question

The question asks for the observable change that occurs when chlorine gas comes into contact with damp starch-iodide paper. This is a classic test for oxidizing agents in chemistry.

2. Explain the concept

Chlorine ($Cl_2$) is a powerful oxidizing agent. Starch-iodide paper contains potassium iodide ($KI$) and starch. When an oxidizing agent like chlorine gas is introduced, it reacts with the iodide ions ($I^-$) in the paper to produce free iodine ($I_2$).

The chemical equation for this reaction is: $Cl_{2(g)} + 2I^-_{(aq)} \rightarrow 2Cl^-_{(aq)} + I_{2(s)}$

Once the free iodine is produced, it reacts with the starch present in the paper to form a deep blue-black complex. This reaction is a standard qualitative test used to detect the presence of oxidizing gases.

3. Show the solution

• Step 1: Chlorine gas acts as an oxidizing agent.
• Step 2: It displaces iodine from the potassium iodide in the damp paper.
• Step 3: The liberated iodine reacts with the starch molecules.
• Step 4: The starch-iodine complex creates a characteristic dark blue (or blue-black) color.

4. Identify the answer

Comparing our findings to the provided options: A. pink B. colourless C. red D. dark blue

The reaction produces a dark blue color. This matches option D.

5. Tip

Think of this as a "color-coded" test: Iodine + Starch = Blue-Black. Whenever you see "starch-iodide" in a question, immediately associate it with the detection of an oxidizing agent and the resulting dark blue color. This is a very common question in JAMB Chemistry, so keep it in your memory bank!