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Chemistry Chapter 7 : Topic 9
Redox Colour Changes
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In a redox reaction, when acidified potassium manganate (VII) solution is added to a solution containing a reducing agent, the purple colour of the solution disappears. This observation is mainly used to identify:
Reasoning: Acidified KMnO₄ (purple) is reduced to Mn²⁺ (colourless), so loss of purple colour indicates a reducing agent.
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A student adds acidified potassium manganate (VII) solution to an unknown liquid. The purple solution becomes colourless immediately. Which conclusion is most appropriate?
Reasoning: Decolourization of KMnO₄ shows reduction of MnO₄⁻, meaning the unknown substance is a reducing agent.
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During a test, potassium iodide solution is treated with an oxidizing agent and the solution turns brown. This brown colour is due to:
Reasoning: Iodide ions are oxidized to iodine, which gives a brown colour in solution.
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When acidified potassium manganate (VII) reacts with potassium iodide, the correct observation is:
Reasoning: KMnO₄ is reduced (purple → colourless Mn²⁺), while iodide is oxidized to iodine (brown colour).
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In a redox reaction involving potassium iodide, starch solution is often used because:
Reasoning: Starch reacts with iodine to form a characteristic blue-black complex, confirming iodine formation.
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A solution of potassium iodide remains colourless until an oxidizing agent is added. The sudden appearance of brown colour confirms that:
Reasoning: I⁻ loses electrons and forms I₂, which is brown in solution.
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Which statement best describes the role of acidified potassium manganate (VII) in redox reactions?
Reasoning: KMnO₄ is a strong oxidizing agent and gets reduced from purple MnO₄⁻ to colourless Mn²⁺.
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A colour change from purple to colourless in acidified KMnO₄ indicates that:
Reasoning: Mn in KMnO₄ is reduced from + 7 to + 2 oxidation state.
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In a laboratory experiment, iodide ions are oxidized. Which observation confirms this most directly?
Reasoning: Oxidation of I⁻ produces iodine (I₂), which is brown.
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Why is acid used with potassium manganate (VII) in redox tests?
Reasoning: Acidic medium ensures correct reduction of MnO₄⁻ to colourless Mn²⁺.
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A student observes that after mixing potassium iodide with an unknown oxidizing agent, adding starch turns the solution blue-black. This indicates:
Reasoning: Blue-black starch complex confirms iodine formation from iodide oxidation.
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Which combination correctly represents a redox reaction?
Reasoning: Redox reactions always involve simultaneous oxidation and reduction.
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In acidified potassium manganate (VII) test, if no colour change occurs, it most likely means:
Reasoning: KMnO₄ remains purple if no reducing agent is present to reduce it.
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The disappearance of purple colour of KMnO₄ is a useful indicator in redox reactions because:
Reasoning: Colour change directly shows reduction from Mn⁷⁺ (purple) to Mn²⁺ (colourless).
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Which statement about potassium iodide in redox reactions is correct?
Reasoning: I⁻ ions donate electrons, so they act as reducing agents.
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A brown solution is observed after adding an unknown substance to KI. Which oxidizing agent behavior is most likely responsible?
Reasoning: Brown colour indicates formation of iodine from iodide oxidation.
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In redox reactions involving KMnO₄ and KI, which pair of changes occur simultaneously?
Reasoning: KMnO₄ is reduced (decolourized) while iodide is oxidized to iodine.
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The best evidence that a redox reaction has occurred between KMnO₄ and KI is:
Reasoning: Dual colour change confirms both oxidation and reduction processes.
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