Test for Reducing Agents via Decolorization of Oxidizing Reagent

Steps:

• Recognize that reducing agents reduce oxidizing reagents, often causing a color change from colored to colorless.
• Examine options for an acidified oxidizing reagent starting purple and turning colorless.
• Identify B as matching: acidified potassium iodide(VII) (an iodine-based oxidant) goes purple to colorless.
• Eliminate others for incorrect reagents, conditions, or color directions.

Why B is correct:

• Reducing agents reduce I(VII) species (like hypoiodite or iodate in acid) from purple I2 complex to colorless I⁻, per the half-reaction: I₂ + 2e⁻ → 2I⁻.

Why the others are wrong:

• A: Manganate(VII) test expects purple to colorless, not the reverse; wrong direction.
• C: Color change is opposite for iodide(VII); reducing agents decolorize, not colorize.
• D: No acidification, so iodide(VII) not activated as oxidant; incomplete test.

Final answer: B