JAMB Chemistry 1988 Question 25

Question 25

Redox Reactions

A mixture of starch solution and potassium iodide was placed in a test tube. On adding dilute tetraoxosulphate (VI) acid and then K₂Cr₂O₇ solutions, a blue-black colour was produced. In this reaction, the

A.iodide ion is oxidized
B.tetraoxosulphate (VI) acid acts as an oxidizing agent
C.starch has been oxidized
D.K₂Cr₂O₇ is oxidized.

Show the answer and explanation

Answer: A (iodide ion is oxidized)

Explanation

Acidified dichromate oxidises iodide ions to iodine, which turns starch blue-black. The dichromate itself is reduced.