The presence of starch in leaves with the iodine test is indicated by a blue‑black colour.

Quick Scoop

When a leaf is boiled, decolorised with alcohol, and then covered with iodine solution, any part of the leaf that contains starch will turn blue‑black, while areas without starch stay yellow‑brown.

In simple terms:
If it goes blue‑black with iodine, starch is present. If it stays brownish, starch is absent.

Why this colour change happens

  • Iodine (in a potassium iodide solution) reacts with the amylose component of starch to form a blue‑black complex.
  • Without starch, the iodine solution keeps its original brownish/yellow colour.

Answer in exam style:
When leaves are tested for starch with iodine, a blue‑black colour indicates the presence of starch.

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