The starch test

The idea

If a leaf turns blue-black when iodine solution is added to it, starch is present.

Is there starch?

A leaf stores starch. But starch cannot be seen with our eyes. We need a test.

What could we add to a leaf to test for starch?

• A few drops of iodine solution
• A few drops of plain water
• Nothing, just look at it

Add the iodine

Add iodine solution to each leaf and watch the colour. The leaves were decolourised first by a teacher.

Blue-black means starch

The leaf from the plant that had sunlight turned blue-black. The leaf from the plant kept in the dark did not.

If a leaf turns blue-black when iodine solution is added to it, starch is present.

• After adding iodine | What it tells us
• Leaf turns blue-black | Starch is present
• Leaf does not turn blue-black | No starch in it

Notes

If a leaf turns blue-black when iodine solution is added to it, starch is present.

Check yourself

Iodine solution is added to a leaf and it turns blue-black. What does this show?

A leaf is decolourised before the starch test. Why?

A leaf from a plant kept in the dark for 36 hours does not turn blue-black. What does this show?

Which colour tells us that starch is present?

• There is no starch. Blue-black shows that starch is present, not that it is missing.
• Starch is present — correct. Yes! Blue-black means starch.
• The leaf is dry. The colour change shows starch, not whether the leaf is dry.
• So that the iodine cannot reach the leaf. Iodine still reaches the leaf. Decolourising just makes the colour change easy to see.
• So that the leaf makes more starch. Decolourising does not make starch. It only removes the green colour.
• So that it is easy to see the colour change — correct. Yes! The change to blue-black shows clearly.
• No starch was produced in it — correct. Yes! No blue-black means no starch.
• The leaf has a lot of starch. A leaf with a lot of starch would turn blue-black.
• The iodine solution does not work. The iodine test works. It turned the sunlit leaf blue-black.
• Pure white. Starch makes the leaf turn blue-black, not white.
• Blue-black — correct. Yes! That is the colour change for starch.
• Bright red. Starch does not make iodine turn red. It turns blue-black.