Testing a leaf for starch
How does anyone know a leaf makes starch, the stored food? They test it.
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The idea
Iodine turns blue-black where there is starch.
First the plant sits in the dark for two days. It uses up its old starch. So the leaf has no starch at the start.
Then cover part of one leaf. Put the plant in the Sun. The covered part gets no light.
A teacher boils the leaf in water. Then the teacher warms it in alcohol. The alcohol sits in a bowl of hot water. The alcohol removes the green colour. So the blue-black is easy to see.
Alcohol catches fire easily. Only a teacher does this test.
Add iodine. The covered part makes no new starch, so it stays brown. The green parts in sunlight turn blue-black. A pale patch has less chlorophyll. In this leaf, it stays brown.
A second plant stays in the dark. Its leaf shows no blue-black.
- Dark cupboard, 2 days: old starch is used up.
- Cover part of one leaf. Put it in the Sun.
- Take the green out. Add iodine.
- Covered part stays brown. The rest turns blue-black.