Test a leaf for starch
Show on a big screenA money plant leaf has green and white patches. The white patches cannot make food. They lack chlorophyll, the green colouring in leaves. A leaf stores extra food as starch. Iodine is a liquid that shows starch.
- One patch of the leaf has chlorophyll. The other has none.
- The plant sits in sunlight for some hours.
- A teacher heats the leaf in alcohol, in a hot-water bath. The colour comes out.
- Iodine is added.
- The patch with chlorophyll shows starch. The other patch does not.
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Switch off one need. Press the test button.
The rule
A leaf stores its extra glucose as starch. Iodine is a test for starch.
Iodine turns blue-black where there is starch. A leaf needs chlorophyll, light and carbon dioxide to make starch. Where there is no starch, the iodine stays brown.
Water is needed too. These set-ups test three of the four needs. Each set-up switches off one need.
First the plant stays in the dark for three days. In the dark it cannot make food. So it uses its old starch. Starch found later is new starch. A teacher then does the test. The alcohol removes the green colour from the leaf. Then the iodine colour is easy to see. Alcohol catches fire easily. The teacher heats the alcohol in a hot-water bath, away from flames. Children do not do this step.
| Set-up | Iodine test | What it shows |
|---|---|---|
| A leaf with green and white patches | The green patch shows starch. The white patch does not. | Chlorophyll is needed |
| A plant in a closed jar with potassium hydroxide | No starch | Carbon dioxide is needed |
| A leaf with a strip of black paper over part of it | The covered strip shows no starch. | Light is needed |
Potassium hydroxide soaks up carbon dioxide from the jar. A second jar has no potassium hydroxide. Its plant is the comparison.
Potassium hydroxide is a harsh chemical. It burns skin and eyes. A teacher handles it. Children do not touch it.
Try the lab to finish this lesson.