Q1.
Keep a leaf in boiling water for five minutes to soften it. Dip this leaf in a test tube containing alcohol. Place the test tube in a beaker containing boiling water. Wait until the leaf becomes colourless (Fig. 10.2a). Take out the leaf and place it on a plate. Now, put a few drops of diluted iodine solution with the help of a dropper on the decolourised leaf (Fig. 10.2b). Wait for a few minutes and observe.
Answer
Observation: the leaf, which was pale cream after decolourisation, turns blue-black where iodine touches it. This indicates the presence of starch in the leaf.
What each step is for:
| Step | What it does |
|---|---|
| Boiling water for five minutes | Softens the leaf and kills it, so that alcohol and iodine can enter easily |
| Alcohol in a test tube, tube kept in boiling water | Hot alcohol dissolves out the green chlorophyll — the leaf becomes colourless |
| Dilute iodine solution on the leaf | Iodine gives a blue-black colour with starch — the test for food stored in the leaf |
Caution: alcohol should never be placed near a heat source directly, as it is highly flammable and can easily lead to fire and burns. That is why the test tube of alcohol is heated indirectly, inside a beaker of boiling water, and why the teacher demonstrates this activity.
Why this matters: a blue-black leaf is direct evidence that the leaf itself made and stored food. It is the test used again in Activities 10.3 and 10.4 to find out what photosynthesis needs.