A covered leaf as evidence
How can we show that light is needed? Scientists change one thing only: the light. They cover part of one leaf, so that part gets no light, and leave the rest of the same leaf in the sun. Everything else stays the same: the same leaf, the same water, the same days. This is a fair test. Afterwards, they test for starch with a chemical called iodine, which turns blue-black with starch. The covered part shows no blue-black, because it made no starch. The starch test needs hot water, alcohol and iodine, which are dangerous, so only trained adults do it, or you read the result from a picture. A safe home version: clip dark card over part of a leaf on a potted plant for a few days and look at the colour. Do not cut the leaf.
| In the test | What we do |
|---|---|
| Change | light: part of the leaf is covered |
| Keep the same | the same plant, the same water, the same time |
| Look for | starch (adult lab test) or colour (home test) |
| Part of the leaf | Iodine result |
|---|---|
| in the light | blue-black (starch) |
| covered strip | stays brown (no starch) |
Plan a test. Write a fair test plan for the card on a leaf: what you change, what stays the same, what you will observe and when. Predict the result before you check. Remember: no iodine, no alcohol and no hot water for children.