What it shows
A leaf partly covered with foil for two days and then tested with iodine turns blue-black only where light reached it. Leaves make starch by photosynthesis, and photosynthesis needs light.
The science
In light, chlorophyll in the leaf uses carbon dioxide and water to make glucose, and the leaf stores extra glucose as starch. The foil-covered part gets no light, so it makes no new starch and uses up what it had. Boiling water kills the leaf and breaks its cell membranes, warm alcohol removes the green chlorophyll so the colour change can be seen, and iodine turns blue-black wherever starch is present.
Materials
Cost: under Rs 100. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A potted plant with broad soft leaves (such as balsam, geranium or tulsi)
- Aluminium foil and paper clips
- Tincture of iodine from a chemist, diluted with water to the colour of weak tea, and a dropper
- A kettle or hot water from an adult, a bowl and a cup
- Rubbing alcohol (isopropyl) and a white plate
Safety
For this project: An adult must handle the hot water and the alcohol. Alcohol is flammable: work in an airy room with no flame, stove or heater nearby, warm it only in a cup standing in hot water, and never heat it on a flame, stove or hot plate. Iodine stains skin and cloth and is harmful if swallowed; wash splashes with water.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Keep the plant in a dark cupboard for a day or two so its leaves use up stored starch (destarching).
Cover the middle of one leaf on both sides with a strip of foil held by paper clips, leaving the ends open.
Put the plant in bright sunlight for 2 days, watering it as usual.
Pick the leaf, remove the foil, and ask an adult to dip the leaf in just-boiled water for a minute to kill it and soften it.
The adult puts the leaf in a cup of alcohol standing in the bowl of hot water until the leaf turns pale and the alcohol turns green.
Rinse the leaf in warm water to soften it again and spread it on the white plate.
Drop iodine over the whole leaf and wait a minute.
Draw the leaf showing which parts turned blue-black and which stayed brown.
What you should see
The uncovered ends of the leaf turn blue-black (starch present) and the part that was under the foil stays yellow-brown (no starch). This shows that the leaf makes starch only where light falls on it.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why keep the plant in the dark first?
So that old stored starch is used up and only new starch made during the test shows.
02What does iodine show?
Starch; it turns blue-black with it.
03Why remove the chlorophyll?
The green colour would hide the blue-black colour of the iodine test.
04What else does photosynthesis need besides light?
Carbon dioxide, water and chlorophyll.
Take it further
Test a leaf from a plant with white patches (variegated) to show that only green parts make starch.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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